For those of you who know me well, don’t be alarmed. I’m not going to talk about bump testing today. Today I’m focused on calibrating gas monitors. Before you over react, yes, I am still committed to the importance of bump testing gas monitoring instruments. But regular calibration plays a role that is just as vital.
Calibration establishes the accuracy of the sensors and the monitor that they are installed in. It provides a reference point for the readings and a degree of confidence that the measured concentrations are in fact what the monitor says they are.
So why does Industrial Scientific recommend calibrating instruments monthly when some others clearly say that longer intervals between calibrations are okay?
Here it is. Calibration is needed to ensure that your instruments perform within the stated specifications. If the sensor manufacture states that the specified signal drift of the sensor itself is +/-2% per month or greater, then it is clear that if you want to maintain an instrument accuracy of +/-5%, that the sensors must be calibrated at least no less frequently than every three months. Now when you take into account that the manufacturer’s specifications are based on laboratory conditions, and your instrument is used in a variety of changing and stressful environments and exposed to many performance altering physical factors, then clearly you must calibrate more frequently than that.
Our studies of sensors used in fielded instruments indicate that more than 50% experience changes in sensitivity of greater than 3% per month. It then seems obvious that if you want your instrument to be accurate to at least +/-5% at any given time, that it should be calibrated on at least a monthly basis.
You can check your instrument at various intervals to determine if it reads as accurately as it is supposed to. But why do this and then make a judgement call as to whether or not it is good enough? It will take the same amount of time and the same amount of gas simply to calibrate the instrument. And the result sets and guarantees the accuracy.
In the end, if the most accurately reading gas monitoring instrument is what you seek, calibration is the only way to go.
Keep it safe, keep your instruments calibrated properly!
Dave
2 comments
Arlin Lagasse says:
February 7, 2012 at 3:19 pm (UTC 0)
When determing which LEL sensor we need – LEL (Pethane) or LEL (Methane) – how do you know?
Dave Wagner says:
February 7, 2012 at 8:40 pm (UTC 0)
Arlin,
Essentially both sensors LEL(methane) and LEL(pentane) are the same. The only difference is in how they are calibrated at the factory when they are installed in the instrument. Once is calibrated to methane and the other to pentane.
As far as which one you choose, I would use these guidelines. If you are going to primarily be detecting natural gas which is mainly methane, choose the methane calibrated sensor and continue to calibrate that way. If you are using the sensor for general detection of combustible gases, choose the pentane calibrated sensor. In this case, your errors when detecting other gases will generally be in the safe direction and your readings will typically be lower than the actual concentration.
Regardless of which one of these sensors you choose, you can calibrate to any known LEL gas concentration without any problems.