SBS - The Best Value in QMS software

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

H

Howard Lee

#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?
 
Elsmar Forum Sponsor

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

Fully vaccinated are you?
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).
 
Thread starter Similar threads Forum Replies Date
M How to read a calibration certificate for thermo hygrometer General Measurement Device and Calibration Topics 2
A Calibration/Verification of thermo-hygrometer in storage areas ISO 13485:2016 - Medical Device Quality Management Systems 9
T Uncertainty in Hygrometer Calibration Certificate General Measurement Device and Calibration Topics 4
N Interpreting Hygrometer Calibration Uncertainty - Temperature Coefficient Measurement Uncertainty (MU) 1
P Thermo-Hygrometer - Measurement of 10DegC @10%RH ISO 17025 related Discussions 4
F Calibratable Thermometer / Hygrometer for Quality Control Room Calibration Frequency (Interval) 1
D How much Deviation is Acceptable when Calibrating a Hygrometer? General Measurement Device and Calibration Topics 3
Q Would you Calibrate this Digital Hygrometer / Thermometer? General Measurement Device and Calibration Topics 17
U Calculate acceptance of equipment reading after calibration General Measurement Device and Calibration Topics 2
D Question on Documented Calibration versus ISO 17025 Accredited Calibration ISO 13485:2016 - Medical Device Quality Management Systems 0
U Calculate acceptance of calibration reading Measurement Uncertainty (MU) 1
D Air hood calibration ISO 13485:2016 - Medical Device Quality Management Systems 1
K Ground Bond Test for Class I Medical Electrical Equipment - calibration problems IEC 60601 - Medical Electrical Equipment Safety Standards Series 2
E Calibration/Verification Report Durometer ISO 17025 related Discussions 2
S Medical device equipment calibration Qualification and Validation (including 21 CFR Part 11) 1
J Scrap Material Scale Calibration Required? IATF 16949 - Automotive Quality Systems Standard 21
S 7.1.5.2.1 Calibration/Verification Records IATF 16949 - Automotive Quality Systems Standard 6
K Is Calibration Required for Non-Adjustable Commercial Inspection Devices? General Measurement Device and Calibration Topics 11
F Calibration in GPC/SEC General Measurement Device and Calibration Topics 1
Q Changing calibration frequency Calibration and Metrology Software and Hardware 18
M IATF 16949 SI # 10: Integrated self-calibration of measurement equipment- Needs for explanation IATF 16949 - Automotive Quality Systems Standard 2
M Storing Calibration Certificate Document Control Systems, Procedures, Forms and Templates 7
B Risk analysis for defective measuring or measuring equipment out of calibration General Measurement Device and Calibration Topics 2
S Placing a calibration sticker with someone other than your initials Records and Data - Quality, Legal and Other Evidence 8
J IATF 16949 Calibration/Verification records question ISO 26262 - Road vehicles – Functional safety 5
M Approval of Calibration Certificates General Measurement Device and Calibration Topics 1
lanley liao Question regarding the calibration of monitoring and measure equipment. Oil and Gas Industry Standards and Regulations 3
N IATF Calibration Lean in Manufacturing and Service Industries 5
Q Do these certificates of calibration meet ISO 9001 requirements for traceability to NIST? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 12
T Plug Gage Calibration Calibration and Metrology Software and Hardware 1
M Load Cell Calibration using a totalizer on a flow meter General Measurement Device and Calibration Topics 0
E Calibration Records needed ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 1
D Limited Range Calibration - 5000 lb Industrial floor scale General Measurement Device and Calibration Topics 3
D Calibration of Small Scales General Measurement Device and Calibration Topics 32
C How to Establish the Calibration & Measurement Capability (CMC)? ISO 17025 related Discussions 1
I IQOQ or just initial calibration required? General Measurement Device and Calibration Topics 3
B Calibration in real life ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 3
J Calibration cycle for monitoring & measuring tools used in medical device manufacturing General Measurement Device and Calibration Topics 5
A Is calibration of test weight required General Measurement Device and Calibration Topics 4
S Calibration Frequency for Slip Gauge Kit used for CMM Calibration? General Measurement Device and Calibration Topics 0
S Calibration/Verification of customer fixtures IATF 16949 - Automotive Quality Systems Standard 6
Ron Rompen Calibration by manufacturer ISO 17025 related Discussions 4
Q Calibration verification records 7.1.5.2.1 IATF 16949 - Automotive Quality Systems Standard 2
B AS9100D 7.1.5.2 Calibration or Verification Method using outside cal lab AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 1
W Next Calibration Due Date Calibration Frequency (Interval) 5
S Where do l get calibration standards to run a calibration lab? Other ISO and International Standards and European Regulations 2
A OEM On-Site Calibration issues during Covid19 ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 12
D Calibration tolerance question using Pipettes Medical Device and FDA Regulations and Standards News 3
M Calibration Certificate Result issued by an accredited external laboratory General Measurement Device and Calibration Topics 9
G Calibration of "Master Parts" Used as Gauges Calibration Frequency (Interval) 5

Similar threads

Top Bottom