Comments Off on Why do readings for H2S & NO2 go up when I breathe on the G460 sensors?
Question: Why do the readings for H2S and NO2 go up when I breathe on the sensors in my G460? I notice they settle back down pretty quickly, but why do they go up in the first place?
Thanks very much for sending us your question.
Electrochemical sensors respond to sudden changes in humidity. They rapidly stabilize at the new humidity, but it can take a few seconds. This is true even for H2S sensors. The air in your exhaled breath is very humid (close to 100% RH) and very warm (98.6°F / 37°C). If you select the “PEAK” hold mode, then breathe on the H2S sensor in a G460, the H2S reading can climb as high as one or two ppm before it settles back down. However, this happens very quickly! If you don’t use the “PEAK” hold button to see the highest reading that is momentarily reached, the sensor stabilizes so quickly you probably won’t notice there was a change in readings.
Reducing gas sensors, (like H2S and CO sensors), use oxygen dissolved in the electrolyte to detect the gas being measured. The oxygen in the electrolyte comes from the atmosphere. Water is used and regenerated during the detection reaction, but the overall reaction does not consume water. The only thing that is used up is oxygen, and the gas that is detected. The overall detection reaction used in an H2S sensor converts hydrogen sulfide into sulfuric acid:
H2S + 2O2 → H2 SO4
H2S sensors exhibit only small transients when the concentration of water in the atmosphere changes, and recover very rapidly.
Oxidizing gas sensors, (like NO2, NO, Cl2, ClO2 and O3), use water from the electrolyte to detect gas. The water in the electrolyte that is used up in the detection reaction is replaced with water from the atmosphere. The overall detection reaction used in an NO2 sensor converts nitrogen dioxide into nitric acid:
NO2 + H2O → H2NO3
Because water is more directly involved in the detection reaction, breathing on oxidizing gas sensors (like NO2) can cause a bigger spike in readings.
Both reducing gas and oxidizing gas sensors stabilize in the new humidity as soon as it stops changing. However, it can take oxidizing gas sensors a slightly longer time to stabilize fully in the new humidity.
GfG Instrumentation mining industry customers use a lot of NO2 sensors. The sensors do very well in underground and high humidity situations. However, please make sure that you don’t hold a sweaty palm over the sensor compartment, and try not deliberately breathe on the sensors when the instrument is in normal operation. Also, if you move from an air conditioned low humidity area to a hot, high humidity area, give the sensors a minute to stabilize in the new humidity before you perform a fresh air zero.
Thanks again for the question.
Comments Off on Why is hydrogen such a big issue for CO sensors?
Question: We recently had a hydrogen leak which affected one of the production units at our refinery. However, the hydrogen leak mostly affected the CO readings, and not the readings from the LEL sensors in the instruments we are currently using. We would pick up 0-5% LEL from time to time, but pretty constantly stay at about 50-250 ppm CO even though we knew that could not be right since CO can only be present as a byproduct of combustion from a heater stack or vehicle exhaust. The wind direction and proximity to either of those things that day would not have allowed such a CO reading to occur.
I guess my questions are, what causes cross sensitivity in a CO sensor, what is it sensitive to, and are there any data I could take a look at on the subject?
Thanks for the great question! Sorry you are having problems due to the response of the CO sensors in your instruments to hydrogen.
Substance-specific electrochemical sensors are available for many of the most common toxic gases including hydrogen sulfide, carbon monoxide, sulfur dioxide, chlorine, chlorine dioxide, ammonia, phosphine, ethylene oxide, nitrogen dioxide, ozone and others. “EC” sensors are compact, require very little power, exhibit excellent linearity and repeatability, and generally have a long life span. The detection technique is very straightforward in concept. Gas that enters the sensor undergoes an electrochemical reaction that causes a change in the electrical output of the sensor. The difference in the electrical output is proportional to the amount of gas present. EC sensors are usually designed to minimize the effects of interfering contaminants, making the readings as specific as possible for the gas being measured.
Effects of interfering gases on electrochemical sensors
One of the chief limitations of electrochemical sensors is the effect of interfering gases – the ones that you are not trying to measure with the sensor – on the sensor readings. Substance-specific sensors are ideally supposed to respond only to the gases they are supposed to measure. The higher the specificity of the sensor, the less likely the sensor will be affected by other gases. The composition of the electrodes and type of electrolyte, as well as the use of selective filters for the removal of interfering gases are all ways to increase the specificity of the sensor. For instance, on the inside, a CO sensor is very similar to a sensor used to measure H2S. The trick is to keep the H2S from reaching the CO sensing electrode. Most substance-specific CO sensors include an internal activated carbon filter designed to remove the H2S and other acid gas interferents before they reach the sensing electrode. Thus, the reading of the sensor is not affected by the presence of H2S in the atmosphere being monitored. While inclusion of a filter is frequently able to increase specificity, removal of a filter may be used to broaden response to a wider variety of gases. For instance, carbon monoxide sensors that do not include a filter are sometimes marketed as “dual purpose” sensors for the simultaneous detection of both CO and H2S. This type of sensor responds to both CO and H2S, but cannot tell them apart. The sensor produces a single signal, which is up to the instrument user to interpret. Even though care has been taken to reduce cross-sensitivity in substance-specific designs, interferences still exist. In some cases, the interfering effect is positive and results in readings that are higher than actual. In other cases, the interference is negative and produces readings that are lower than actual. It’s important to understand clearly the effects of potential interferents on the output of the sensors installed. Users should consult the owner’s manual or contact the manufacturer of the instrument they will be using to verify the correct values to use when making decisions based on interfering contaminants.
