How aware of cross-sensitivities when using gas detectors are you?

In a perfect world, gas detector sensors would identify, isolate and measure specific gases and give precise readings for each gas in any context. Unfortunately, technology allows us to come close to that but not to achieve it completely. That is why, when dealing with electrochemical toxic sensors, we have the challenge of cross-sensitivities, sometimes known as ‘interfering gasses’.

Gas detectors generally detect a specified gas and give an alarm and/or reading in proportion to the level present. Cross-sensitivity occurs when a gas other than the gas being monitored/detected can affect the reading given by an electrochemical sensor. This causes the electrode within the sensor to react even if the target gas is not actually present, or it causes an otherwise inaccurate reading and/or alarm for that gas. Obviously, this puts the person using the sensor at risk.

Inaccuracies caused by cross-sensitivity

Cross-sensitivity may cause several types of inaccurate reading in electrochemical gas detectors. These can be positive (indicating the presence of a gas even though it isn’t actually there, or indicating a level of that gas above its true value), negative (a reduced response to the target gas, suggesting that it is absent when it is actually present, or a reading that suggests there is a lower concentration of the target gas than there is), or the interfering gas can cause inhibition.

Inhibition occurs when the sensor simply fails to register the target gas when it is exposed to the target gas and the inhibitor together, or the inhibitor causes the sensor to stop registering the target gas for some time (which may be hours or even days) after exposure to the inhibitor.

Here are some examples of each error type:

Positive response error: a CO sensor has a positive response to H₂ at a rate of 60%. Thus, when the sensor detecting CO sees 200ppm of H₂ it indicates 60% of 200ppm (around 120ppm).
Negative response error: an SO₂ sensor has a –120% response to NO₂. So, if it sees 5ppm of NO₂ at the same time as 5ppm of SO₂, the reading is reduced by 6ppm, which (depending on the type of sensor involved) gives a 0ppm reading or negative value.
Inhibition: SO₂ sensors may be inhibited by NH₃, and take many hours to recover and respond to SO₂

All of these errors can have adverse effects. Clearly, danger arises when toxic gas is present and the sensor does not read correctly. But even when cross-sensitivity causes an over-reading or false positive, time and resources can be wasted by needless evacuations, ventilation and other unscheduled downtime.

Some manufacturers publish cross-sensitivity data and charts, and these can give some indication of how cross-sensitivities may influence readings in those products. However, it is important not to rely on these too heavily: there can be huge differences between electrochemical sensors, manufacturers may change their sensor designs and specifications at short notice, and scientific understanding is constantly evolving. So, it is a good idea to maintain dialogue with the manufacturer’s technical support team, who will be aware of the latest information and best placed to advise on a particular sensor. It is also sensible to ensure that any staff involved in gas detection are aware of the nature of cross-sensitivity and interference, and alert to its likely effects.

Leave a reply

Keeping your gas monitors clean during COVID-19

During this challenging time, keeping your gas monitor clean is more important than ever to ensure you’re keeping yourself, and others, safe.

Cleaning your monitor

The following procedure and precautions should be noted if you intend to clean your Crowcon gas monitor to protect against COVID-19 transmission.

Gas monitors contain sensors that may be affected by the chemicals in cleaning compounds. In general Crowcon recommends cleaning with mild soap and a soft cloth taking care not to introduce excessive amounts of liquid into the product/sensors.

Alcohol-based cleaning products may cause a temporary response on some electrochemical sensors; potentially leading to false-alarms. It is recommended monitors are switched-off before cleaning and not switched back on until the alcohol has fully evaporated.

Cleaning agents that contain chlorine and/or silicones must be avoided, especially on monitors that contain pellistor-type flammable gas sensors as these compounds will ‘poison’ the sensor leading to permanent loss of sensitivity to gas.

Where gas monitor cleaning regimes are introduced or increased Crowcon strongly recommends that sensors are bump tested with the target gas periodically to ensure that sensors remain operational. Pellistor-type sensors in portable monitors should be tested every day before use as prescribed in the European standard EN60079-29 Part 1.

