confusion calibrating the hygrometer...

[mrphilips]

i have been struggling with hygrometer calibration since i started on this journey of sausage curing.
i have found several "tried and true" methods:

a) put the hygrometer and a small bottle cap of salt, moistened with a few drops of water in a ziplock bag for 12-24 hours. the humidity should read 75%

b) (similar to above) put the hygrometer and a coffee cup with 1/2c of salt and 1/4c of water in a ziplock bag for 12-24 hours. the humidity should read 75%

c) wrap the hygrometer in a wet towel for 30-60min. the humidity should read 98%


my problem is, every time i run these tests i get different responses. i have used four different brands of hygrometer - analogue and digital, and yet none of them produce the same results.
for example, if i use the salt and ziplock bag test, i may have to tweak the unit to get it to reach 75%. but then, if i let it dry out and then test it again using the wet towel method, i still have to tweak it by maybe 15%

does anyone have a suggestion?

this is my current setup:
i've calibrated two hygrometers (one digital, one analogue) using the coffee cup of salt method and am using a humidifier in my curing chamber (a wine cooler, about 3.2 cubic feet) to keep the humidity between 70-75%, temperature fluctuating betwen 50-60`F (maintaining an IT of about 55` in a jar of water).
i have a small CPU fan coming on for about 30min in every 2.5 hour period (when the humidifier comes on) to dissipate the humidity.
this level of humidity seems to leave droplets of water everywhere in the chamber, but both hygrometers seem happy with the level - assuming my calibrations are accurate!

[wheels]

So, are we saying that both the analogue and digital are at least showing the same readings when in the same environment?

If we are then it's just a case of finding how far 'out' that reading is?

If so, all I could suggest is to determine the margin of error using a Mason's thermometer or (preferably) sling hygrometer - maybe borrowed? Do you know anyone with an amateur weather station?

Phil

[mrphilips]

yes, exactly! i have found two analugue hygrometers and two digital hygrometers that read within about 3-4% of each other when tested and when left in various environments.

finding out how far "out" is the point... i can calibrate the analogue ones or use the digital ones with a bias, but only if they're reading the same thing correctly.

no, i have no friends with super powers or more advanced technology than mine own, unfortunately.


the thing that really shaves my goat is how prolific these "tried and true" tests are, but they are not producing the expected results... if all three methods work, i should be able to calibrate using one method and then get the expected results without calibrating using either of the other two... but they are off by well over 10% each

[wheels]

I'm at a loss what to suggest. Maybe try them outdoors and compare the results with the local weather station...

...I know, it's really not very good. I think that Mason's thermometers can be bought fairly cheaply. Not the best accuracy, but it should give some idea.

Hopefully, someone else will have a brainwave!

Phil

[vagreys]

Calibrating hygrometers to a high degree of accuracy is considered difficult, even by professionals. Sling psychrometers are the easiest to calibrate, because they are thermometers.

None of the 'tried and true' methods you mentioned will yield accurate results, because they depend on uncontrolled variables. How much is 'a small capful' or 'a few drops'? Do 1/2 cup of salt and 1/4 cup of water combine to produce a saturated solution, or will it be supersaturated? How damp is damp? How wet is a wet towel?

If I were you, I would invest in an inexpensive hygrometer calibration kit. People who are serious about their cigars and tobacco keep them in a humidor and monitor the humidity with a hygrometer. High-end cigar and tobacconist shops that sell portable humidors and/or install walk-in humidors will likely have hygrometers and hygrometer calibration kits, or you can order them on-line. The Boveda One Step Calibration Kit is available on Amazon for US$5.45.

That's how I would do it.

[SausageBoy]

None of the 'tried and true' methods you mentioned will yield accurate results, because they depend on uncontrolled variables. How much is 'a small capful' or 'a few drops'? Do 1/2 cup of salt and 1/4 cup of water combine to produce a saturated solution, or will it be supersaturated? How damp is damp? How wet is a wet towel?

It's my understanding that it is fairly accurate to use a saturated salt solution to calibrate a hygrometer.
I've used the Boveda One Step Calibration Kit, I saw no major difference from using a saturated salt solution.