The reason that CO sensors are potentially susceptible to hydrogen (H2) interference is the reaction that is used to detect gas. Hydrogen is actually part of the detection reaction. The relative response to hydrogen depends on the brand and model of sensor. Some commonly used CO sensors show a relative response to hydrogen as high as 60%. (Not GfG sensors, of course!) Sometimes the cross sensitivity is presented as an advantage to customers. In some cases the response is so high the manufacturer tells customers the sensors should not be used at all in the presence of hydrogen.
GfG instruments can be equipped with a number of different models and types of single and dual channel CO sensors. For applications where hydrogen may be present we use specially designed CO sensors with a very low relative response to H2. For refinery applications we would normally specify a two electrode “hydrogen nulled” 2CF sensor with a catalyst system designed to limit the response of the sensor to hydrogen. The 2CF sensor has a relative response to hydrogen of about 5%. You can’t completely eliminate the relative response, but you can certainly reduce it far below what you are currently seeing!
How electrochemical CO sensors detect gas
Gas that enters the sensor undergoes an electrochemical reaction that causes a change in the electrical output of the sensor. The difference in the electrical output is proportional to the amount of gas present. Gas enters the sensor through an external diffusion barrier that is porous to gas but nonporous to liquid. An internal organic vapor filter is usually included in the CO sensor to remove or at least reduce the interfering effects of solvents, alcohols and other unsaturated hydrocarbon vapors. (The 2CF sensor has a robust internal organic vapor filter which further improves its fitness for use in refinery applications.)
Carbon monoxide that enters the sensor is oxidized at the surface of the sensing electrode, causing the potential of the sensing electrode to rise relative to that of the counter electrode. Current collecting filaments connect the electrodes with the external pins of the sensor. The instrument supplies power to the sensor, and interprets the output of the sensor by readings obtained through the external pins. In two-electrode sensor designs, the potential of the sensing electrode is compared directly to that of the counter electrode. In three electrode designs, what actually is measured is the difference between the sensing electrode and reference electrode.
The electrodes include materials necessary to catalyze the detection reaction. Without the presence of the catalyst the reaction would not occur. Changing the type or relative proportions of the metals in the catalyst system can profoundly affect both the speed of response as well as the relative response of the sensor to interfering contaminants.
The CO detection reaction is a two-step process. The electrolyte in which the reaction occurs is a weak solution of sulfuric acid. In the first step the carbon monoxide is oxidized at the sensing electrode to produce CO2. The reaction generates two electrons of electricity (2e-) for each molecule of CO detected, (which is how the concentration of CO is measured). The reaction uses one molecule of water from the electrolyte, and produces two hydrogen protons (2H+) for each molecule of CO reacted.
CO + H2O → CO2 + 2H+ + 2e-
The second step occurs at the counter electrode. In the second step the hydrogen protons (2H+) produced in the first step react with oxygen in the electrolyte to produce water. The oxygen in the electrolyte comes from the air in which the sensor is located. Electricity from the instrument’s power supply is used to provide the necessary electrons (2e-). Since the molecules of water consumed in the first step are regenerated in the second step, the reaction is said to be “balanced” by this second step.
½ O2 + 2H+ + 2e- → H2O
When the two “half-cell” steps are added together, the overall (or net) detection reaction becomes:
CO + ½O2 → CO2
The working efficiency of the sensing electrode is very high. This means the sensor is usually easily able to oxidize the incoming CO as fast as it reaches the sensing electrode. If the concentration of incoming gas exceeds the ability of the sensing electrode to oxidize the gas, the sensor becomes saturated, in which case the output reaches a maximum value and can’t rise any higher. However, as soon as the concentration of gas in the atmosphere drops below this critical concentration, the sensor rapidly recovers with no damage done to the sensor.
Water from the electrolyte is used, but is regenerated during the course of the reaction. The CO2 produced in the reaction accumulates in the acid electrolyte as carbonic acid. The only materials consumed during the detection reaction are the molecules of carbon monoxide, power from the battery of the instrument and oxygen. As long as the sensor is located in an atmosphere containing even small amounts of oxygen, the sensor is able to replenish itself directly from the atmosphere. This is the reason that non-consuming electrochemical sensors have such long life spans. The lifespan of the sensor is not affected by exposure to the contaminant that it measures. No part of the sensor is consumed during the detection reaction. You can expose the sensor to CO calibration gas every single day without shortening or affecting the lifespan of the sensor.
