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Why six month calibration in one country and monthly. Why would a portable gas detection manufacturer recommend differently on the same product? What do the actual sensor manufacturer say about loss of signal over a six month span?

 

I welcome your ideas.

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I believe that 6 month calibration intervals stem from the European tradition of having equipment calibrated and maintained by the manufacturer or a third party. If you look at the specifications from various sensor manufacturers, you will see some specifications that state long term sensitivity drift at 5% or 10% per year. But you will also see many others that specify long term sensitivity drift of 2%, 3% or even 5% per MONTH. With that specification, it would seem impossible to maintain accuracy specified at +/-5% over a 6-month period. You will also see that sensor manufacturers state that their long term drift specification apply to exposure in clean laboratory air, not the typical working environments. Here is an example. If you are using a City Technolgy 4P-50 combustible gas sensor, the long term sensitivity drift specification is <5% per month. How can you realistically expect to maintain any reasonable measured accuracy over a 6-month period in that instance?

In all of these cases, only the sensor's conditions are considered without concern for any other factors that might affect the ability of the sensor to detect gas in the instrument in which it is installed. All combined it doesn't seem possible at all to consciously extend calibration intervals on safety critical instruments to six months or longer.
What heppens to the accuracy if a sensor if it has seen high concentrations and/or high temperatures during that time?
It is not easy to say and it depends what type of sensor we are talking about. Catalytic bead sensors will suffer a loss of sensitivity after repeated exposures to high concentrations of gas. Some electrochemical sensors with consumable electrolytes or catalysts will lose sensitivity after high gas exposures as well. Some electrochemical sensors are not really affected by high concentrations of gas. Exposure to high temperatures in humid environments may cause sensors to absorb moisture and eventually leak and fail. Hot, dry environments may cause sensors to dry out and lose sensitivity.

The bottom line is that it is very difficult to predict what will happen to a sensor once it is outside a controlled environment. This is why it is imperative that you test and calibrate them regularly.

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Calibration of portable gas monitors 3 Replies

Why six month calibration in one country and monthly. Why would a portable gas detection manufacturer recommend differently on the same product? What do the actual sensor manufacturer say about loss…Continue

Started by Carl Mondy. Last reply by David Wagner May 11, 2010.

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