Uncertainty in Hygrometer Calibration Certificate

[t.PoN]

I received a calibration certificate for my Hygrometer (which is a device that is used to monitor the humidity in lab room). The reported uncertainty is +-5%.

Going back to the test method, the required humidity in the lab room should not exceed 50+-5%.

The question is that if the hygrometer gave me a reading of 53%. Then according to the calibration certificate the reading may be 58% or 48%.
Which make me somehow not sure if I am complying with the test method requirement?!!!

should i accept the uncertainty in the calibration certificate or should i try to using different device with better readability or resolution to improve the uncertainty?

thanks

 

[Jerry Eldred]

That's a pretty high uncertainty. I am familiar with accredited uncertainty calculations and with RH calibrations, just not with both together. I am thinking you should be able to get better uncertainties than +/-5%. That almost looks like the lab calibrated your unit using another hygrometer (which to me, personally, is not a good method). There are a number of methods available to calibrate hygrometers. And yes, the "TOLERANCE" of some hygrometers are often as poor as +/-5%RH. But there are better hygrometers; and there is better uncertainty I think.

The two pressure method used in Thunder Scientific chambers can provide a calibration accuracy (not total expanded uncertainty) of as good as +/-0.3%RH. Chilled mirror method should certainly be down around +/-1%RH or so, used in conjunction with a humidity generator.

I just checked the vendor we use to have our hygrometer data logger calibrated (accredited) by. Their uncertainty listed in their CMC ranges from a little over +/-0.5%RH at the low end of scale to at worst +/-1.13%RH at the high end of scale. So an uncertainty of +/-5%RH is in my opinion pretty high. If the standard the lab used to calibrate your hygrometer with as another hygrometer, I believe use of a Thunder Scientific 2500 series would definitely be preferable.

As for what model you are using for your measurements. That is a second topic. If the tolerance limits on your hygrometer is +/-3%RH or +/-5%RH and that is not good enough for your requirements, there are more accurate hygrometers. Two of the best brand names for hygrometers are Vaisala and Rotronic. DISCLAIMER: I don't work for any hygrometer company or sell them. This is my non-commercial opinion. There are quite a few other brand names out there.

If you want to know the lab we use, send me a PM and I'll share it (so as to keep this board non-commercial).

 

[BradM]

Agree in entirety with Jerry's post. We have a Thunder Scientific chamber with fairly small tolerance. We are calibrating field instruments to ?2.5% RH. I would think you should have no problem finding a better solution.

To confirm, the ?5% RH is for the standard they used, or the uncertainty/ tolerance of the instrument they calibrated for you?

 

[Hershal]

Jerry is correct.

Remember that the CMC in the scope of your calibration provider is likely NOT the uncertainty you will receive. It should be fairly close, but will almost certainly be a bit higher. That will likely be driven by your instrument.

Another consideration is how the calibration was performed. It is possible that the lab may be mixing what are sometimes called "slushies" which is a salt solution type of mix. NIST used to sell the mixes, but creating them from local sources was also common. That would drive the uncertainty quite high also.

The calibration provider should be able to explain the reasons for the high uncertainty.

Until then, you do in fact have to live with the potential of being outside your own tolerance. As a result, you may want to keep the environmental controls set toward the low end to minimize the risk of an out of tolerance situation.

Hope this helps.

 

[john ambrose]

Wanted to look up the cal cert for our lab's temperature/humidity monitor:

Omega OM-73 specified accuracy is +/-2% RH and +/-1 degree F Temperature

To calibrate, Omega used a RHCL-2 RH/Temp Calibrator with a maximum system uncertainty of +/-0.5% RH and +/- 0.2 deg C for temperature. They tested at 3 humidity points and 1 temperature point.

We do have a less expensive Temp/Humidity meter for our cabinet. This has an accuracy for RH of +/-8% and appears to read 8% low all the time. I don't bother calibrating it, since the cabinet it is monitoring is in the lab with the accurate Omega. The cabinet just needs to be less humid than the lab.

It appears you need to upgrade your T/H meter.