Effect of hydrogen on CO sensors
The first step in the CO detection reaction generates hydrogen protons. The second step uses hydrogen protons to regenerate the water used in the first step of the reaction. The materials in the sensor are designed to facilitate this process. Even though the sensor is designed for the detection of CO, hydrogen gas can react (and be detected) to at least some extent at the sensing electrode. The reaction for H2 at the sensing electrode is:
H2 → 2H+ + 2e-
The second step at the counter electrode is the same reaction as the second step in the CO detection reaction:
½ O2 + 2H+ + 2e- → H2O
The relative response of the CO sensor to hydrogen is determined by how well the catalyst and electrode system facilitates the hydrogen detection reaction compared to the CO detection reaction. It is possible to reduce the relative response of the sensing electrode to hydrogen by using a slightly different catalyst, and/or using a two electrode rather than three electrode sensor design. (It is very important, of course, to choose a catalyst system that is still a good choice for CO detection!)
The “hydrogen nulled” GfG 2CF sensor is a two electrode design, in a standard size 0.4 inch radius (4 Series) formatted housing. Three electrode single channel CO sensors, as well as four electrode “dual channel” CO / H2S sensors generally have a higher relative response to hydrogen. However, while the 2CF sensor has a much lower relative response to H2, it takes a little longer to clear back down to zero once the exposure is completed.
Most instrument users find that the performance of the 2CF hydrogen nulled sensor is an outstanding improvement over the performance of the standard CO sensors they have used in the past.
The following chart compares the relative response of the hydrogen nulled 2CF sensor and our standard 4CM CO sensor when the sensors are exposed to 1000 ppm hydrogen. The relative response of the 2CF sensor is less than 5%!
The second chart compares the performance of the 2CF hydrogen nulled sensor, and a standard 4 electrode dual channel CO / H2S “COSH” sensor exposed to 1000 ppm hydrogen calibration gas.
The third chart compares the performance of the GfG 2CF hydrogen nulled sensor, and a standard GfG 4CM CO sensor exposed to 200 ppm CO calibration gas. While the hydrogen nulled sensor takes a little more time to recover after exposure, the time to alarm is virtually the same for both sensors.
What other gases and vapors can have an interfering effect on CO sensor readings?
Although the internal organic vapor filter included in most CO sensors reduces the effects of interfering contaminants, once the filter is saturated, breakthrough can occur. Once breakthrough occurs the CO sensing electrode responds to a wide variety of interfering contaminants including alcohols, (such as methanol and isopropyl alcohol), solvent vapors, (such as toluene and MEK), combustible liquids, (such as kerosene and jet fuel), and unsaturated hydrocarbon gases (such as ethylene, propylene and isobutylene). Many CO sensors also show a significant response to nitric oxide, (NO). CO sensors also respond strongly to acetylene gas. Since the internal filter does not absorb acetylene, the sensor shows a very strong and immediate response to this gas. GfG Technical Note 2019, “G460 and Micro IV electrochemical (EC) toxic sensor relative response matrix” includes a chart listing some of the most common documented interferences for the electrochemical sensors used in GfG products. The note is posted on our www.goodforgas.com website at the following link: TN2019
Unfortunately, although CO sensors recover rapidly from exposure to hydrogen, it can take hours or even days for the sensor to recover completely after exposure to solvent vapor or acetylene. Although CO sensors are not usually harmed by exposure to low concentrations, exposure to very high concentrations of solvent or alcohol vapor can permanently damage the sensor. Never use alcohol or solvents to clean the instrument housing or sensor compartment area! Sample draw tubing or filters that are contaminated by solvent or exposure to heavy fuels (such as diesel vapor) should be discarded and replaced.
The following chart shows the response of an LEL sensor and a 2CF “hydrogen nulled” CO sensor to 10% LEL methanol (= 6,000 ppm). Note that this is a very high concentration of methanol! In the following chart you can see when the methanol starts to break through the internal filter in the CO sensor. The CO reading does not begin to rise until almost 4.5 minutes after the methanol exposure is ended. It took several hours for the CO sensor to finish stabilizing back on zero after the exposure to methanol. It should be noted that the final stable reading of 13 ppm for the 2CF CO sensor is extremely low compared to the response of most other CO sensors.
The following chart shows the response of the CO channel of a dual channel “COSH” type sensor for the measurement of CO and H2S to 5% volume (= 50,000 ppm) ethanol. Note that this is a very high concentration of ethanol! The red colored line on the chart shows the response of the LEL sensor. Because the COSH type sensor has a less robust internal filter, the relative response to the alcohol is quite a bit higher (450 ppm) than the response of the 2CF CO sensor in the previous example. It also takes a lot longer for the CO channel of the COSH sensor to recover after the exposure ends.
GfG supplies a substantial number of instruments equipped with the hydrogen nulled 2CF CO sensors for use in oil refinery applications. We receive very positive reviews for instruments equipped with this sensor. Please let us know if you would like to evaluate instruments that have been equipped with this type of sensor.
