Question:

Can the LEL sensors in the GfG instruments detect gasoline and kerosene? And can they measure the vapors of these petroleum derivatives?

Answer:

Catalytic LEL sensors can be used for gasoline, but are not recommended for kerosene.  The problem is the size of the molecules in the kerosene.  The larger the molecule, the slower the sensor responds, and also, the lower the relative response.  Another problem is that the toxic exposure limit for heavy fuels like diesel, jet fuel and kerosene is very low.  The TLV for gasoline is 300 ppm.  The TLV for kerosene is only 30 ppm.

The 100% LEL concentration for kerosene is 0.7% volume (= 7000 ppm).  That means 10% LEL kerosene = 700 ppm.  If you use a catalytic LEL sensor, and set the alarm at 10% LEL, even if the sensor responded perfectly for kerosene, it would take a concentration over 23 times higher than the exposure limit to activate the alarm.

For this reason, we usually use a PID sensor to measure kerosene and gasoline vapor, which allows us to set the alarm at the ppm toxic exposure limit.  You still have an LEL sensor installed in the instrument, but you take action at the toxic exposure limit concentration.

Also, if you decide to use a catalytic sensor to measure gasoline you should use the non-filtered version of the CC LEL sensor.  The standard CC LEL sensor is warranted for 3 years , but the unfiltered LEL sensor is warranted for 2 years.

Another approach is to use an infrared (IR) LEL sensor to measure explosive range concentration gasoline and kerosene.  The IR LEL sensor responds better than the CC LEL sensor to these vapors.  However, you still have the issue of taking action at the TLV rather than taking action at 10% LEL.  Also, since IR LEL sensors cannot detect hydrogen, we normally include an electrochemical hydrogen sensor in any instrument that includes an IR LEL sensor.

There are some Application Notes that discuss these issues in greater detail:

AP1001 discusses how the various detection technologies are used to measure gas:

Choosing Best Technologies for Comb Gas and VOC Measurement

AP1018 discusses catalytic (CC) LEL sensors in detail:

Combustible Sensor Performance

AP1014 discusses using PID sensors to measure heavy fuel and toxic VOC vapors:

Monitoring Toxic VOCs in Oil Industry Applications

Question:

I was just reading your information on gases produced by lead acid batteries and came across this in the “Ask Bob” blog:

For LEL range measurement, using a standard catalytic combustible gas (CC) sensor with a range of 0 – 100% LEL is a good approach. For situations where you need to take action at a lower concentration, using an electrochemical (EC) toxic gas sensor to measure the hydrogen may be a better approach. The typical range for an EC hydrogen sensor is 0 – 2,000 ppm. (This is equivalent to a range of 0 – 5.0% LEL.)………..Should that be 0–0.05?

Answer:

This is a good question.

It’s important not to confuse parts-per-million or percent by volume measurements with percent LEL measurements. And decimal places can be tricky!

Hydrogen (H2) can be measured by means of catalytic type percent LEL combustible gas sensors, or by means of substance-specific electrochemical sensors.

Catalytic LEL sensors detect gas by oxidizing or “burning” the gas. Catalytic LEL sensors require the presence of oxygen in order to detect gas. Standard catalytic LEL sensors cannot detect gas if the atmosphere contains too little oxygen. Most of the time measurements for combustible gas are given in percent LEL (% LEL). The reading provides a comparison of the measured concentration against the LEL concentration of the gas used to calibrate the sensor. Deciding which combustible gas or “scale” to use when you calibrate the sensor is an important issue. These issues are discussed in detail in Application Note 1018, “Understanding catalytic LEL combustible gas sensor performance.” The note is posted at the following link: https://goodforgas.com/wp-content/uploads/2013/12/AP1018_Combustible-sensor-performance_6_30_13.pdf

Electrochemical sensors use a substance-specific chemical reaction that causes a change in the electrical output of the sensor that is proportional to the concentration of the measured gas. The electrochemical hydrogen sensors in GfG instruments can be used for extended periods of time to measure H2 in oxygen free atmosphere. Most substance-specific electrochemical (EC) sensors read in percent volume or parts-per-million (ppm) units. GfG offers several different versions of electrochemical hydrogen sensors for different applications.

For low range leak detection we usually use an electrochemical hydrogen sensor with a measurement range of 0 – 2,000 ppm. The readings for this sensor are given in 1.0 ppm increments. The sensor is able to detect changes in concentration of ± 1.0 ppm.

When the application calls for a wider measurement range we use an electrochemical hydrogen sensor with a measurement range of 0 – 4.0% volume. The readings for this higher range hydrogen sensor are given in 0.01 percent volume increments. One percent (1.0%) volume is equal to 10,000 ppm. One hundred percent (100%) volume is equal to 1,000,000 ppm. So, 0.01% volume = 1,000,000ppm X .0001 = 100 ppm.