It is extremely likely that any viral agent could get trapped within the pump or filters within an instrument. Maintenance procedures should continue to be performed as described in the Operation and Maintenance Manual for the product and in-line with operating company policy.

For more information on how to keep you or your business safe during the COVID19 pandemic, get in touch and we’d be more than happy to help.

1 Reply

What is the life expectancy of my sensors?

Given the critical nature of gas detectors, it is important to know they are working correctly at all times. Many factors can affect the performance of gas detection sensors, and all sensors will fail eventually, so users must be vigilant and prepared to change their sensors when required. But changing sensors too early, when they actually have plenty of life left, can be a waste of time and money.

A further issue arises with purchasing and storing spares. Replacement sensors have a finite shelf life, which begins from the moment they are made. As time passes, they can degrade even if kept in ideal conditions (i.e. in a contaminant-free, temperature and humidity controlled environment so the period between purchase and first use should be brief.

So, what should users do to extend the life of their sensors without putting people at risk?

Factors affecting sensor life

The life and/or performance of gas detection sensors can be affected by various factors, including:

Temperature
Humidity
Interfering gases
Physical factors, e.g. excessive vibration or impact
Contamination of or damage to the sensor e.g. by incorrect cleaning products
Contamination of filters or sinters e.g. by dust, sand or pests (yes spiders!)
Exposure to poisoning/inhibiting compounds even when the sensor is not powered.

There are multiple sensing technologies available and the life expectancy of a sensor is commonly linked to the technology employed. Electrochemical sensors tend to have a shorter life expectancy as compared to Infrared (IR) or catalytic sensors. The type of gas being detected can also have an impact of the life expectancy, the more ‘exotic’ gases (for example chlorine or ozone) tends to be shorter than that of sensors monitoring the more common gases (carbon monoxide, hydrogen sulphide for example).

Most sensors will also suffer general wear and tear, and the damage caused is not always easy to detect, so the first rule for keeping sensors safe and in good working order is to undertake regular maintenance. This should include scheduled bump testing (also known as a gas or functional test) and calibration; while exposure to substantial volumes of gas may harm some sensors, the small amounts used in bump testing and calibration are absolutely fine

It is not always easy to tell that a sensor has failed; some of the techniques suggested are unreliable and this is not an area in which to take risks. The only sure-fire way to know a sensor is working correctly is through application of the target gas(es) in bump testing/calibration.

Planning gas sensor replacement

It makes sense for users to extend the life of their sensors as far as possible; they cost time and money to replace, after all. The ability to forward-plan and predict sensor consumption also makes sensor purchasing more efficient and helps to reduce the time spare sensors are kept in storage.

To predict and plan sensor replacement, users must understand the factors that influence their sensors’ performance. These will be specific to their own setting, which is why users must also be able to draw upon knowledge and experience built up through regular testing and calibration of sensors in their particular environment and applications.

Good quality sensors will come with a warranty, but while this may indicate a general life expectancy there are too many variables and too much at stake for it to stand alone. There really is no substitute for user knowledge and regular maintenance: with these in place, gas detector sensors are far more likely to live long and prosper.

Leave a reply

What’s the difference between a pellistor and an IR sensor?

Sensors play a key role when it comes to monitoring flammable gases and vapours. Environment, response time and temperature range are just some of the things to consider when deciding which technology is best.

In this blog, we’re highlighting the differences between pellistor (catalytic) sensors and infrared (IR) sensors, why there are pros and cons to both technologies, and how to know which is best to suit different environments.

Pellistor sensor

A pellistor gas sensor is a device used to detect combustible gases or vapours that fall within the explosive range to warn of rising gas levels. The sensor is a coil of platinum wire with a catalyst inserted inside to form a small active bead which lowers the temperature at which gas ignites around it. When a combustible gas is present the temperature and resistance of the bead increases in relation to the resistance of the inert reference bead. The difference in resistance can be measured, allowing measurement of gas present. Because of the catalysts and beads, a pellistor sensor is also known as a catalytic or catalytic bead sensor.