Thanks again for the question!
Comments Off on What is the best concentration of calibration gas to use for instruments and for bump tests?
Question: We own several different brands of direct reading gas detectors. Different manufacturers seem to use different concentrations of gas to calibrate their instruments. Is there a rule of thumb regarding the best concentration of the calibration gas? Also, do I need to use different concentrations of gas for bump tests?
At GfG the rule of thumb is to use a calibration gas concentration that is near 50% of the full linear range of the sensor. Part of the rationale is that testing a sensor at 50% of full range verifies that the sensor still has enough remaining reaction efficiency to handle higher gas concentrations within the performance specifications of the sensor. This is similar to why a medical examination frequently includes putting the patient on a treadmill for a “stress test”. You may not get the complete picture if all you do is test the patient’s blood pressure while sitting still.
However, you can generally use any concentration within the linear range of the sensor. Sometimes the stability and availability of the calibration gas dictates the concentration we use. For chlorine we generally use 10 ppm gas to calibrate the sensor, even though the full linear range of the sensor is only 10 ppm. (For the chlorine sensor the over-limit concentration is about 12.5 ppm.) Sometimes the odor or toxic nature of the gas determines the concentration. For instance, the standard ranges for GfG H2S sensors are 0 – 100 ppm or 0 – 500 ppm. However, we normally use 20 ppm H2S to calibrate the sensor.
Calibrating with a gas concentration near the alarm concentration is not necessary. I’ve used H2S sensor calibration in the following example.
Calibration is normally a two-point adjustment procedure. In the first step the sensor is “fresh air” zero-adjusted in atmosphere that contains no measurable contaminants. In the second step the instrument is “span” adjusted using calibration gas that contains a precise concentration of the toxic gas. Most calibration gas is manufactured and packaged by specialty suppliers to traceable reference standards. The accuracy and the dating (shelf-life) over which the accuracy statement applies are normally printed on the cylinder label. Several calibration gas manufacturers offer H2S calibration gas with ±3.0% accuracy with 6-month shelf life dating. They also offer ±10.0% accuracy with up to two-years shelf life dating.
The accuracy of the reading is determined by the accuracy of the sensor (= ±5% of reading) plus the effects of the accuracy of the calibration gas (= ±3% of reading). The effect of these relationships on the accuracy of readings can be illustrated graphically.
The instrument is calibrated using 20 ppm H2S calibration gas. The minimum resolution of the H2S sensor in this example is set at 0.2 ppm. The full linear range of the H2S sensor of 0 – 100 ppm.
At 20 ppm the accuracy of the properly calibrated sensor is ±8% of 20 ppm = ±1.6 ppm. When the instrument is exposed to 10 ppm H2S, the accuracy of the reading = ±8% of the reading, which is ±0.8 ppm.
When the instrument is later exposed to 1.0 ppm however, the concentration is much closer to the minimum unit of resolution. Since 8% of 1.0 ppm = 0.08 ppm, (which is less than the minimum unit of resolution); at 1.0 ppm the accuracy of the reading becomes ± 0.2 ppm.
So even though 20 ppm gas calibration gas is used to adjust the sensor in this example, at 1.0 ppm the accuracy of the reading is still ±0.2 ppm (the minimum resolution of the sensor).
GfG Application Note “AP1019 Setting the alarms in electrochemical sensor equipped toxic gas instruments” explores this issue in greater detail. The note is posted on our www.goodforgas.com website at the following link: AP1019
Part of the reason for performing a bump test is to verify that the sensors properly respond when exposed to test gas. However, an equally important part of the test is to verify that the instrument alarms and indicators work properly when the instrument is exposed to gas. The best practice when performing a bump test on a direct reading portable gas monitor is to use concentrations of gas high enough to test the function of both the “low” (A1) and “high” (A2) peak concentration gas alarms for all the sensors installed in the instrument. The gas concentrations should minimally be high enough to at least activate the A1 “low” peak alarms.
Fresh air contains 20.9% oxygen. During the fresh air AutoCal procedure the oxygen sensors in GfG instruments are automatically adjusted to match this concentration. In North America the A1 (descending) peak alarm is normally set at 19.5%. When the concentration drops below this concentration, the first (A1) alarm is activated. In North America the second (A2) descending alarm is normally set at 18%. The standard GfG “Quad Mix” calibration gas normally used to test G450 and G460 instruments in North America includes 50% LEL combustible gas, 200 ppm CO, 20 ppm H2S and 18% O2. The concentration in this mixture is low enough to activate the second descending (A2) oxygen deficiency alarm. However, even for customers who set the A2 alarm at 17%, I suggest using calibration gas that contains at least 18% oxygen, as using a concentration of oxygen lower than 18% can have an effect on the accuracy of the calibration of the LEL sensor. (It’s very easy for customers who set the descending A2 alarm for oxygen at 17% to test the alarm by exhaling on the sensor.)
