Thursday, June 14, 2012

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.

1 comment:

  1. The first reason is that calibration performs a basic function check to ensure that the equipment is working properly. NDT calibration