View Full Version : Calibrating a hygrometer

11-22-2014, 07:54 AM
Just purchased a caliber IV and am wondering how best to calibrate it, or whether I should even bother with calibrating it.

The salt test is at 75%, and hygrometers don't measure linearly so not sure how useful that is to ukes. Boveda has calibrating kits for 75% and 33%, and a salt test using potassium carbonate can get you to 43% RH but where the heck do you get it lol.

According to the link below, there's data to show the caliber IV is pretty accurate or accurate enough out of box without calibration.


Burgess violins also sells a calibrated hygrometer that looks like just a caliber IV and it also appears to be set to be accurate between 40-60% RH.

11-22-2014, 09:48 AM

Measuring the humidity accurately is pretty tough. A wet-dry bulb hygrometer is pretty accurate, I would say, within 2%? Using the saturated NaCl solution (table salt) is OK, but it only calibrates at 75% so IMHO, not very useful as you need your hygometer to be accurate in a 30 - 50% range. I didn't know about potassium carbonate, but it is cheap. If you type in "potassium carbonate" on E-Bay, there are many available. Or, maybe check a local pet shop (aquarium)? or a farm supply store?

Good luck!
Chief (Physical Chemist!)

11-22-2014, 10:06 AM
Having two points identified on the scale is calibration. When I calibrate a thermometer I do it with ice water and at boiling, (these are exact values as long as you have two phases of liquid/vapor.
I would use the 75% 33% RH as the endpoints for your humidity range. A linear gradation between the two is sufficient for your purposes. Adding additional data points won't make your scale any more accurate, especially if you use different testing methodology for the various points. If you need more accuracy obtain a Sling Psychrometer. You can purchase one for about $40, no messy chemicals. Better still if you know a retired engineer ask them if they have one.

11-22-2014, 10:24 AM
Thanks guys for the info. I called David burgess who is a violin maker and asked him about his calibrated hygrometer he sells. He calibrates them to 33% and 54%, but he did say the hygrometers he uses, the caliber IV, are pretty accurate out of the box. He mentioned a few he shipped out today required no offset adjustment, but a few last week were off by about 5% points.

Rather than go through the hassle of buying a pound of these random salts, I went ahead and ordered a caliber IV from Amazon and one from burgess violins. I'll adjust the Amazon one to match the calibrated one and should be all set.

11-22-2014, 10:35 AM
This is a nice website that has some different salts which produce different RH levels.


It appears magnesium chloride produces 33% and magnesium nitrate produces 54%, which makes for 2 very nice reference values that make sense.

11-22-2014, 10:40 AM
If you are considering magnesium nitrate for use, check the MaterialSafetyDataSheet. IIRC water and nitrates yield nitric acid.

11-22-2014, 10:54 AM
> water and nitrates yield nitric acid.

I do not think so. You can write a chemical equation but the reaction favors the reactants by a wide margin, ie., equilibrium stays on the reactant side. My students would say "the reaction is not spontaneous"


11-22-2014, 11:01 AM
By the way, it is a very good idea to check MSDS. Just google "magnesium nitrate MSDS". Nitrates can affect you.


11-22-2014, 11:10 AM
No need to check - David burgess has already calibrated the unit with those salt compounds. I'm assuming he was safe about it :)

I'll use that calibrated one to reference the other one. I am glad I just paid the extra dough for the burgess one considering some of the volatility magnesium nitrate :)