GfG Application Note “AP1007 Calibration Requirements for Direct Reading Portable Gas Monitors” explores this issue in greater detail, and provides the procedures and definitions that are used in North America. The note is posted on our www.goodforgas.com website in the “Support Materials” section along with the other Application and Technical notes.
Thank you for the question.
Comments Off on What could cause a negative reading on a CO sensor?
Question: Here is my situation. We use G460 instruments for testing the atmosphere in an underground mine. Our instruments are set up to measure O2, LEL, CO, H2S and SO2. I did air sampling with one of our GfG G460 instruments. At the end of the sampling period (about 10 hours), when I picked it up, it displayed a -4 ppm value for carbon monoxide. I’ve never seen negative readings on the CO sensor.
My question: What could cause this to happen?
It was 114 degrees Fahrenheit in the workplace where the GfG instrument was being used. Could this be a factor? The G460 had been bump tested prior to use and is less than two years old.
Thanks very much for your question.
The ambient temperature should not be an issue. G460 instruments are CSA® Certified for use in ambient temperatures up to 55°C (131°F). The age of the sensor should not be an issue either. The dual-channel sensor used to measure CO and H2S in your instrument is warranted for three years, but expected to last even longer.
Carbon monoxide sensors are normally very stable. They are cross sensitive to a couple of potential interfering gases, however.
Usually negative readings are the result of the CO sensor being fresh air adjusted while in the presence of a detectable interfering gas, or when the sensor is fresh air adjusted before it has completed recovering from a prior exposure to an interfering gas.
The CO sensor is always producing a signal as long as the instrument is turned on. In the presence of CO the signal goes up. In fresh air, you still get a signal, but the value is lower. When you fresh air zero the instrument you are telling the instrument to use the signal from the sensor at that moment as the point of comparison.
You always want to make sure that the instrument is located in an area where the air is known to be fresh before making a fresh air “zero” calibration adjustment. If you can’t be sure that the air is fresh you may need to use air from a cylinder of “fresh air” calibration gas. If the instrument uses the signal of the sensor while it is in the presence of a detectable gas, later on, when the sensor is located in an area where there is no detectable gas, the signal will be lower, causing a negative reading.
Sometimes the problem is due to the presence of small concentrations of CO being present in the area where the instruments are being calibrated. Make sure that any possible sources of CO are eliminated. Also, make sure that the calibration gas that used to span adjust the CO sensor is not allowed to accumulate in the area where you fresh air adjust your instruments. Even though four or five ppm of CO is not harmful or dangerous, the presence of the gas can have an effect on the accuracy of the fresh air zero procedure.
The most common interfering gas to have an effect on CO sensors is hydrogen. I am sure you have many intrinsically safe, rechargeable battery equipped devices, such as cap lamps, tractors, etc. around the mine. Try to make sure that the instruments are not stored or calibrated in areas where you are recharging large numbers of batteries, especially the lead acid batteries that are used to power larger types of equipment such as tractors and carts.
The hydrogen does not harm the CO sensor. However, the CO sensors in your instruments will show a slight response. If you fresh air zero adjust the instruments in an area that has 10 or 20 ppm of hydrogen around due to a nearby battery charging station the CO reading can wind up being off by a few ppm later on.
Another type of interfering gas that can have an effect on CO sensors is the vapor produced by solvents. Many paints and degreasers contain solvents such as methanol and toluene. CO sensors have an internal organic vapor filter that is able to remove small concentrations before the sensor is affected. However, in higher concentrations, if the filter becomes saturated, the solvent can fully penetrate the sensor, in which case the reading may start to become affected. Typically, when the CO sensor is exposed to alcohol or solvent vapor, the readings slowly climb for a period of time even after exposure to the solvent vapor has stopped. In the case of heavy exposure it can sometimes take hours, or even a full day (or more) for the sensor to recover completely, and for the fresh air reading to stabilize completely back on zero. The acetylene gas used in welding and hot work procedures has a similar effect, but can take even longer for the sensor to clear.
If the CO sensor is fresh air adjusted before it has completed recovering, say when it is still showing a reading of four or five ppm; later on, when it has finished recovering, it will show four or five ppm negative.
It is important to wait until the sensor has stabilized completely before making a fresh air zero adjustment. If you see the sensor reading is continuing to drop, even slowly, try to wait before performing the fresh air adjustment. If you don’t you may need to readjust the fresh air reading to correct the response.
GfG has an Application Note that discusses the issue of negative readings in greater detail that is posted on our www.goodforgas.com Internet site at the following link: https://goodforgas.com/wp-content/uploads/2013/12/AP1012_meaning_of_negative_readings_02_04_13.pdf
I hope this information is useful. Thanks again for the question.
Comments Off on Can Alcohol vapors affect electrochemical CO sensors?
Question: I was at a chemical factory for explosivity testing and I used a G450 multi gas meter (last factory calibrated earlier this year and field calibrated on site). The chemicals they had were mostly alcohol based liquids. I expected it would pick up some LEL%, but LEL% stayed at zero even when I placed the probe inside the alcohol-based liquid containers. However, elevated CO levels were detected instead. There were neither gasoline-operated equipment nore combustion sources in the area. I am wondering if alcohol vapor can trigger CO sensors?