Originally created in the 1960’s by British scientist and inventor, Alan Baker, pellistor sensors were initially designed as a solution to the long-running flame safety lamp and canary techniques. More recently, the devices are used in industrial and underground applications such as mines or tunnelling, oil refineries and oil rigs.

Pellistor sensors are relatively lower in cost due to differences in the level of technology in comparison to IR sensors, however they may be required to be replaced more frequently.

With a linear output corresponding to the gas concentration, correction factors can be used to calculate the approximate response of pellistors to other flammable gases, which can make pellistors a good choice when there are multiple flammable vapours present.

Not only this but pellistors within fixed detectors with mV bridge outputs such as the Xgard type 3 are highly suited to areas that are hard to reach as calibration adjustments can take place at the local control panel.

On the other hand, pellistors struggle in environments where there is low or little oxygen, as the combustion process by which they work, requires oxygen. For this reason, confined space instruments which contain catalytic pellistor type LEL sensors often include a sensor for measuring oxygen.

In environments where compounds contain silicon, lead, sulphur and phosphates the sensor is susceptible to poisoning (irreversible loss of sensitivity) or inhibition (reversible loss of sensitivity), which can be a hazard to people in the workplace.

If exposed to high gas concentrations, pellistor sensors can be damaged. In such situations, pellistors do not ‘fail safe’, meaning no notification is given when an instrument fault is detected. Any fault can only be identified through bump testing prior to each use to ensure that performance is not being degraded.

IR sensor

Infrared sensor technology is based on the principle that Infrared (IR) light of a particular wavelength will be absorbed by the target gas. Typically there are two emitters within a sensor generating beams of IR light: a measurement beam with a wavelength that will be absorbed by the target gas, and a reference beam which will not be absorbed. Each beam is of equal intensity and is deflected by a mirror inside the sensor onto a photo-receiver. The resulting difference in intensity, between the reference and measurement beam, in the presence of the target gas is used to measure the concentration of gas present.

In many cases, infrared (IR) sensor technology can have a number of advantages over pellistors or be more reliable in areas where pellistor-based sensor performance can be impaired- including low oxygen and inert environments. Just the beam of infrared interacts with the surrounding gas molecules, giving the sensor the advantage of not facing the threat of poisoning or inhibition.

IR technology provides fail-safe testing. This means that if the infrared beam was to fail, the user would be notified of this fault.

Gas-Pro TK uses a dual IR sensor – the best technology for the specialist environments where standard gas detectors just won’t work, whether tank purging or gas freeing.

An example of one of our IR based detectors is the Crowcon Gas-Pro IR, ideal for the oil and gas industry, with the availability to detect methane, pentane or propane in potentially explosive, low oxygen environments where pellistor sensors may struggle. We also use a dual range %LEL and %Volume sensor in our Gas-Pro TK, which is suitable for measuring and switching between both measurements so it’s always safely operating to the correct parameter.

However, IR sensors aren’t all perfect as they only have a linear output to target gas; the response of an IR sensor to other flammable vapours then the target gas will be non-linear.

Like pellistors are susceptible to poisoning, IR sensors are susceptible to severe mechanical and thermal shock and also strongly affected by gross pressure changes. Additionally, infrared sensors cannot be used to detect Hydrogen gas, therefore we suggest using pellistors or electromechanical sensors in this circumstance.

The prime objective for safety is to select the best detection technology to minimise hazards in the workplace. We hope that by clearly identifying the differences between these two sensors we can raise awareness on how various industrial and hazardous environments can remain safe.

For further guidance on pellistor and IR sensors, you can download our whitepaper which includes illustrations and diagrams to help determine the best technology for your application.

5 Replies

You won’t find Crowcon sensors sleeping on the job

MOS (metal oxide semiconductor) sensors have been seen as one of the most recent solutions for tackling detection of hydrogen sulphide (H₂S) in fluctuating temperatures from up to 50°C down to the mid-twenties, as well as humid climates such as the Middle East.

However, users and gas detection professionals have realised MOS sensors are not the most reliable detection technology. This blog covers why this technology can prove difficult to maintain and what issues users can face.