The higher range sensor cannot detect changes in concentration less than 0.01% (or 100 ppm). If you need to measure smaller changes in concentration, you should choose the lower range version sensor.

According to OSHA, “Lower explosive limit (LEL)” means the minimum concentration of vapor in air below which propagation of a flame does not occur in the presence of an ignition source. When the concentration of gas reaches 100% LEL, it can be ignited if a source of ignition is present. Different combustible gases have different lower explosive limit concentrations. For instance, the LEL for methane (CH4) = 5.0% volume, while the LEL for propane (C3H8) = 2.1% volume.

The lower explosive limit (100% LEL) concentration for hydrogen is about 4.0% volume, which is equal to 40,000 ppm.

So, 1.0% LEL hydrogen = 40,000 ppm X 0.01 = 400 ppm.

So, a concentration of 2000 ppm hydrogen = 2000 ÷ 400 = 5.0% LEL.

Question:

We manufacture an ozone based air and water purification system.  We need to determine the concentration of any bacteria that remain after the purification process.  We have been using a luminometer to determine the bacterial count, and verify the effectiveness of our system on solid surfaces, as well as in water.  However, it is very difficult to determine the concentration of bacteria present in the air.

Bacterial respiration produces a mixture of methane, CO2 and volatile organic gases. Is it possible to measure the waste products produced by bacterial respiration, and use this as an indicator?

We were wondering whether we might be able to use a photoionization detector (PID) for this purpose.  When we spoke with one of the other leading PID manufacturers we were told this probably would not work.  What do you think?

Answer:

I agree with the answer you received from that “other” PID manufacturer.  Direct reading instruments can be used to measure by-products of microbial respiration, but it takes a lot of microbes to produce a meaningful change in concentration.  In order to produce contaminants microbes have to be actively metabolizing.  Most of the bacteria present in the air are in the form of dormant spores, and are not actively respiring.  Spores are smaller, lighter, and remain suspended in the air for a much longer period.  You can easily culture any viable spores that remain after sterilization, but measuring atmospheric contaminants produced by the microbes while they are still in the air is next to impossible. Even when they are present on solid surfaces or in water, the bacteria have to be actively respiring in order to produce detectable metabolic by-products.

In a confined space, where there is no mixing with fresh air, microbial decomposition can easily create hazardous atmospheric conditions.

There are many different types of bacteria and microbes involved in this process.  Some types of “aerobic” microbes use oxygen, and produce carbon dioxide.  Other types of “anaerobic” bacteria that do not use oxygen produce methane and hydrogen sulfide.  Which types of bacteria are active at any moment depends on the type of organic material that is present in the confined space, the oxygen concentration in the space at that time, and other environmental conditions such as humidity and temperature.

The effects of microbial decomposition on the atmosphere in the space often (but not always) follow the same sequence.  Aerobic respiration, which utilizes oxygen, is the most efficient way to convert organic material into energy.  That’s why human beings are aerobic organisms that require oxygen.  When not actively metabolizing, bacteria and microbes are present in the form of dormant spores.  The still atmosphere in a confined space initially contains plenty of oxygen.  These early conditions are good for aerobic decomposition.  Oxygen using bacteria and microbes become active, and begin to proliferate.  Aerobic bacteria deplete the oxygen, and generate CO₂.  Being much heavier than fresh air, the CO₂ tends to accumulate in the bottom of the space, creating locally anaerobic conditions.  Anaerobic bacteria remain in the form of inactive spores until conditions become agreeable for their metabolism.  As the atmosphere becomes increasingly oxygen deficient, anaerobic microbes germinate and begin to metabolize.

Anaerobic microbes do not require oxygen.  Anaerobic decomposition is less efficient, and proceeds more slowly than aerobic decomposition.  The metabolic byproducts of anaerobic respiration include methane (CH₄) and hydrogen sulfide (if the organic material in the space includes sulfur).  The more sulfur the organic material in the space contains, the greater the concentration of H₂S that is likely to be produced by anaerobic bacterial action.  Being heavier than air, the H₂S also tends to accumulate near the bottom of the space.  Methane, being lighter than air, tends to rise, and accumulates near the top of the space, or escapes from the space, if there are any openings.

I would suggest continuing to assess the bacterial count on solid surfaces and in the water by means of the luminometer.  Unless you leave the sterilized area alone for a lengthy period of time, you are unlikely to see anything going on in the air, even if the purified armosphere includes viable spores.  The spores have to germinate to have an effect on the atmosphere.

On the other hand, a prime application for the G450 and G460 is to monitor the atmosphere where microbial action can cause dangerous conditions.  Anaerobic fermentation is used to produce alcohol, wine, distilled spirits and beer.  It is highly associated with the presence of dangerous levels of CO₂, as well as oxygen deficiency.  Hydrogen sulfide is highly associated with sewage and wastewater treatment, oil production and refining, commercial fish and meat processing, and many other industrial applications.  Methane produced by microbial action is highly associated with many types of confined spaces, including sewers, manholes, digesters, vaults and tunnels.