All electrochemical CO sensors, regardless of the brand or model, respond to high concentrations of alcohol vapor.
CO sensors have an internal filter designed to absorb interfering vapors from VOCs such as alcohols. The filter is sufficient to protect the sensing electrode in the CO sensor from low concentrations, but once the filter is saturated, the alcohol will break through the filter, and the sensor will begin to respond. Once breakthrough has occurred, CO sensor readings typically continue to slowly climb even after the source is removed from the atmosphere being sampled. This is due to VOC slowly being released from the saturated filter into the sensor. Given time, the sensor usually recovers in fresh air without permanent damage. However, depending on the concentration of the exposure, complete recovery can sometimes take several hours, or even overnight. So, it is better not to unnecessarily expose the CO sensor to high concentration alcohol vapor.
The response of the LEL sensor to alcohol vapor is affected by the calibration gas that is used to calibrate the instrument, and the measurement scale you use to display the LEL readings.
The standard calibration gas used to calibrate G450 instruments used in general confined space entry applications contains 50% LEL (2.5% volume) methane. LEL readings are normally displayed in %LEL methane (CH4) increments. Compared to methane, the relative response of the LEL sensor to ethanol is about 80%. That means, the uncorrected reading for ethanol will be lower than the reading for an equivalent concentration of methane. More importantly, however, is the speed of response of the sensor to ethanol compared to methane.
In general, the larger the molecule, the lower the relative response, and the slower the response of the CC LEL sensor. Application note AP1018 that posted on our www.goodforgas.com website that explains combustible sensor performance in greater detail.
Also, on a standard basis, the LEL sensors in general purpose G450 confined space instruments are equipped with a protective external filter designed to remove silicone vapors, (a virulent combustible sensor poison). The presence of the filter slows the diffusion of ethanol vapor into the sensor. The following two graphs show the response of the catalytic combustion (CC) LEL sensor with and without the protective filter. As you can see, with or without the filter the CC LEL sensor responds accurately to ethanol, but with the filter in place it takes longer. The instruments in the charts were equipped with PID and infrared (IR) LEL sensors as well as standard CC LEL sensors. Neither the PID nor the IR LEL sensor is equipped with an external filter, so they provide a good baseline for comparison with the CC LEL sensor.
For your client, assuming that alcohols are the most significant LEL vapors of interest, I would make the following recommendations:
- The instrument should be calibrated and operated using the propane scale. I would suggest using GfG 50% LEL propane equivalent calibration gas for this purpose. You will need to change the instrument settings for the combustible sensor scale from “methane” to “propane”.
- I suggest removing the protective external silicone filter from their LEL sensor. This is a very simple procedure. Our Service Department can talk you through the procedure over the telephone. All you need to do is access the combustible sensor, peel back the protective moisture barrier filter, (which exposes the silicone filter), remove the silicone filter material, and replace the moisture barrier filter.
- We have an alternative CO sensor available for use with the G450 that is more resistant to exposure to alcohol vapors. It also recovers more quickly after high exposures. If response of the CO sensor to alcohol appears to be an issue, I suggest replacing the standard CO sensor in your customer’s instrument with our CO-H (City Technology 2CF) CO sensor.
- When measuring ethanol vapor you must give the CC LEL sensor enough time to reach a final stable reading, however much time it takes, before taking action based on the readings. Remember to build this into your customer’s procedures. Also, remember that the amount of time it takes for the instrument to respond is also affected by the length of sample tubing. The motorized pump draws the sample through the tubing at a rate of about 1 foot per second. The sample has to reach the sensors before they can begin to respond.
Please call the GfG Service Department at 800-959-0329 for assistance with making these modifications, or ordering the new propane scale calibration gas.
Thank you for your question.
Question: I am a Canadian instrument user with a GfG G460 multi-sensor instrument. The instrument is equipped with sensors for the measurement of LEL combustible gas, O2, CO, H2S and hydrogen cyanide (HCN). I am currently performing a bump test on all of the sensors every single day. The biggest issue is the cyanide sensor. It takes longer to complete the bump test for the cyanide sensor than the other sensors, and the cyanide calibration gas is very expensive. What are the manufacturer requirements in regard to performing a daily bump test?
Answer: Thank you for your request for clarification regarding GfG’s requirements for performing a bump test before each day’s use. As a manufacturer, to a certain extent our hands are tied by the standards to which our instruments are certified. However, the requirements may not be as onerous as they appear at first glance.
GfG does not explicitly require that a bump test (function test) be performed on all sensors before each day’s use. The exact wording we use in the G460 manual is:
GfG recommends that you “bump test” the sensors before each use to confirm their ability to respond to gas. To do this, expose the detector to a gas concentration that exceeds the alarm set points. Manually verify that the audible and visual alarms are activated. Calibrate if the readings are not within the specified limits.