One of the major drawbacks of the technology is the liability of the sensor “going to sleep” when it doesn’t encounter gas for a period of time. Of course, this is a huge safety risk for workers in the area… no-one wants to face a gas detector that ultimately doesn’t detect gas.

MOS sensors require a heater to equalise, enabling them to produce a consistent reading. However, when initially switched on, the heater takes time to warm up, causing a significant delay between turning on the sensors and it responding to hazardous gas. MOS manufacturers therefore recommend users to allow the sensor to equilibrate for 24-48 hours before calibration. Some users may find this a hinderance for production, as well as extended time for servicing and maintenance.

The heater delay isn’t the only problem. It uses a lot of power which poses an additional issue of dramatic changes of temperature in the DC power cable, causing changes in voltage as the detector head and inaccuracies in gas level reading.

As its metal oxide semiconductor name suggests, the sensors are based around semiconductors which are recognised to drift with changes in humidity- something that is not ideal for the humid Middle Eastern climate. In other industries, semiconductors are often encased in epoxy resin to avoid this, however in a gas sensor this coating would the gas detection mechanism as the gas couldn’t reach the semiconductor. The device is also open to the acidic environment created by the local sand in the Middle East, effecting conductivity and accuracy of gas read-out.

Another significant safety implication of a MOS sensor is that with output at near-zero levels of H₂S can be false alarms. Often the sensor is used with a level of “zero suppression” at the control panel. This means that the control panel may show a zero read-out for some time after levels of H₂S have begun to rise. This late registering of low-level gas presence can then delay the warning of a serious gas leak, opportunity for evacuation and the extreme risk of lives.

MOS sensors excel in reacting quickly to H₂S, therefore the need for a sinter counteracts this benefit. Due to H₂S being a “sticky” gas, it is able to be adsorbed onto surfaces including those of sinters, in result slowing down the rate at which gas reaches the detection surface.

To tackle the drawbacks of MOS sensors, we’ve revisited and improved on the electrochemical technology with our new High Temperature (HT) H₂S sensor for XgardIQ. The new developments of our sensor allow operation of up to 70°C at 0-95%rh- a significant difference against other manufacturers claiming detection of up to 60°C, especially under the harsh Middle Eastern environments.

Our new HT H₂S sensor has been proven to be a reliable and resilient solution for the detection of H₂S at high temperatures- a solution that doesn’t fall asleep on the job!

Click here for more information on our new High Temperature (HT) H₂S sensor for XgardIQ.

Leave a reply

An ingenious solution to the problem of high temperature H2S

Due to extreme heat in the Middle East climbing up to 50°C in the height of summer, the necessity for reliable gas detection is critical. In this blog, we’re focusing on the requirement for detection of hydrogen sulphide (H₂S)- a long running challenge for the Middle East’s gas detection industry.

By combining a new trick with old technology, we’ve got the answer to reliable gas detection for environments in the harsh Middle Eastern climate. Our new High Temperature (HT) H₂S sensor for XgardIQ has been revisited and improved by our team of Crowcon experts by using a combination of two ingenious adaptations to its original design.

In traditional H₂S sensors, detection is based on electrochemical technology, where electrodes are used to detect changes induced in an electrolyte by the presence of the target gas. However, high temperatures combined with low humidity causes the electrolyte to dry out, impairing sensor performance so that the sensor has to be replaced regularly; meaning high replacement costs, time and efforts.

Making the new sensor so advanced from its predecessor is its ability to retain the moisture levels within the sensor, preventing evaporation even in high temperature climates. The updated sensor is based on electrolytic gel, adapted to make it more hygroscopic and avoiding dehydration for longer.

As well as this, the pore in the sensor housing has been reduced, limiting the moisture from escaping. This chart indicated weight loss which is indicative of moisture loss. When stored at 55°C or 65°C for a year just 3% of weight is lost. Another typical sensor would lose 50% of its weight in 100 days in the same conditions.

For optimal leak detection, our remarkable new sensor also features an optional remote sensor housing, while the transmitter’s displays screen and push-button controls are positioned for safe and easy access for operators up to 15metres away.

