Hygrometer Calibration - Best way to verify accuracy of a 'reference' Hygrometer?

Howard Lee

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#1
I picked up a small hygrometer just to use around the plant for spot checks. We have hygrometer chart recorders around but I wanted womething to carry around. How would be the best way to verify its accuracy...just for the sake of argument?
 
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BradM

Staff member
Admin
#2
Re: Calibration of a Hygrometer

Without sounding overly simplistic... send it off and get it calibrated. In the end, you may spend more time messing with an inaccurate unit, that will more than offset the calibration cost.

You can try to mix up some salt solutions and test it that way. You have to mix the solutions just right, and generally you want a small chamber/ covered type setup to test it in.
 

Marc

Hunkered Down for the Duration
Staff member
Admin
#3
Re: Calibration of a Hygrometer

Is the Hygrometer to be used for 'Reference'? That's what it sopunds like. You could use your hygrometer chart recorders as 'intermediates'. My only point being, if you do not need it calibrated, don't spend the money.
 
D

DsqrdDGD909

#4
Re: Hygrometer Calibration - Best way to verify Hygrometer accuracy

I picked up a small hygrometer just to use around the plant for spot checks. We have hygrometer chart recorders around but I wanted womething to carry around. How would be the best way to verify its accuracy...just for the sake of argument?
You can do a one point calibration with a saturated salt solution.

"The recommended and most accurate calibration method is to place the hygrometer together with a salt solution into a small plastic box. One should take a cup or spoon filled with salt and moisten it with a few drops of water. The salt should not dissolve, but merely be damp. Afterwards, place the hygrometer and the salt in a well-sealed plastic box and wait. After about 8 hours the humidity within the box should have reached a humidity of 75%. Then adjust the needle of the hygrometer with a screwdriver at the rear to indicate 75% humidity.
Alternatively, remove the hygrometer from the humidor, wrap the hygrometer in a thoroughly moistened cloth, wait about one hour and then adjust the hygrometer on the rear with a screwdriver to 96% rel. humidity. " http://www.humidor-guide.com/hygrometer-calibrating-4-1.php3


No precision or uncertainty noted in methods above.
 

Jerry Eldred

Forum Moderator
Super Moderator
#5
Re: Hygrometer Calibration - Best way to verify Hygrometer 'reference' accuracy

I have the unfortunate "baggage" of having extensive experience in calibration of hygrometers. So since it appears this is a reference instrument, I'll avoid the technical descriptions of the various methods.

I'll simply say to beware of a single point calibration of a hygrometer. They are infamous for linearity issues. One particular model transmitter I cal has about four adjustments to linearize the scale, and some brands of humidity chart recorders accuracy specs degrade depending on what humidity level you measure.

Another simple method to check it is with desiccant. I experimented with some recently and found I needed a VERY GOOD seal to make it work. You can place the probe NEAR (not in) desiccant, in a sealed container, wait a few hours to stabilize, and use that to verify close to zero %RH.

And one further "soap box" while I'm on the topic... Also, beware of psychrometers (wet bulb/dry bulb liquid-in-glass thermometers with a fan motor and conversion chart). I've found they are not as accurate as many believe (I'll avoid the technical part unless someone wants to hear it).
 

BradM

Staff member
Admin
#6
Re: Hygrometer Calibration - Best way to verify Hygrometer 'reference' accuracy

Jerry,

I'm not sure about the OP, but I would be interested in your experience with the sling pyschrometers (sp?). I have always been leary of them, and have stayed away from them.
 

Jerry Eldred

Forum Moderator
Super Moderator
#7
Re: Hygrometer Calibration - Best way to verify Hygrometer 'reference' accuracy

I won't mention brand names, but I learned when I questioned the specs on dual liquid-in-glass psychrometer a number of years ago. The accuracy of a psychrometer depends on:

1. Wet Bulb Temp accuracy
2. Dry Bulb Temp accuracy
3. Air flow rate
4. Barometric pressure
5. Conversion Chart accuracy

The air flow rate is variable on a sling type, it is unmeasured in an electric fan type. Barometric pressure is not accounted for in either, and the conversion charts are ambiguous (on one particular mfr/model I used, you could calculate %RH two different ways; depending on which way you used, there was as much as 5%RH difference).