From the GfG manufacturer standpoint, use of the term “recommends” indicates the decision is up to the customer. In the United States, instructional guidance letters published by OSHA suggest performing a bump test before each day’s use. However, the OSHA guidance letters are non-mandatory, and in most jurisdictions in the USA, it is still up to the customer. Provincial authorities in Canada generally view the issue differently.
To be legal for sale in Canada, all instruments that include a sensor for the measurement of LEL combustible gas must be certified as Intrinsically Safe according to Canadian requirements. Canadian Standards Association C22.2 NO. 152-M1984 (R2001), “Combustible Gas Detection” is the CSA standard that covers the details of construction, performance, and test procedures for portable instruments used to detect or measure combustible gases in hazardous locations characterized by the known or potential presence of combustible gas. Section 5.3, “Instruction Manual” lists the minimum information and warnings that must be included in the owner’s manual of gas detectors that are compliant with this standard. Paragraph (k) includes the following statement:
CAUTION: BEFORE EACH DAY’S USAGE SENSITIVITY MUST BE TESTED ON A KNOWN CONCENTRATION OF _____ (SPECIFY GAS) EQUIVALENT TO 25-50% OF FULL SCALE CONCENTRATION. ACCURACY MUST BE WITHIN -0-+20% OF ACTUAL.
In other words, to comply with Canadian requirements, the performance of the combustible sensor must be verified by exposure to known concentration combustible gas before each day’s use. This does not come from the manufacturer, it comes from CSA. It is this statement from CSA 22.2 that makes testing the LEL sensor before each day’s use mandatory for many Canadian customers.
You will notice that the standard only refers to testing the performance of the combustible LEL sensor. It does not include a requirement to test other sensors installed in the same instrument. However, some provinces locally require users to perform a bump check on all sensors, (especially the basic sensors in a typical four gas LEL / O2 / CO / H2S instrument), before each day’s use. It may be a good idea to contact the local provincial Ministry of Labour office to confirm the requirements in your area.
While CSA 22.2 defines the pass fail criteria to use when testing the LEL sensor, the standard does not provide guidance on the pass fail criteria to use when testing other types of sensors. GfG uses the definition of “bump test (function check)” developed by the International Safety Equipment Association, (ISEA) and referenced by USA OSHA in instructional letters to USA instrument users. Per the ISEA / OSHA definition, during a bump test (or function check), the accuracy of the sensor readings is not verified or adjusted. The bump test simply verifies that the sensors respond and the alarms are properly triggered when the instrument is exposed to a concentration of gas high enough to cause the activation of the alarms. For the LEL sensor you should stick with CSA pass / fail criteria. For the other sensors, from the GfG standpoint, all you need to do is verify that the alarms are properly activated when the sensors are exposed to gas. Any sensor which fails a bump test must be calibrated, and be found to perform properly before further use.
From the GfG standpoint, as long as the local Canadian authorities agree, based on user experience with the instrument, it may be possible to lengthen the interval between performing a bump test on sensors not used to measure LEL explosive gas. When we are asked for guidance, GfG suggests using the following criteria to help decide whether or not it is prudent to lengthen the interval between bump tests:
- During a period of initial use of at least 10 days in the intended atmosphere, calibration should be verified daily to be sure there is nothing in the atmosphere which is poisoning the sensor(s). The period of initial use should be of sufficient duration to ensure that the sensors are exposed to all conditions that might have an adverse effect on the sensors.
- If the tests demonstrate that it is not necessary to make adjustments, then the time interval between checks may be lengthened but should not exceed 30 days.
- The history of the instrument since last verification can be determined by assigning one instrument to one worker, or by establishing a user tracking system such as an equipment use log.
- Any conditions, incidents, experiences, or exposure to contaminants that might adversely affect the calibration should trigger immediate verification of calibration before further use. Most importantly, if there is any doubt about the calibration of the sensors, expose them to known concentration test gas before further use.
GfG DS-404 automatic test and calibration docking stations significantly simplify calibration and testing procedures, and can greatly reduce the cost by reducing the amount of gas and time required. For instance, GfG docking stations can be set up to test the LEL sensor on a daily basis using inexpensive single-component LEL test gas. The docking station can be set up to test the other sensors using gas from different cylinders on a less frequent basis. There is no need to include all of the sensors every time the docking station performs a bump test on the instrument. Which sensors are tested is up to our customers.
Thanks again for your inquiry.
What could be the cause of suddenly fluctuating or negative readings from a chlorine dioxide (ClO2) sensor?
Background: One of our distributors had just completed testing a different, (not GfG), brand of single-sensor ClO2 instrument with 10 ppm of chlorine dioxide. He confirmed a problem that his customer had brought to his attention. As our distributor put it:
“When continuously exposed to 10 ppm ClO2 the meter runs accurately for about 30 minutes, then begins a nose dive that drives the reading negative (with associated negative drift alarm) over the next 20 minutes. Is it possible that the self-protection is kicking in? This is a very serious issue with use of this meter.”