So, while a few dormant spores may not cause a measurable change in the atmosphere, large numbers of actively metabolizing bacteria can rapidly produce deadly conditions!

Thank you for the question.

Question: I’ve spoken with a number of other gas detection suppliers. They always seem to run up against the same problem. We have both CO₂ and HF that is produced in our process. The other suppliers say their SO₂ sensor has a relative response to HF of 1:1. Is there anything we can do? Are your sensors any different than those of the other manufacturers?

Answer:

The answer is to use multiple sensors to rule in or rule out potential interfering gases.

CO₂ does not affect SO₂ or HF sensors.  We use an IR sensor to measure CO₂.  The IR CO₂ sensor is not affected by exposure to SO₂ or HF.

The relative response of the HF sensor to SO₂ is about 0.8.  This is the only situation where you have a high relative response between one and another of the gases on the customer’s list.

The relative response of the SO₂ sensor to HF is very low.  For short exposures to HF gas, the SO₂ sensor relative response should be close to zero.  If the internal filters become saturated due to high exposure, the HF should theoretically produce a slight negative interfering effect on the SO₂ sensor.  However, I did not get a response even when I exposed an SO₂ sensor to 10 ppm HCl gas (which we use as a surrogate for HF) for over 10 minutes.

Putting this information into a logic chart allows you to determine which gas is likely to be present:

Thus, in the event of simultaneous exposure to both SO₂ and HF, the true concentration of SO₂ is determined by the SO₂ reading.  The true concentration of HF is determined by subtracting the 80% of the value of the reading of the SO₂ sensor from the reading from the HF sensor.

Let’s say you have a reading of 20 ppm from the HF sensor, and a reading of 10 ppm from the SO₂ sensor.  The true concentration of SO2 = 10 ppm.  The true concentration of HF = (20 ppm) – (10 ppm X 0.8) = 12.0 ppm.

Relative responses vary between sensors, and can change a little over the life of the sensor.  When you use relative response factors to calculate the actual concentration remember that relative response factors are approximate values!

HF sensors require a little extra effort when it comes to calibration.  Also they usually need to be replaced after one year, when the warranty expires.

We have a Technical Note which discusses HF sensor calibration issues that is posted on our website www.goodforgas.com at the following link:

TN2014 Calibrating GfG instruments equipped with HF sensors 1-14-13 (pdf)

The note discusses HF sensor calibration from the standpoint of the Micro IV, but the issues are exactly the same when the HF sensor is installed in a G460 multi-sensor instrument.  It is likely that at least some of the customer’s prior negative experience with HF sensors had as much to do with calibration as with issues related to relative response.  Also, HF sensors are very prone to exhibiting short term fluctuations due to abrupt changes in humidity.  Breathing on the sensor, or putting your hand over the HF sensor causes the reading to fluctuate.  The reading stabilizes as soon as the humidity stabilizes.  Customers need to be careful to avoid abrupt changes in humidity, and to give the sensor time to stabilize when exposed to new conditions in the ambient atmosphere.

I hope this information was helpful.

Question: I have a customer who is using an electrochemical sensor for CO to comply with NIOSH Method 6604, however they are using Nitrogen instead of air as Zero Gas; they have Nitrogen for Gas Chromatography purposes. This Nitrogen has 99.9% purity. An ISO 17025 auditor told them that this could affect the calibration process for this sensor, so the customer is asking me if this is true.

Do you think that the Nitrogen could affect the sensitivity of CO for this electrochemical sensor? Or is this procedure correct?

Thanks in advance for your help!

Answer:

The first step in the calibration process is the “fresh air calibration” or “fresh air zero adjustment”.  In this step the instrument looks at the electrical output of the CO sensor while it is located in contaminant free fresh air, and adjusts the instrument reading to 0 ppm.  This provides a point of comparison when the sensor is exposed to atmosphere that contains carbon monoxide.  The electrical output of the sensor when it is exposed to CO is higher than the electrical output of the sensor in fresh air.  The difference in electrical output is proportional to the concentration.  Calibration gas that contains CO is used to adjust the reading of the sensor while it is exposed to gas.  This is referred to as the “span calibration” or “gas calibration” step.

The electrochemical reaction used to detect CO requires oxygen.  However, the sensor does not require or use O₂ during the fresh air zero adjustment step.

Given the tolerances and accuracy of the electrochemical CO sensors used in health and safety instruments, using nitrogen instead of fresh air to adjust the CO sensor should not materially affect the accuracy of the fresh air zero adjustment. However, it is always better to use zero contaminant fresh air, not pure nitrogen for this calibration step.

