Calibration of Humidity Instruments

[D.Hutti]

I've recently been tasked with setting up our Laboratory to handle the calibration of Humidity Test Equipment. I ahave a couple of questions concerning this. 1)What are the governing documents for this type of calibration process? 2)What are the basic requiremnts for a Temp/Humidity Chamber?

 

[Jerry Eldred]

I believe there are ASTM specs for RH cals. Although I haven't used them because I have always had GIDEP procedures available, our RH tech should have those documents (he isn't here at the moment).

As for chamber requirements, there a number of options for your equipment. If you have chart recorders that do NOT have a detachable/movable RH probe (i.e.: the old Honeywell style, or similar), you will need a chamber (which I'll get to in a moment). If you have meters and/or chart recorders that have removable/detachable probes (i.e.: chart recorders similar to the Dickson or Omega or other brand name), and/or meters with probes, you may not need a full chamber.

Some of the options for the probe type meters are the HMK11 salt solution blocks made by Vaisala, or Rotronic makes some single use disks. General Eastern has a desiccator/RH generator that uses a combination of dry nitrogen to dry air, and saturation (it is normally externally monitored with a chilled mirror dewpoint meter (be careful on NIST traceability if you use that one). I am trying to be fair by mentioning all the vendors I know that have cal equipment for these. I am not recommending any of them.

If you need a larger chamber, Thunder Scientific is the only vendor I am familiar with that makes a large two pressure chamber.
This is considered by many to be top of the line for doing in-chamber cals of chart recorders.

Depending on your requirements, another option is to build a plexiglass chamber. If you build one of these, it involves taking sheet clear plastic, cutting it into squares and gluing together a cube shaped chamber with a hinged, airtight sealed door. Probably about 24-30 inches on each side, with a shelf about 6 inches below the bottom with many holes drilled through it, and with a fan to circulate the air. You use a pyrex or corning ware type lab quality glass cake pan, and pour salts or Assay grade in the pan, and spray distilled water on them. You then need to qualify the chamber with a high accuracy hygrometer, externally calibrated to verify uniformity. A good quality thermometer needs to be mounted in the chamber to monitor temperature as well. I don't reommend this method if it is a critical part of your process. Some places where this is not critical choose this method to save costs. I recommend the better methods.

Anyway, just a few ideas.

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