Let's talk a little bit about recalibration.
Probes that are use for ASTM F2170 according to the ASTM standard must be checked within 30 days before use. There's a movement and it's likely that the standard would be change very soon so that probes have to be checked within 90 days before use.
Checking them every month is just a big burden on the user. But, like any scientific instrument, we want to be sure that the instrument that we're using is reliable, that it's accurate and precise. And if you have a probe that you've used in the field multiple times, how do you know that it's actually working accurately?
Well, the standard requires this rechecking of the probes. Checks must be done traceable to a national standard. So as the US Government National Institute of Standards and Technology, in order to do that, generally you have to buy a kit for recalibration from the manufacturer of the probes or send the probes back to the manufacturers for checking.
Checking is different from recalibration. In the checking process, the user simply verifying that the probe is working accurately. Recalibration means, actually sending the probe to the manufacturer and having them certify its accuracy traceable to a national standard.
So, how do we do checking?
Well, here are a couple of examples. This is a kit made by one of the manufacturers of probes. The kit comes with a series of containers. You can unscrew the cap and inside of there you pour in distilled water and put in a salt solution, which will create a saturated solution of known constant relative humidity. Certain commonly available reagent salt produce humidity levels such as 75, 85 or 97% RH. Once the solution is placed in here and stable, we take the probe and insert it into the top and it sits stand in there and you allow it to equilibrate, generally over-night. Plug-in your meter and take the readings and see how it's reading. You do this at several points to assure yourself that the probe is working over the range of interest.
So, normally we're measuring floors that might have humidity from 75% to 95%, so that means you're going to want to check them and at least two solutions ranging from 75 to 95% to be sure they're working accurately. But, its very important and it's actually a requirement of the standard, in order to know that the probes are checked before use.
Now, what's the difference between checking and recalibration?
I touched on that before. Checking is something a user can do, and in fact, must do before using the probes. Recalibration is actually taking a probe and having the manufacturer check it in their factory and providing certificate traceable to a national standard and adjusting the probe if necessary in order to get an accurate answer. If you find that your probe, when you checked it, is out of accuracy by more than 3%, then the standard requires that it be recalibrated or discarded. And recalibration generally is something that users cannot do themselves.
What about shelf life?
Probes that come from the manufacturer in an original sealed containers, generally manufacturers will say that their initial calibration is good for about two years. Once the probe is been used, the variety of usage situation is beyond the manufacturer's control. If you put a probe into a very wet environment, or very cold environment, or very dirty environment, you may have affected the accuracy of the probe. And therefore, that checking really is important before you go use it on your next job. Only the manufacturer can provided a certification that it's calibrated accurately. And often that calibration cost more than the initial price of some of the probes. Some of the manufacturers will say our probes are really intended for use up to a year until it needs to be recalibrated. And at that point, you discard it and but a new probe.