All electrochemical CO sensors use the same two part reaction to detect gas.  The first half-cell reaction occurs at the sensing electrode.  The second half-cell reaction occurs at the counter electrode.

Carbon monoxide is oxidized at the sensing electrode.  Water from the sensor electrolyte is used up during this first step. The electrolyte in the sensor is an aqueous solution of sulfuric acid.

2CO  +  2H₂O    →    2CO₂  +  4H⁺  +  4e-

The counter electrode acts to balance out the reaction at the sensing electrode by reducing oxygen present in the air to water.  The water that is consumed in the first step is regenerated in the second step.  The oxygen used in this step comes from O₂ dissolved in the electrolyte.  The oxygen dissolved in the electrolyte comes from the atmosphere in which the sensor is located.

O₂  +  4H⁺  +  4e-    →   4H₂O

When you add the two half-cell reactions together you get the overall reaction:

2CO + O₂      →    2CO₂

For every 2 molecules of CO you detect you use up one molecule of O₂.

Note that unless the sensor is in the presence of a high concentration of CO, very little O₂ is actually used by the sensor.  Even when the atmosphere is oxygen deficient there is usually more than enough oxygen in the air to replace O₂ consumed in the detection reaction.

Long term exposure to pure nitrogen eventually depletes the oxygen in the electrolyte.  If the oxygen is depleted, the sensor is unable to detect gas.  However, this takes quite a while to occur.  The electrochemical sensors used in GfG portable instruments are full sized, “4 Series” sensors.  Span calibrating or zero-adjusting a GfG electrochemical CO sensor with pure nitrogen, or using CO in nitrogen to span adjust the sensor will not materially affect the accuracy of readings.

The time it takes for the electrolyte to become oxygen depleted is affected by the volume of electrolyte in the sensor.  Miniaturized or low volume (flat) CO sensors that contain less electrolyte, (and less dissolved oxygen) can become O₂ depleted more quickly.

So, as a general rule, it is better when possible to use contaminant free fresh air to zero-adjust the sensor, and CO calibration gas that includes oxygen when a span calibration adjustment is performed.

Question: Can hydrogen have an effect on CO readings, and can downloading the data help determine what’s going on?

Answer:

The attached .txt file is a very cool data download from a G460 instrument (the file is posted at the following link: 06262015.txt ). The readings are from an instrument that belongs to Robert Lindley, the Eastern Zone Manager for GfG.  Robert was in his garage, getting ready to calibrate his instruments prior to leaving on a business trip for GfG.  When he turned his G460 on he was shocked to see high readings for carbon monoxide.  He left the garage, opened the garage door from the outside, and did not re-enter until the readings returned to fresh air values.  The response of the instrument provides a good example of conditions that can affect readings, and how to interpret the results.  The data-logging interval is set to 1.0 minute, which means our “G450 / G460 Data Viewer Software” is perfect for looking at the results.  If you don’t yet have a copy, the software is posted in a .zip file on our website at the following link:  https://goodforgas.com/wp-content/uploads/2015/05/DDS_G450-G460_v4395.zip

What caused the readings?

Robert has a golf cart with a faulty battery charger.  Instead of the charger shutting down or going into trickle when the battery finished charging, it was continuing to pump power into the battery at the full charging rate.

We have a blog entry on www.goodforgas.com that talks about what gases to look for in areas where lead acid batteries are being charged:  https://goodforgas.com/hazardous-gases-associated-lead-acid-battery-charging-stations/

During charging, (especially in the event of overcharging), lead acid batteries produce oxygen and hydrogen.  These gases are produced by the electrolysis of water from the aqueous solution of sulfuric acid.  Since the water is lost, the electrolyte can be depleted.  This is why you need to add water to non-sealed lead acid batteries.  When a lead acid battery cell “blows” or becomes incapable of being charged properly, the amount of hydrogen produced can increase catastrophically.

Hydrogen is not toxic, but at high concentrations is a highly explosive gas.  The 100% LEL concentration for hydrogen is 4.0% by volume.  At this concentration, all it takes is a source of ignition to cause an explosion.  Sparking from a battery terminal as it is connected or disconnected from the charging system is more than adequate as a source of ignition energy.  That’s why lead acid batteries should only be charged in well ventilated areas.

Why did the hydrogen have an effect on the CO readings?

This is the chemical reaction that is used in electrochemical CO sensors to detect gas:

CO Sensing Electrode Reaction:    2CO + 2H₂O   →    2CO₂ + 4H⁺ + 4e^–

CO Counter Electrode Reaction:    O₂ + 4H⁺ + 4e^–   →   2H₂O

In the first part of the detection reaction, the sensor consumes water from the electrolyte (the solution in the sensor) and generates hydrogen (H⁺).  In the second part of the reaction, the hydrogen reacts with oxygen in the electrolyte to produce water.  Because the CO detection reaction includes hydrogen, it’s easy for hydrogen in the ambient air to have an effect on readings.

