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The Role of Calibration in Gas Detection

Everyone who uses gas monitors is aware of the need to calibrate them, but we are often asked why this is.

The first reason is that calibration performs a basic function check to ensure that the equipment is working properly. ChemDAQ sensors are generally very reliable and probably could be used for a long time without significant reduction in sensitivity. However, as with any device, there is always a risk of failure. Periodic calibration provides a basic function check to make sure that the sensors respond to gas correctly.

The second reason for calibration is ensure that the gas monitor reads the correct value when exposed to the target gas or vapor. Using electrochemical sensors as an example, the output current from the sensors is proportional to the gas concentration and calibration provides that proportionality constant. In principle it is possible to derive this calibration parameter from the diffusion properties of the sensor components, such as the membranes and spaces etc. of a sensor and calculate the steady state response by applying the relevant laws of electrochemistry and diffusion; however this method is neither practical on a regular basis nor particularly accurate.

Instead, a more practical approach is to exposure the sensor to clean air (zero air) and set the baseline to zero, and then apply the span gas and adjust the output of the sensor module so that the reading on the monitor matches the concentration of the gas applied.

Reactive gases cause difficulties in calibration for several inter-related problems. The first is that reactive gases are more hazardous and so must be detected at lower concentration than less hazardous gases, for example the OSHA PEL for ethylene oxide and hydrogen peroxide are both 1 ppm, whereas the PEL for the less toxic carbon monoxide is 50 ppm.

Secondly, the lower the concentration, the more severe adsorption effects will be. Materials compatibility is also critical. Pass 50 ppm carbon monoxide down a well used vinyl tubing and 50 ppm will come out the other side. Pass 1 ppm hydrogen peroxide down a used or pristine vinyl tubing and only air will come out. Even tubing that is chemically compatible with hydrogen peroxide or other reactive gases may still remove the test gas if the tubing is contaminated or damp. The more reactive the gas, the more difficult it is to ensure the calibration process is not flawed. For the manufacturers of monitors for these reactive gases, a large percentage of the technical support issues concern calibration problems.

For many gases, such as ethylene oxide, the gas is delivered in cylinders along with a certificate certifying what the gas concentration is. For other gases and vapors, such as hydrogen peroxide, we have to generate the hydrogen peroxide in-house and then calibrate the hydrogen peroxide test gas against a known standard before we can use it to calibrate the sensor modules.

It is primarily for these reasons that ChemDAQ provides factory calibration for all its customers. Factory calibration ensures that the calibration is performed correctly and accurately. In addition, factory calibration is much more convenient to the end user since they can simply swap out a precalibrated sensor instead of havin to work with compressed gas cylinders or other gas sources. ChemDAQ's monitors mainly detects sterilant gases, highly reactive gases and vapors used to sterilize medical and food equipment and supplies. While some of these gases are available in calibration cylinders, many of the others are not. ChemDAQ equipment is therefore designed to be factory calibrated to avoid problems with calibration using very reactive gases.

ChemDAQ's SXP® calibration service offers other benefits as well. Tracking is one major advantage. Some facilities are very good in tracking when their gas monitors are due to be calibrated, but many others are not so diligent and uncalibrated sensors are a regular cause of error in gas detection systems. ChemDAQ's SXP service includes tracking the calibration status of the sensors, and ChemDAQ will contact each customer when their sensors are due for exchange.

One of the more widely used methods to improve the sensitivity of gas sensors is to use a chemical filter. A chemical filter in front of the sensor reacts with the gas or vapor one does not want to detect so that only the target gas reaches the sensor. Chemical filters have a capacity beyond which they no longer react with the interfering gas or vapor. Depending on the chemistry, the capacity of some filters is large and the filter rarely needs to be changed, for others the capacity is more limited and the filter needs to be changed more frequently.

One of ChemDAQ's claims to fame is our specific filter for the EtO sensor. This patented filter reacts with alcohols, many VOCs and even carbon monoxide (there are not too many compounds that react with carbon monoxide at room temperature); but it still allows EtO to pass through unhindered. This filter works remarkably well, but it does have a limited capacity. Therefore this filter is replaced every calibration cycle.

In summary, calibration is an essential part of gas detection as a basic function check and to ensure that the gas monitor reads the correct value. Some manufacturers have their customers calibrate on-site, which is OK for stable gases but becomes increasingly problematic for reactive gases. ChemDAQ's solution to this problem is to factory calibrate all monitors which ensures that the calibration is performed correctly and by tracking each customer, ensure that the calibration is performed when due. Factory calibration offers other benefits as well such as ease of operation for the end user and automatic replacement of consumables such as chemical filters.