The results of our new HT H₂S sensor for XgardIQ speak for themselves, with an operating environment of up to 70°C at 0-95%rh, as well featuring a 0-200ppm and T90 response time of less than 30 seconds. Unlike other sensors for detecting H₂S, it offers a life expectancy of over 24 months, even in tough climates like the Middle East.

The answer to the Middle East’s gas detection challenges fall in the hands of our new sensor, providing its users with cost-effective and reliable performance.

Click here for more information about the Crowcon HT H2S sensor.

Leave a reply

What you need to be aware of when…

…zeroing your CO2 detector

Without wishing to sound accusing, where were you the last time you zeroed your CO2 detector? In your vehicle? In the office before you travelled to the location you were working in?

It might not have caused you problems so far, but the air around you can make a big difference to the performance of your CO2 detector.

What is zeroing?

Zeroing your detector means calibrating it so its ‘clean air’ gas level indication is correct.

When is zero not really zero?

Many CO2 detectors are programmed to zero at 0.04% CO2 rather than 0%, because 0.04% is the normal volume of CO2 in fresh air. In this case, when you zero your detector, it automatically sets the baseline level to 0.04%.

What happens if you zero your CO2 monitor where you shouldn’t?

If you zero your detector where you shouldn’t, the actual CO2 concentration could be much higher than the standard 0.04% – up to ten times higher, in some cases.

The end result? An inaccurate reading, and no true way of knowing how much CO2 you’re actually exposed to.

What are the dangers of CO2?

CO2 is already in the earth’s atmosphere, but it doesn’t take much for it to reach dangerous levels.

1% toxicity can cause drowsiness with prolonged exposure
2% toxicity is mildly narcotic and causes increased blood pleasure, pulse rate, and reduced hearing
5% toxicity causes dizziness, confusion, difficulty in breathing, and panic attacks
8% toxicity causes headaches, sweating and tremors. You’ll lose consciousness after five to ten minutes of exposure.

What can I do to make sure I’m safe?

Only zero your instruments if you really have to, and make sure you zero your detector in fresh air – away from buildings and CO2 emissions, and at arm’s length to make sure your own breath doesn’t affect the reading.

What if I think my zero reading is incorrect?

It’s best to test the instrument with 100% nitrogen to check the real zero point, and then with a known level of CO2 test gas. If the zero gas reading is incorrect, or any other gas reading for that matter, the detector will need a full service calibration – contact your local service provider for help.

If you have a Crowcon detector, you can use our Portables Pro software to correct its zero reading. For further information, call Crowcon customer support on +44 (0)1235 557711.

1 Reply

Just what are Intrinsic Safety Barriers?

In your industry, you may have heard of Intrinsic Safety barriers, commonly known as I.S. barriers. But what are they, exactly?

I.S. barriers are protection devices for electrical equipment such as gas detectors, fire detectors, alarms etc mounted in a hazardous area. They protect equipment from current surges, which would otherwise run the risk of turning the equipment into an ignition source – disastrous when the detector is in an area where there may be explosive gases.

A good analogy is a steam engine with a pressure relief whistle – when the engine is under too much pressure, it’s relieved through the whistle by literally letting off steam.

How do they work?

I.S. barriers work by limiting the energy available to the I.S. device. Here at Crowcon, we use two types of I.S. barriers – zener barriers and galvanic isolators.

Zener barriers contain zener diodes which divert any excess energy to earth – so you need to make sure that there’s an intrinsically safe earth point available. When you don’t have an earth point, you can use a galvanic isolator, which provides electrical isolation between the hazardous area and the safe area circuits via a transformer.

When do you need to use them?

Basically, when you’re using certified devices that use the I.S. protection method. If your device uses this method, you’ll see the following in their ATEX and IECEx certificates:

‘ia’ or ‘ib’ in their certification classification
For example – Ex ia IIC T4 Ga (the classification for our Xgard Type 1 fixed detector)

Some products might use more than one protection method – a common example is I.S. and flameproof protection. In these cases, the product is unlikely to require the use of an external I.S. barrier. However, as always, we recommend that you consult your product manual for guidance.