At GfG we use CO sensors that are deliberately designed to limit the response to hydrogen.  However, they still show at least some response.

The following chart shows the response of a substance-specific, electrochemical hydrogen sensor to 1,000 ppm hydrogen calibration gas (the green colored response curve).  The readings of the H₂ sensor are very accurate when the sensor is exposed to gas.

The blue colored line shows the response of the CO channel of a COSH sensor to the same 1,000 ppm hydrogen gas.  COSH sensors simultaneously measure both CO and H₂S.  We use the “two-in-one” COSH sensor whenever we need to save space in the G460.  Using the COSH sensor means we have another sensor position available to measure a different gas.

COSH sensors have one sensing electrode for H₂S, and a separate sensing electrode for CO.  The sensor is split into two chambers.  H₂S is measured in the outer chamber, and CO is measured in the inner chamber.  The sensor is designed for the rapid diffusion of atmosphere that contains CO into the inner chamber.  The CO channel of the COSH sensor has a relative response to hydrogen of about 33%.  As you can see, when we exposed the COSH sensor to 1,000 ppm hydrogen, it produced a reading of 330 ppm on the CO channel.

We only use the COSH sensor when we need to save space.  When we don’t need to save space, we use a single-channel CO sensor that is specifically designed to have the lowest possible response to hydrogen.  The single-channel “hydrogen nulled” CO sensor is the standard sensor we use in all 4-gas G450 instruments.  Even though the sensor is designed to show the lowest possible response, it still shows a relative response to hydrogen of 56 ppm (= 5.6 %).

Robert’s results:

“Session 5” started when Robert turned the instrument at 8:28 AM on June 26, 2015.  The session ended when he turned the instrument back off at 10:18 PM on the same date.  In the following set of graphs, the actual CO readings are shown in blue, the 15-minute STEL calculation is shown in black, and the 8 –hour TWA calculation is shown in green.  The software lets you view or hide each of the graphs by clicking on the legend for the graph at the bottom of the screen.

Let’s start by hiding the STEL graph.

When you look at the real-time CO readings, the concentration was well above 0 ppm from the moment Robert turned the instrument on.  But at 10:50 AM something really interesting happened.  Robert had been moving the instrument around the garage, trying to find the source of the CO readings.  You can see the concentration at any given moment by moving the cursor over the graph to the point of interest.

At 10:50 AM Robert positioned the instrument close to the lead acid battery in the golf cart, and the readings immediately climbed to 127 ppm.  Given that the CO channel of the COSH sensor has a relative response to hydrogen of about 33%, the true concentration of hydrogen in Robert’s garage where the instrument was located at 10:50 AM was about 100/0.33 ≈ 300 ppm H₂.

The readings rapidly went up and down when Robert moved the instrument back and forth near the battery.  Because hydrogen is actually part of the electrochemical CO sensor detection reaction, the sensor responds and recovers very quickly.  This is different than the response of the sensor to other gases (such as acetylene) that can interfere with CO sensor readings.  It can take hours for the sensor to recover after exposure to acetylene.  In the case of hydrogen the response and recovery is very rapid, which is exactly what you see in the graph.

When you look at the table (instead of the graph) of the logged results for Session 5, you see something else that is very interesting.  In the “Table View” of the session, any intervals where the concentration exceeds the alarm settings are flagged in red.  The low (A1) alarm for CO was set at 35 ppm.  The STEL alarm was set at 50 ppm, and the TWA alarm was set at 35.  There are three intervals in the table (flagged in red) where the A1 alarm was exceeded.  There were no intervals where the STEL or TWA alarms were exceeded.

Now for the interesting part.  At the exact moment the CO reached 127 ppm, the H₂S reading started to rise, reaching a concentration of 0.1 ppm.

You can also see this when you look at the H₂S graph for Session 5:

This makes perfect sense, and helps confirm that the CO readings were actually due to the presence of hydrogen.  The hydrogen sulfide (H₂S) channel of the COSH sensor has a very small relative response to hydrogen of about 0.03%.  If the true concentration of hydrogen in the area of the instrument at 10:50 AM was about 300 ppm, you would expect the H₂S channel of the COSH sensor to read about 300 x .0003 ≈ 0.09 ppm H₂S.   The actual reading was 0.1 ppm, which is extremely close to the expected value.  Sweet!

Why didn’t the LEL sensor detect the presence of hydrogen?

Hydrogen is a flammable gas.  The lower explosive limit (LEL) is the minimum concentration of gas that can explode if a source of ignition is present.   The LEL concentration for hydrogen is 4.0% volume, (which is equal to 40,000 ppm).

The LEL alarm is normally set at 10.0% LEL.  Assuming the instrument is properly calibrated, it would take about 4,000 ppm hydrogen to activate the LEL alarm.  However, Robert’s instrument was calibrated to methane, which has a slightly different response.