Answer: The exposure limit for ClO2 is 0.1 ppm (TWA) and 0.3 ppm (STEL). GfG uses Sensoric 3E 1 O chlorine dioxide sensors in our instruments. The sensors are optimized for performance at exposure limit concentrations. The resolution is normally set to 0.01 ppm, over a full linear range of 0 – 2.0 ppm. The sensor goes into over-range alarm when the concentration exceeds the linear range limit by more than 30%, (for ClO2 this would be about 2.6 ppm). We normally use a chlorine generator set to 1.0 ppm to calibrate ClO2 sensors.
Electrochemical sensors used to measure oxidizing gases like ClO2 utilize water molecules in the detection reaction. That makes oxidizing gas sensors more susceptible to humidity related fluctuation than the sensors used to measure reducing gases such as CO and H2S. Our distributor stated that when his customer’s instruments were exposed to 10 ppm ClO2 the readings were initially stable, but then, after 30 minutes, the readings suddenly started counting downwards, and eventually wound up in negative alarm. This would be consistent with the reaction having consumed all of the available water in the sensor electrolyte. The sensor takes on water as necessary from the surrounding atmosphere. But if the available water in the electrolyte is consumed faster than it can be replaced, eventually you run out of water, and the detection reaction comes to a halt until the sensor has a chance to replenish itself.
An additional factor is the dryness of the gas and air used to test the sensor. Gas from a cylinder contains no moisture whatsoever. We normally recommend using either a chlorine generator, or a chlorine dioxide generator to test and calibrate ClO2 sensors. Cl2 and ClO2 generators mix the test gas into a stream of ambient air. The humidity of the ambient air in which the generator is located may vary from one day to the next, but even on a very dry winter day, it is never zero. However, the drier the air, the harder the sensor will have to work to replenish the water used up in the detection reaction. It doesn’t surprise me that using a very high concentration of test gas, mixing the gas into dry winter air, and prolonging the exposure over a 30 minute period, eventually led to fluctuating or declining readings.
Both the GfG Micro IV single-sensor and G460 Multi-sensor gas detector can be equipped with ClO2 sensors. GfG instruments perform very well when equipped with ClO2 sensors. Prolonged exposure to ClO2 at concentrations within the linear detection range should not cause the GfG sensor to lose sensitivity, or begin to display negative readings. Deliberate exposure to concentrations that exceed the upper range limit are not recommended.
The only other time you are likely to see negative readings is when the ClO2 sensor is exposed to a high concentration of a reducing gas (such as H2S).
The ClO2 sensors we use in GfG products are pretty resistant to most interfering gases. However, H2S has a negative interfering effect on ClO2 sensor readings. Exposure to 10 ppm H2S will produce a reading of about – 2.5 ppm. On the other hand, high concentrations of ClO2 or chlorine have very little effect on the H2S sensors we use in GfG products. GfG technical note TN2019 lists relative response values for the electrochemical toxic gas sensors used in GfG products. The note can be downloaded by clicking here.
I normally suggest using a multi-sensor instrument equipped with both ClO2 and H2S sensors when both hazards are potentially present. If you see a high reading on the H2S sensor at the same time you see a negative reading on the ClO2 sensor, you can be reasonably certain of the cause.
Our distributor also asked if using a colorimetric measurement tube might be the best solution for high-range ClO2 measurement. If the customer needs to take action at 10 ppm ClO2 (or higher), using a tube would probably make sense.
An alternative approach for this customer would be to continue to his current instruments, but use them for periodic rather than continuous sampling. As long as you operate the instrument only for brief intervals, and give the sensor time to recover between readings, you will probably not deplete the electrolyte sufficiently to run into any problems. The customer should be careful to discontinue use the instrument if the readings suddenly begin to fluctuate or decline.
OSHA uses the term Permissible Exposure Limit (PEL) to define the maximum concentration of a listed contaminant to which an unprotected worker may be exposed. Depending on the contaminant, the PEL may reference an eight-hour, time-weighted average (TWA), a 15-minute short-term exposure limit (STEL) or an instantaneous ceiling (C) concentration that cannot be exceeded for any period of time. Individual states either follow federal regulations, or follow their own, state-specific permissible exposure limits. States may not publish or follow exposure limits that are more permissive than federal OSHA limits.
The ACGIH TLVs® are guidelines for workplace exposure to toxic substances. TLVs® are developed and designed to function as recommendations for the control of health hazards, and to provide guidance intended for use in the practice of industrial hygiene. But ACGIH TLVs® are frequently incorporated by reference into state, federal and many international regulations governing workplace exposure. They may also be cited or incorporated by reference in consensus standards of associations such as the National Fire Protection Association (NFPA), or American National Standards Institute (ANSI).
Given the potential for lawsuits, many employers have made the strategic decision to base their corporate health and safety programs on conservative applicable recognized standards. Since ACGIH recommendations are frequently more conservative than OSHA PELs, many programs, especially the programs of multinational or prominent corporations, use the ACGIH TLV®.