What is Instrument Calibration?

Dear Dr. Zoomie –

I keep hearing about instrument calibration and don’t quite know what that means. Can you tell me what it is, when we have to do it, and whether it’s something I can do myself or should hire a contractor?

Let’s start off with why we have radiation instruments at all – it’s because we have no way of sensing radiation ourselves. No matter how high the rad levels are, you won’t feel your skin tingle, you won’t get a strange taste in your mouth, and you certainly won’t see or hear anything. That’s why we have instruments – to make up for this deficit in our senses. So if we’re using instruments to tell us whether or not we’re at risk – or even just to meet regulatory requirements – it behooves us to make sure that the instruments are working properly and to make sure we can trust their readings. That’s what calibration is for.

So calibration makes sense from a practical perspective, but it’s also a regulatory requirement – you’re required to perform a yearly calibration on all radiation instruments that are used for health and safety purposes or to perform surveys used to meet regulatory requirements. So every one of your radiation meters that’s used for routine radiation or calibration surveys has got to be calibrated every year. You should also have your instruments calibrated anytime they undergo extensive repairs – changing the batteries or cables is OK, but replacing probes or repairing the inner workings calls for calibration to make sure it’s still working properly.

If you’ve got a lot of radiation meters you might want to calibrate them yourself, but you’ll need to amend your radioactive materials license to let you do so, you’ll need to purchase the appropriate equipment, and you’ll need to get trained up so you can do it properly. Unless you have a few hundred instruments or more it probably doesn’t make sense to go through this process – but if you have a lot of instruments, calibrating them in-house can save you enough money to make it worth considering.

If you only have a few instruments you should send them out for calibration. Here you need to be careful to send them only to a facility that’s licensed to do the calibrations – most of the major instrument manufacturers can do this, but there are a lot of other facilities as well who can help you out. Doing an on-line search for “instrument calibration services” will give you a bunch of options; you can also look through the companies listed under Instrument Calibration Services in the Affiliates section of the Health Physics Society’s website (http://hps.org/aboutthesociety/affiliates/services.html).

One last thing – in addition to the annual calibrations there are quick and simple checks you should be performing every day that you use your instruments. Check to make sure the batteries are charged (many meters have a “Bat Test” button or switch position), make sure the meter’s physical condition is OK, and (for contamination detectors) check the meter for proper response against a check source to make sure your meter reads within 20% of the expected reading. For this one you can purchase a “button source” that can be mounted on the side of the meter; when your instrument is calibrated the calibration certificate will include information on the expected count rate. So, for example, if the expected count rate is 1000 cpm, you should get a reading that’s between 800-1200 cpm when you do the response check.

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