Assuming the highest true concentration of hydrogen in Robert’s garage was about 300 ppm, it’s easy to see why the LEL sensor didn’t show a reading.  Even if the instrument had been correctly calibrated for hydrogen, the concentration in Robert’s garage was still less than 1.0% LEL.  The concentration was just too low to register on the LEL sensor.

What if I wanted to directly measure ppm hydrogen?

No worries!  All you need to do is install a 0 – 2,000 ppm range, (= 0 to 5% LEL) substance-specific hydrogen sensor in your G460.

Robert’s downloaded results illustrate how useful it can be to actually look at the graphs and tabular response of the sensors over time.   With a little bit of experience the shape of the curves can provide all sorts of additional critical information.

Thanks again to Robert for sending the download to my attention.

Bob Henderson

President

GfG Instrumentation, Inc.

Question: One of our Canadian friends in the Industrial Hygiene community sent us the following question:

A colleague of mine was talking the other day about the presence of H₂S in crude oil.  In July, 2013 we had a terrible rail accident involving crude oil that occurred in the town of Lac-Mégantic in Quebec.   I have heard no mention of H₂S in official reports about the accident.   Nevertheless, some unofficial data is indicating that H₂S may have been present in very high concentrations, perhaps even reaching the flammable limit of 4%.  You have mentioned that H₂S is a poison to catalytic combustible LEL sensors.  Am I correct in guessing the LEL sensor would not be dependable in an H₂S LEL environment?   Also, if H₂S is damaging to the sensor, what concentration of H₂S can the LEL sensor handle, and for how long?

Answer:

This is a great question.  I think the answer will make a good post for our gas detection blog on the company website ( www.goodforgas.com ).

It is true that H₂S is a potentially flammable gas.  The LEL concentration for H₂S is only 4.0%.  H₂S can easily reach 100% LEL in many confined space and oil industry settings, especially when the situation involves “sour gas” or “sour crude” that contains high concentrations of sulfur.

Exposure to high concentrations of H₂S can inhibit or poison catalytic LEL sensors.   To keep this from happening, most LEL sensors, especially the ones used in portable instruments, have an internal filter to remove the H₂S before it can reach and damage the active bead in the sensor.  This is why you can use multi-component calibration gas that includes both H₂S and LEL gas to calibrate the combustible gas sensor without causing damage to the sensor.

Catalytic LEL sensors detect gas by catalytically oxidizing or “burning” the gas on an active bead or “pellistor” located within the sensor.  The sensor contains two coils of fine platinum wire which are coated with a ceramic or porous alumina material to form beads. The beads are wired into opposing arms of a balanced Wheatstone Bridge electrical circuit. The “active” bead is treated with a platinum or palladium-based catalyst that facilitates the oxidation of combustible gas on the bead. A “reference” bead in the circuit that has not been treated with catalyst provides a comparison value.  As oxidation occurs the active bead is heated to a higher temperature. Since heating due to oxidation of the combustible gas only occurs on the active bead, the difference in temperature between the two beads is proportional to the concentration of gas in the area where the sensor is located.  Because the two beads are strung on opposite arms of the Wheatstone Bridge circuit, the difference in temperature between the beads is registered by the instrument as a change in electrical resistance.

The LEL sensor depends on the activity of the catalyst to detect gas.  If the catalyst is harmed by exposure to sensor poisons or inhibitors, the sensor’s ability to detect gas is affected.  In the case of sensor inhibitors the effects may be reversible, and the sensor may recover over time (at least to an extent).  In the case of virulent LEL sensor poisons like tetraethyl lead and vapors that contain silicone, the damage is rapid and irreversible.  H₂S functions both as a poison and as an inhibitor.  The sensor may recover some of its lost sensitivity over time, but once the bead has been exposed to H₂S, there will be permanent harm.

The internal filter used to remove H₂S has a capacity on the order of 20,000 ppm minutes.  Each time you expose the LEL sensor to gas that contains H₂S you use up a little of the remaining capacity of the filter.  For most instrument users the capacity  of the filter is sufficient to protect the LEL sensor for the entire expected life of the sensor.  Bump testing the instrument before each day’s use has very little effect on shortening the life of the LEL sensor.

During the bump test the sensors are exposed to known concentration test gas.  The concentrations used in the test gas need to be high enough to activate the alarms.  In Canada, CSA takes the requirements a step further.  Because LEL sensors can be poisoned or damaged by exposure to sensor poisons like H₂S, it’s not enough simply to activate the LEL alarms.  To pass the test CSA requires that the reading of the LEL sensor is between  “minus 0% and plus 20%” of the concentration of gas applied.  In other words, if you use 50% LEL gas to test the combustible sensor, to pass the CSA version test the readings must stabilize between 50% LEL and 60% LEL.

