Surrogate calibration

The term “surrogate calibration” refers to a practice in instrument calibration whereby a standard different from the entity to be measured is utilized.

For example, thermal flowmeters are based on convective heat transfer effects, and can be calibrated at ambient conditions on a specific gas, and then be used at process conditions to make accurate mass flow measurements. However, if the process gas is highly reactive or toxic, it may be difficult or impossible to perform a calibration, even at ambient conditions.

In such cases, it is common practice to calibrate the flowmeter with a substitute (surrogate) gas, that matches the thermal characteristics of the target gas as closely as possible, while acknowledging that error may be introduced.

In the world of LEL (lower explosive limit) gas detection, catalytic-bead pellistor type sensors respond to many combustible gases. Ideally, they should be calibrated with the target gas—if it can be identified. Sometimes, though, this is not possible, and methane, propane, or pentane standards are used as surrogates. Fortunately, relative response data, based on how the sensor would respond to a given target gas—calibrated with each surrogate—is available.

As it happens, calibrating with pentane causes enhanced response for most gases, methane causes a lowered response for most gases, while propane strikes a middle ground.

For toxic gases, calibration standards—either cylinder gas, permeation devices, or on-demand calibration gas generators—are available in virtually all cases, and surrogate calibration is not needed.  However, this has not stopped certain gas detection companies from advocating surrogate calibration for what one might call “convenience” reasons.

They may advocate the use of a carbon monoxide standard to calibrate an ethylene oxide instrument since CO standards are cheaper and easier to come by, even if ethylene oxide standards ARE readily available.  They may also advocate the use of an SO2 standard to substitute for hydrogen peroxide.

There are some dangers here, though. At best, the surrogate conversion factor (1 ppm surrogate registers “x” ppm as the target gas) is a single number, based on testing of new sensors, maybe with a fudge factor built in to adjust for how this ratio changes with sensor age. Maybe. The problem is that no one can tell you at any particular time how YOUR sensor, installed in the field, and calibrated with a surrogate, will react to the target gas.

As you might suspect, such uncertainties do not bode well if litigation should arise. Be assured that plaintiff’s lawyers know all the right questions to ask about calibration and maintenance of gas detection systems.

Best practice calls for a rigorous chain of custody of calibration, traceable all the way back to some recognized standards body, such as NIST in the United States. It is unlikely that this can be achieved with surrogate calibration.  In those very rare instances where a surrogate calibration is necessary, a responsible manufacturer will disclose all the details and pitfalls.

We do note that no commercial standards are available for hydrogen bromide, hydrogen peroxide, or peracetic acid.  Interscan has developed methods to calibrate these sensors in-house, and recommends SENSOR EXPRESS®.

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