A question and answer session with Greg Shires, Managing Director of CAC GAS & Instrumentation.
First let me by clear on this subject: Gas detection instruments are life saving devices.
They can make the difference between a worker going home at night or not. If I was required to use a gas detection instrument in my work place each day I would make sure it was calibrated weekly and bump tested prior to every use. This would make sure it was accurate and I was safe.
So, let’s discuss this further.
Q: Why should a user calibrate their gas detector?
That’s it…there is no other reason.
Calibration is the process of applying a known concentration of a specific gas mixture to an instrument while the instrument is in the calibration mode. The instrument analyses the gas and adjusts the instrument response to match the gas value(s). This process maintains the accuracy of the instrument.
The process also checks all other aspects of the instruments, alarms, display, the general condition of the instrument but you don’t have to calibrate an instrument to check those functions.
Q: So, how often should a gas detector be calibrated?
A: Before we can answer that, a few more questions need to be asked.
- How accurate to you want your readings to be?
- What is the purpose of the gas measurement?
- What gas(es) are you measuring?
All these issues play a part in answering your question.
Decisions on the frequency of calibration are directly dependent upon your objectives, your application and the gases which you are measuring.
If you’re an Industrial Hygienist and you are measuring gases in the workplace to determine worker exposure (data logging, TWA, STEL, PEAK information) then you would want the most accurate information. You should be calibrating before and after each measurement period. The problem with today’s instruments is the calibration process normally hides the actual response of the instrument to the gas mixture. To determine if your instrument has drifted during the monitoring period you should verify that the instrument is reading accurately. To do this you would apply the calibration gas for a specific length of time (sensor dependent) while the instrument is in the normal operating mode. You would then record the readings to see if the instrument is still within accuracy specifications. In this way you can provide details that your instrument was accurate before and after the measurement period.
In other cases, instruments are used for confined space entry measurement, pre-entrance testing and then continuous personal monitoring or area monitoring. If you are measuring and recording those reading on a legal document, then one would think you would want the readings to be accurate. I know I would. If I’m signing my name to a legal document, I’d want to be sure the information is correct.
Still further, many instruments are used for personal safety meaning the purpose is for an alarm to go off if the gas concentrations exceed the alarm point. In these cases, the numerical value is not recorded so an inaccuracy may be justified, knowing that the instrument will alarm within a safety zone of measurement. So in these cases you might calibrate less often.
Accuracy will be dependent upon many factors. The instrument itself (how much it drifts), exposure to gases and many other factors. Understanding your instrument: how it is used; by whom; the abuse it is taking in the field; the gases the instrument will be in contact with, will all impact your need to calibrate your instrument more or less often.
Stability data from manufacturers are based upon laboratory environments. The real world is much different and your work environment is the key to calibration frequency decisions.
Years ago, I was working with a national regulatory agency who required carbon monoxide detectors. The spent a year analysing instruments for stability, so they could send the instrument out to the field for 6 months at a time. Within a month of sending out the instruments they complained the instruments were drifting. Upon investigation we found out their staff tested the instruments using the exhaust of their vehicles…this caused the instruments to over range and drift. All their great plans of calibrating every six months came to an end. Education and providing calibration equipment in the field solved the issue.
Q: Should you calibrate more often if the instrument is used for personal safety?
Not necessarily. If the purpose of the instrument is to provide an alarm when gas concentrations increase (toxics and explosive gases) or decrease (O2), then the accuracy of the reading is not critical. Meaning it is not to determine whether the concentration is 10 or 11. The instrument alarm points are set to relatively low concentrations, 5% or 10% LEL for example. An instrument which is bump/response tested daily at a value of 50% LEL will still go into alarm even if the sensor is reading 100% lower than actual. In this case it might take 20% LEL to create an alarm at 10%. This might all sound bad, but the reality is the instrument will alarm and allow the worker to leave the hazard area. Of course, you can only be sure the instrument will provide you the safety required if you have bump/response tested the instrument prior to use.
For safety purposes, if the instrument is bump tested prior to each use, then the need for frequent calibration is reduced. It all comes down to what is the purpose of the instrument.
Q: Could your instrument be calibrated every day prior to use?
Yes, there is no technical reason why you could not calibrate your instrument every time prior to use.
You can calibrate the instrument as often as you like.
A very large airline company that I know of calibrates their instruments every day. They do so to maximize accuracy so they can minimize risk.
The reason for not calibrating tends to be linked to not having trained personnel capable of calibrating, lack of time and the cost. In my opinion, time and cost are not an acceptable reason for not calibrating. Instruments today take little time and cost to calibrate.
Calibration though, must be completed by a trained, competent person who is capable of calibrating an instrument correctly. This is critical, as the calibration process can change the way the instrument works. If the person uses the incorrect gas mixture or incorrect gas concentration the instrument will not work as intended.
One of the reasons Docking Stations have become so popular is that the device becomes the trained individual. Having said that, the docking station must still be set up using the correct calibration gas and concentration values.
Whether it is a person or a machine, the system must be trained and set up correctly to achieve your goal of a properly calibrated gas detection device.
Q: Why is Bump/Response Testing so important?
There are many reasons why your gas detector may not work correctly.
Testing the instrument to a gas mixture is the only method of determining whether your gas detector is responding to a gas exposure.