Typical “four gas” multi-gas atmospheric monitors include sensors used to measure O₂, LEL, CO and H₂S.  The gas typically used to perform bump checks on these meters is a multi-component mixture that is designed to test all of the sensors at the same time.  The gas (depending on the manufacturer) usually includes 20 ppm or 25 ppm H₂S.

It typically takes less than 30 seconds to perform a manual bump test.  Thirty-seconds of exposure to 20 ppm H₂S is the same thing as “10 ppm minutes” of exposure.  Since the capacity of the filter is 20,000 ppm minutes, it takes around 2,000 bump tests to saturate the filter.   Even if you bump test the instrument every day, five days a week, 50 weeks per year, it will still take over 8 years before you begin to cause damage to the LEL sensor due to the H₂S in the calibration gas.

Docking stations are quite a bit faster, and expose the sensors to less gas, which is one of the reasons gas detector manufacturers encourage their use.   They save money as well as time, improve the accuracy of the test results, and simplify documentation.

By definition, “sour” natural gas contains at least 4 ppm H₂S.  However, the concentration can be much higher.  The gas from one well in Canada is known to contain 90% hydrogen sulfide, while other wells are documented have H₂S in the tens of percent range.  Some oil industry customers have specialized procedures that can expose workers (while wearing SCBA and other protective equipment) to H₂S concentrations that can reach several thousand ppm.  Even a short period of use in high concentration H₂S can rapidly saturate the filter, and damage the LEL sensor.  Twenty minutes of continuous exposure to 1000 ppm H₂S would be enough to saturate the filter.  Once the filter is saturated, the active bead can be damaged very rapidly by further exposure.  This is one of the reasons it is so important to perform a bump test to verify the performance of the LEL sensor before each day’s use.

We normally expect (and warrant) GfG catalytic LEL sensors to last for at least three years.  However, some of our sour gas customers need to replace LEL sensors on a yearly basis.

It’s hard to predict in advance when (or if) the internal filter is close to saturation.  While damage to the bead is electronically detectable, saturation of the filter is not.

The important thing is to verify that the readings of the LEL sensor are accurate by testing or calibrating the sensor before each day’s use!  Sensors that fail a daily bump test need to be calibrated or replaced before further use.

Thanks again for a great question!

Question: I work at a fire department.  We recently ran into a problem in the area where we charge lead acid batteries that we use in our engines and boats.  One of the batteries became overheated while being recharged.  What gases or hazardous atmospheric conditions are associated with lead acid battery charging stations?  Can batteries release flammable gas?  If the batteries contain sulfuric acid, does that mean they can produce and release toxic H₂S?  We thought we smelled something.  What should we measure?

Answer:

Thanks for your very good question.

Lead acid batteries are used to power forklifts, carts and many other types of machinery in many industrial settings.  Many facilities have charging areas where multiple heavy duty lead acid batteries are recharged at the same time.  In some cases facilities maintain large banks of lead acid batteries that are used to provide backup power to critical systems during an emergency.  Fire engines, HAZMAT and emergency response vehicles frequently include banks of lead acid batteries for the same purpose.  Gases produced or released by the batteries while they are being charged can be a significant safety concern, especially when the batteries are located or charged in an enclosed or poorly ventilated area, or on the truck.

Sulfuric acid contains sulfur, and hydrogen sulfide (H₂S) is a possible by-product of over-charging and battery decomposition. If you smell the rotten egg odor of H₂S in the charging area, you should assume that this very dangerous gas is a possibility.  You should leave the area, and use a gas detecting instrument with an H₂S sensor to confirm whether the gas is present before returning.

However, H₂S is not the most common gas associated with charging or discharging lead acid batteries that contain sulfuric acid.  Given the over-heating and other problems you mentioned, you may not find H₂S, but you probably will find the presence of other atmospheric hazards when you test the atmosphere in the area with your gas detector.

The most common reaction byproducts associated with sulfuric acid (H₂SO₄) are hydrogen and sulfur dioxide.  Overcharging, or lead acid battery malfunctions can produce hydrogen.  In fact, if you look, there is almost always at least a little H₂ around in areas where lead batteries are being charged.

Overcharging, especially if the battery is old, heavily corroded or damaged can produce H₂S. Deteriorated, old or damaged lead acid batteries should be removed from service, as damaged batteries are much more likely to be associated with production of H₂S.

Sulfuric acid reacts with a number of metals and substances to produce SO₂ as well as other “sulfur oxides” (SOx) such as SO₃, SO₄, S₂O, etc.  Many sulfur oxides have a pungent odor, but they are NOT H₂S.  H₂S is a reduced sulfide, not an oxide.  When you have a spill, SO₂ is generally the most common gaseous sulfur reaction by-product.

During discharge of a lead acid battery you have the following two half-cell reactions.  Neither SO₂ or H₂S are normally produced, even by catastrophic discharge!