The Correct Placement of Gas Monitors

We are often asked about the correct placement of area gas monitors. Area monitors monitor an area and tell workers whether it is safe to be in that area.

Area monitors should be placed close to potential sources of the gas or vapor being monitored, so for example if the gas is ethylene oxide used for hospital sterilization, then the monitors should be placed close the sterilizer, the abator (if used) and locations where the ethylene oxide is stored. An area monitor should detect the gas before it reaches the people working in the area and so can give more advanced warning than a personal monitor.

The monitors should be placed at a height similar to breathing zone for most people, typically about five feet (1.5 m) off the floor. The sensor or sampling point should be open to air and not covered or blocked.

A common question is how far can a gas monitor detect gas, and the accurate answer for diffusion type sensors is that the sensors detect only the gas that reaches them. It is possible to model the gas diffusion in static air, but the results are of little use because this model does not apply to real life. In a typical hospital sterilization department, aseptic food processing or commercial sterilization environment with large air turn overs (e.g. > ~10 turnovers/hr) it is difficult to predict how far the gas will travel because of the rapid turbulence of the air. We typically use a rule of thumb that gas monitors have a maximum radius of detection of about 5 feet and so two gas monitors should be no more than 10 feet apart (~3m). However, the optimum design of the gas monitoring system must take into account the place of the gas sources, e.g. (sterilizers), abators and ventilation.

We are often asked about the best placement of a monitor on or near a sterilizer (typically small dishwasher sized sterilizers, not the large commercial ones). If the sterilizer is wall mounted, then we recommend placing a monitor on the wall close to the front of the sterilizer. Sterilizers can potentially leak from the door when it is opened and so we need a monitor close by to provide suitable warning. This is particularly true in situations where users regularly open the sterilizer door and reach in to remove the load.

If the sterilizer is a free standing unit, the sensor should be placed above the door (ChemDAQ's remote sensor) and have a monitor on a nearby wall. If above the door is not possible, then as close to the front of the sterilizer as possible. Sterilizers can sometimes emit gas or vapor when the door is first opened after a cycle. We have seen 30 to 40 ppm hydrogen peroxide be emitted by some sterilizers each time the door was opened, significantly higher than the OSHA PEL of 1 ppm (8 hr TWA) and similar magnitude to the NIOSH Immediately Dangerous to Life and Health value (75 ppm).

Even though the OSHA PEL is not reached, other authorities that have updated their occupation exposure levels more recently than OSHA have introduced a STEL for hydrogen peroxide, for example 3 ppm in the State of Washington and 2 ppm in the United Kingdom While these limits are not enforceable outside Washington State and the UK respectively, they do indicate that exposure to high concentrations of vapors like hydrogen peroxide is best avoided.

Where there are several free standing sterilizers in a row, the best configuration is to have a sensor over every door to provide protection as described above. In some cases, cost demands that a monitor is placed between each pair of sterilizers. This configuration is will detect major leaks of gas or vapor but will not provide warning against small releases of gas or vapor when the sterilizer is opened.

Since hydrogen peroxide has almost no odor, workers would be exposed to this vapor cloud if they were not protected by a monitor. If the remote sensor is placed above the door, then workers can step away from the sterilizer after opening the door and return once the monitor says it is safe to do so. It is important that the remote sensor be placed above the door and not be pushed back since the plume of gas coming from the sterilizer is likely to completely pass by a sensor that has been pushed away, but the person leaning in to unload the sterilizer may not be so fortunate.

Another potential sources of leakage is at the rear of the sterilizer, from leaks in the plumbing or exhaust. If the sterilizer exhaust goes to a separate abator (EtO only), then the abator should be monitored too. There have been several cases of ethylene oxide leakage from holes in the duct between the sterilizer and abator, or failure of fans to exhaust the gas.

Another common question is whether a monitor for heavier gases and vapors should be placed low because the gas sinks in air. Those who did Chemistry may remember the experiments with heavy gases and vapors like nitrogen dioxide and bromine that form brown clouds that sink to the floor. In some applications where the air is static (man-holes, grain silos and other confined space entry) stratification of gases by mass is an important issue. People who go down manholes sometimes put their monitors at ankle height while descending, so that they will hear an alarm before their head reaches that level. For most common applications with high air turnover, the air turbulence is so great that stratification is not significant. The monitors should therefore be placed at breathing height.

In conclusion, area gas monitors should be placed near the sources of gas so that they can inform people whether it is safe to be in the area and when it is safe to return to an area after a release of gas or vapor. The monitors should be placed at breathing height, ~ 5 feet, and where there are multiple sources close together, the monitors should be no more than 10 feet apart. Every facility is different and so your ChemDAQ rep. will work with you to design a system that meets your needs.