Bump/Response/Function/Challenge Testing are all terms used for testing your instrument with a known concentration of test gas to determine if the instrument is working correctly and if it will respond to a gas exposure in the work place. Typically, the instrument is exposed to the test gas while in the normal operating mode. The gas concentrations will cause the toxic and explosive sensors to increase in value and the oxygen to decrease. The user should observe this occurring with all sensors in the instrument and that the alarms (audible and visual) come on correctly. Once this is completed, the gas is turned off and the instrument allowed to stabilize. If the instrument readings do not come back to zero (toxics/combustibles) and 20.9% O2, then the instrument is normally zeroed in clean breathing air. The entire test would normal take less than 30 seconds.
You should bump test prior to every use. That includes changes of user from one part of the day to another. If your mate used the instrument this morning, you should bump test the instrument before you use it this afternoon. Your life depends on that instrument.
By testing the instrument prior to each use, you are assured the instrument is working and will respond to gas exposure at that time. This does not test for the accuracy of the instrument.
Bump Testing is about safety – does the instrument work and will the instrument alarm if needed.
If you want to determine whether the instrument is reading gas values accurately, then you would either calibrate or complete a verification, which we reviewed earlier.
If you want both accuracy and safety, calibrate more often and bump test prior to EVERY USE.
Q: Are my toxic sensors specific to one gas.
The short answer is no. The sensors used for the measurement of toxic gases in detection instruments are normally electrochemical. They will have varying cross interference capabilities and stability characteristics. Sensor manufacturers may have many sensors for one particular gas type each with their own specifications. Often the cross sensitivities to other gases will not impact your readings but you should obtain the specification details of the sensor you will be using to determine if your environment will have any cross interference with that sensor. Not all cross sensitivities create a positive response on the instrument reading. Some create negative responses which could result in a zero reading.
Cross sensitivities can be managed provided you know what you are dealing with. Ask your supplier.
Q: What gas should I use for calibrating a combustible gas sensor?
The correct choice of combustible gas for calibration is directly dependent upon your application. What is your business, your application and what are the potential gas hazard(s).
If you are a propane distributor, then your hazard is propane. Propane would be the logical choice for calibrating your combustible sensor.
If you are entering sewers, then the hazard is methane. Methane would be the correct choice.
If though, you encounter many different combustible gases, then you need to consider a number of issues.
- What are the gases?
- Which gas is the most prevalent?
- What are their LEL concentrations?
- What type of sensor do you have?
For this discussion we will assume you have a catalytic combustible gas sensor.
All combustible gases have a Lower Explosive Level (Limit), LEL. A catalytic sensor can measure any combustible gas. How it measures is based upon which gas you calibrate to. In general terms, if you calibrate to a gas which has a low LEL, then the sensor will respond with more sensitivity to all gases which have an LEL higher than the calibration gas. If, on the other hand, you select a gas which has a high LEL, then the sensor will be less sensitive to all gases with a lower LEL.
LEL of Four Combustible Gases: (NIOSH)
- Pentane: 1.4 % LEL
- Propane: 2.1% LEL
- Hydrogen: 4% LEL
- Methane: 5% LEL
As an example,
If you calibrated your instrument to pentane, and then entered an atmosphere of 10%LEL methane your instrument would read around 19%LEL (almost double what is actual).
If on the other hand, if you calibrated your instrument to methane and then entered an atmosphere of 10% pentane your instrument would read 5%LEL (half of what is actual).
Neither readings are accurate but the pentane calibration would make your instrument safer, while the methane calibration seems unsafe.
From this, it would seem that you should select pentane as your calibration source.
That might be the correct choice, but the decision does not stop there.
Catalytic sensors are affected by poisons and inhibitors. Poisons (silicon spray) kill the sensor. Inhibitors (sulphurs, chlorinated compounds) reduce the sensitivity of the sensor.
This reduced sensitivity will be noticed sooner when using methane as your calibration and bump test gas before it is noticed when using pentane.
It is possible that a sensor can measure pentane accurately but not be capable of measuring methane due to the reduced sensitivity caused by the inhibitor(s). This is because methane is very difficult to burn or oxidize, so reduced sensitivity of the sensor will show up first with methane.
So, if you have poisons or inhibitors in your atmosphere, then methane is the better choice to determine the negative impacts on the sensor performance.
As always, the gas hazard(s) and the atmosphere in your applications will dictate which calibration gas is the best choice for calibrating your detector.
Q: What is a pentane equivalent calibration?
We have found out that calibrating to a low LEL gas (such as pentane) will make your instrument more sensitive to all other gases with LEL values higher than the calibration gas.
We also know that methane provides the best indication of reduced sensitivity.
To obtain the benefits of a pentane calibration with the benefits of methane protection, manufacturers have calculated values to calibrate your sensor to methane artificially creating the pentane sensitivity.
Each manufacturer is different but, as an example, if you set your instrument to 58% LEL and calibrated to 1.45% CH4 you created a pentane equivalent calibration.
1.45% methane would normally create a reading of 29% LEL. By telling the instrument this is 58% you just doubled the sensitivity which gives you the benefits of pentane with the benefits of methane.
The negative issue here is your inaccuracy could be as high as +/-25%. Speak to your instrument manufacturer to get the specifics prior to going this way.
In theory, though, it is a great idea.
If you have questions, send them to CAC GAS at firstname.lastname@example.org