Negative plate reaction:

Pb (solid) + HSO₄^– (aqueous) → PbSO₄ (solid) + H⁺ (aqueous) + 2e⁻

Positive plate reaction:

PbO₂ (solid) + HSO₄^– (aqueous) + 3H⁺ (aqueous) + 2e⁻ → PbSO₄ (solid) + 2H₂O

The total reaction can be written as:

Pb (solid) + PbO₂ (solid) + 2H₂SO₄ (aqueous) → 2PbSO₄ (solid) + 2H₂O

During charging, (especially in the event of overcharging), lead acid batteries produce oxygen and hydrogen.  These gases are produced by the electrolysis of water from the aqueous solution of sulfuric acid.  Since the water is lost, the electrolyte can be depleted.  This is why you need to add water to “wet” (flooded type) non-sealed lead acid batteries.  When a lead acid battery cell “blows” or becomes incapable of being charged properly, the amount of hydrogen produced can increase catastrophically:

Water is oxidized at the negative anode:  2 H₂O (liquid) → O₂ (gas) + 4 H⁺ (aqueous) + 4e⁻

The protons (H⁺) produced at the anode are reduced at the positive cathode:  2 H⁺ (aqueous) + 2e⁻ → H₂

So, in an area where lead acid batteries are being charged, the first gas to measure is H₂.

Hydrogen is not toxic, but at high concentrations is a highly explosive gas.  The 100% LEL concentration for hydrogen is 4.0% by volume.  At this concentration, all it takes is a source of ignition to cause an explosion.  Sparking from a battery terminal as it is connected or disconnected from the charging system is more than adequate as a source of ignition energy.  That’s why lead acid batteries should only be charged in well ventilated areas.

The best way to measure hydrogen in an area where you are charging batteries is with a permanently installed monitoring system.  You can use a standard catalytic LEL sensor, or you can measure the hydrogen by means of a substance specific electrochemical sensor.  The sensor and housing need to be designed and certified for installation and use in hazardous locations characterized by the potential presence of combustible gas.  Since hydrogen is lighter than air, H₂ sensors are usually mounted to the wall or ceiling at a height at least slightly above the source of gas.

Readings from the H₂ sensors can displayed right where the sensor is located, or on a remotely located controller or monitor.   Readings from the sensors can be used to activate relays, fans or alarms, or the information can be transmitted and integrated into the facility’s overall environmental health and safety and fire detection systems.

For LEL range measurement, using a standard catalytic combustible gas (CC) sensor with a range of 0 – 100% LEL is a good approach.  For situations where you need to take action at a lower concentration, using an electrochemical (EC) toxic gas sensor to measure the hydrogen may be a better approach.  The typical range for an EC hydrogen sensor is 0 – 2,000 ppm.  (This is equivalent to a range of 0 – 5.0% LEL.)

In the event of a sulfuric acid spill, or where the sulfuric acid is coming into contact with metals and / or other materials, you may need to measure SO₂ as well.  Although this is normally not necessary in charging areas where the acid is fully contained in the batteries.

If you are concerned with aerosol droplets of sulfuric acid, you can directly measure H₂SO₄ as well.  Once again, this is not normally a concern in battery charging areas.

Deteriorated, old or damaged lead acid batteries should be removed from service, as damaged batteries are much more likely to be associated with leakage leading to the production of SO₂, or charging malfunction which could lead to the production of H₂S

I hope this info is helpful!

Question: Why is the ammonia in the calibration cylinder odorless? and is the concentration of gas used to calibrate gas dangerous?

Answer:

Some people can smell ammonia down to 0.04 ppm.  However, according to OSHA the odor threshold for most people is between 5 and 50 ppm.  We normally use 100 ppm NH₃ to calibrate Micro IV and G460 ammonia sensors.  One hundred ppm is above the odor threshold for most people, so you would think you could smell the gas.  But there are other factors to consider.

Ammonia has a desensitizing effect on the sense of smell for people who work around it for prolonged periods of time.  There is a noticeable odor of ammonia throughout the facility where the NH₃ Micro IV detectors are being used.  Because of this on-going exposure, many people in the facility may be partly, and some may be almost completely desensitized to the odor of the gas.  For these people it can take a concentration higher than 100 ppm for the odor to be detectable.

However, a more important factor is the dilution effect when the ammonia is released into the air.

A cylinder of 100 ppm ammonia calibration gas doesn’t contain very much ammonia.  Remember that it takes 10,000 ppm to equal 1%.  Thus 100 ppm = 0.01% by volume.  When you release this already low concentration into the air it is rapidly diluted to a level well below the odor threshold.

One of the instructional GfG webinars we offered last year, “Gas detection myths and misconceptions”, included the following slide.  You can see in my example that even when you release the NH₃ calibration gas in a small enclosed space like the interior of a passenger vehicle, because of the dilution effect, the concentration is below the odor threshold.

Thank you for asking this question.