How do you use them?

I.S. barriers should be located between the devices in the hazardous area and the control equipment (installed in a safe area). The I.S. barrier needs to be within the safe area.

The ATEX certificate for the I.S. device will stipulate acceptable parameters for the I.S. barrier.

When should they be avoided?

Detectors which don’t use the ‘intrinsic safety’ method of protection shouldn’t be used with an I.S. barrier.

For example, the Xgard type 5 uses the flameproof (Exd) method of protection – so it doesn’t need an I.S. barrier. However, not all versions of the Xgard have flameproof protection, so do need an I.S. barrier – it all boils down to the product you’re using.

When your detector and control equipment are both installed in the safe area, you don’t need I.S. barriers.

One thing you should remember – using an I.S. barrier with a detector that doesn’t use the intrinsic safety method of protection doesn’t make the detector intrinsically safe.

1 Reply

Electrochemical sensors: how long on the shelf, and how long in the field?

You might have heard the term ‘shelf life’ and ‘operational life’ before in reference to electrochemical sensors. They’re the type of terms that lots of people know, but not everybody knows the finer details of what they mean.

How long on the shelf?

For the purposes of this piece, “shelf life” is the time between manufacture of a product and initial operation.

Electrochemical sensors typically have a stated shelf life of six months from manufacture, provided they’re stored in ideal conditions at 20˚C. Inevitably, a small proportion of this period is taken up during the manufacture of the gas detector and in shipping to the customer.

With that in mind, we’d always advise that when acquiring sensors and any spare parts during its lifetime, you plan and time your purchases for minimal delay between storage and usage.

How long in the field?

Again, “operational life” in this context refers to the time from when a sensor starts being used, until it’s no longer fit for purpose.

In absolutely ideal conditions – stable temperature and humidity in the region of 20˚C and 60%RH with no incidence of contaminants – electrochemical sensors have been known to operate in excess of 4000 days (11 years)! Periodic exposure to the target gas doesn’t limit the life of these tiny fuel cells: high quality sensors have a large amount of catalyst material and robust conductors which don’t become depleted by the reaction.

However, absolutely ideal conditions don’t always exist, or stay that way, so it’s vital to err on the side of caution when it comes to gas sensors.

With that in mind, electrochemical sensors for common gases (for example carbon monoxide or hydrogen sulphide) have a typical operational life of 2-3 years. A more exotic gas sensor, such as hydrogen fluoride, may have only 12-18 months.

Leave a reply

Why you shouldn’t spark up

Think back to the last time you wanted to test your flammable gas detector. You’re busy; you want something quick and convenient. An obvious answer is a cigarette lighter, isn’t it? A quick squirt of gas should do the job. Shouldn’t it?

If ‘the job’ is ruining your detector’s sensor at the flick of a switch, then yes!

If you use a cigarette lighter to test your sensors, you run the risk of:

Rendering your sensor useless
Compromising your warranty – carbon deposits are a dead giveaway for manufacturers who then won’t honour your claim due to incorrect testing

Why cigarette lighters are bad news for your sensors

Pellistor-type sensors (also known as catalytic beads) are used in industrial gas detectors to detect a wide variety of gases and vapours. The sensors are made up of a matched pair of ‘beads’ which are heated to react with gases. The sensors operate in the ‘Lower Explosive Limit’ (LEL) range, so provide a warning well before a flammable level of gas concentration accumulates.

Periodic and irregular exposure to high gas concentrations is likely to compromise sensor performance, and cigarette lighters expose the sensor to 100% gas volume. Not only that, but this exposure can potentially crack the sensor beads. Cigarette lighters also leave damaging carbon deposits on the beads – leaving you with useless sensors, and potentially putting your life at risk.

How to safely test your sensors

Bump test! Or you can calibrate using 50% LEL gas – but make sure you’re using the correct gas calibration adaptor from your gas cylinder, and that your cylinder’s flow is regulated to 0.5 to 1 litre per minute.

Leave a reply