I will certainly concede that some of the very nice types that use high accuracy built-in SPRTs are much better. But using the simple sling-type or dual liquid-in-glass types are less accurate than a decent hygrometer.

As for my general opinion about sling psychrometers, they are useful as a low cost reference device. But if you require pretty good accuracy, I would personally not use them due to their inherent accuracy (which I would say is typically at best around +/-5%RH).
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#8
Re: Hygrometer Calibration - Best way to verify Hygrometer 'reference' accuracy

The previous replies are really good.....take their advice.....
A word about the slings in particular.....they work on an impossibility principle.....100% RH is not hard to achieve, but 0% RH is (I believe) impossible in our known atmospheric conditions, hence your error at 100% is very small but increases as you head toward zero.....as an quick check instrument in a field setting it is adequate for the use......never, ever, trust it for precise work.

Hershal
 
J

Jim Mathwig

#9
Re: Hygrometer Calibration - Best way to verify Hygrometer 'reference' accuracy

I have a cigar humidor in operation for two weeks now. The hygrometer seemed inaccurate so I did the wet towel test and recalibated, then did the salt test and it read 70% when it should be 75%, so I thought I'd go with it and just add 5% (or 4%) to the readings. Put in the humidor it has been reading 60%. Today I added a digital hygrometer and it is reading 82% after 4 hours. I thought the digital was supposed to be accurate within 2%, but now I don't know what to think or where to begin. Any ideas?
 

Jerry Eldred

Forum Moderator
Super Moderator
#10
Re: Hygrometer Calibration - Best way to verify Hygrometer 'reference' accuracy

THe first point is that humidity measurement is very quirky. It is a secondary (or perhaps tertiary) measurand, which means at best, it is a combination of all its cumulative inaccuracies.

The wet towel test is a generally accepted functional check. However, I wouldn't depend on its reading to very accurately set a hygrometer. Reason being that many hygrometers are the least linear at the ends of their ranges. So if you use the wet towel method to establish 100% RH on a (for example) +/-3%RH hygrometer, you might be +/-5 or more.

Then, checking with NaCl can be tricky. You have to be sure it is all set up properly to get accurate results. In a previous context (when I used salts), I had a chamber and a pyrex dish to create saturated salt solutions. You need a good airtight seal and fully saturated NaCl in distilled water, and plenty of time to stabilize. If your ambient humidity is significantly less than 75%RH, if you had any leaks in your chamber you likely would not reach 75% with the NaCl.

Regarding the digital Hygrometer, I calibrate lots of them. at 80%, you are in the grey area (unless you have a good quality cal with data that you trust) - I mentioned above that the extremes of range in hygrometers is where they are most infamously non-linear.

Short of paying to get something properly certified by a lab, I will assume that is not a reasonable option. I recommend you discount the wet towel test, as it is inherently troublesome (end of range). If this is a home hobby type application (not spending any money), I recommend fine tuning the 75%RH measurement. Get a bottle of distilled water and some non-iodized salt (not sea salt).

Please pardon if I provide details you already understand. I want to be sure and cover it completely (also for the sake of others who may be interested).

Here is the slightly tricky part...

To make a proper saturated salt solution, pour the salt into a very clean pyrex glass container. Add enough distilled water to saturate/cover the salt, with a little extra. Stir and be sure there is not enough water to dissolve all the salts.

Suspend your hygrometer probe immediately above the salt solution (not touching) seal the container airtight. I would allow a few hours stabilization time. Remember that every detail is important (non-iodized, airtight, enough salt, distilled water, enough time, etc.). Also, you need some idea of the ambient temperature. NaCl is 75.3% @ 15 Deg C, 75.5% @ 20 Deg C, and 75.8% @ 25 Deg C. So if ambient is in that area, there are your numbers.

Monitor the hygrometer for further drift in one direction until it stabilizes completely. Whatever your hygrometer reads is what it is. This isn't quite up to requirements for a lab, but should yield pretty good results.

The other check is with an airtight container filled partially with good quality desiccant and properly sealed will give you a NEAR zero check (again, in the non-linear region so don't expect high accuracy.

If you already covered as I described above (for NaCl check at 75%RH) and your digital hygrometer reads 82%, the hygrometer is inaccurate. One caution on humidity accuracy of hygrometers is they are pretty sensitive to temperature variations. Handling or holding the probe can shift readings (varies by different mfrs).
 
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