From the Omega Engineering website....

"Saturated Salt Solutions
A very convenient method to
calibrate humidity sensors is the use
of saturated salt solutions. At any
temperature, the concentration of a
saturated solution is fixed and does
not have to be determined. By
providing excess solute, the solution
will remain saturated even in the
presence of modest moisture
sources and sinks. When the solute
is a solid in the pure phase, it is easy
to determine that there is saturation.
The saturated salt solution, made up
as a slushy mixture with distilled
water and chemically pure salt, is
enclosed in a sealed metal or a glass
chamber. Wexler and Hasegawa
measured the humidity in the
atmosphere above eight saturated
salt solutions for ambient
temperatures 0 to 50°C using a dew-
point hygrometer. Later, Greenspan
compiled, from the literature, data on
28 saturated salt solutions to cover
the entire range of relative humidity.
Using a data base from 21 separate
investigations comprising 1106
individual measurements, fits were
made by the method of least squares
to regular polynomial equations to
obtain the “best” value of relative
humidity in air as a function of
temperature. These values are
summarized in the table shown."

http://www.omega.com/temperature/z/pdf/z103.pdf

[mrphilips]

so sausageboy, it seems like your response is essentially is support of the wet-salt method...

i've seen the "capful" test used mostly in small bags with small humidor-style hygrometers.

my adjustable hygrometers are bigger (coupled with a thermometer), so i usually use the 1/2c salt + 1/4c water method, which is also very heavily supported in many online resources.

the wet towel method - though faster and therefor easier - never seems to match the same results, but there's plenty of online resources backing up that one as well, including many of the guys on this forum, who's opinion means a lot to me!


well, thanks to everyone who's piped in, and since he's given some sciene to back up the theory - i'm gonna go with sausageboy for this round.
i will trust my 1/2c+1/4c wet salt method for the time being.

thanks again guys.

[vagreys]

...It's my understanding that it is fairly accurate to use to calibrate a hygrometer.
I've used the Boveda One Step Calibration Kit, I saw no major difference from using a saturated salt solution...

Using a saturated salt solution is a fairly accurate way to calibrate a hygrometer. That was not the question. The variation in the OP's results were, and methods described seemed to me to be contributing factors to the variability. A "capful of salt" combined with "a few drops of water" does not create a saturated solution. A 1/2 cup of salt combined with 1/4 cup of water does not yield a saturated solution. Not in the lab sense, and your quotes are coming from laboratory results using terms of chemistry.

The Boveda kit shouldn't yield significantly different results from a saturated salt solution, because it is a saturated salt solution. So, your results are not surprising.

I suggested the kit because it is easy, available, and offers reproducible results. You can also make your own saturated salt solution by dissolving 30-35g of salt in 100g of room-temperature or warm (37°C) distilled water. The solution is saturated when all the salt is dissolved and no more salt will dissolve into the solution, and there are no undissolved solids in the solution. Some will counter that you don't need to use distilled water, that tap water will do fine. That's up to you.

If you're concerned about an error of 1-2%, and want to be precise, then make a true saturated solution, yourself, add water to a kit, or use a kit where the saturated solution is already made. It's up to you.

[mrphilips]

i understand what you're saying, but i think the margins of error are small enough to ignore...

i'm fairly sure that a super-saturated solution (when there's still some undissolved salt in the container holding the liquid) would yield the same results as a saturated solution. if that excess salt was actually a significant factor in the experiment, then the physical composition of the container would also be a significant factor, the amount of air, the prcise time it's left before evaluating, and so on... so i'm fine with the loseness of 1/2cH20+1/4cHCl/12-24h.

and the proof? no matter how i've done it, i get the same results within a few %. to me, the margin of error isn't really worth the scrutiny.


my question is more about the nature of the test. essentially, the different tests themselves seem to yield the greater variances of results, and i'm unsure which is actually reliable - without a weatherstation, as wheels suggested - not the precise measurements of bag size, g's of salt, ml's of water, etc.

so many people swear by each test, and i can test and test over and over and get well aligned results, but each type of test gives a different base-line of results...


vagreys, i've seen those bags recently but opted out as my preffered device is actually too large for the bag, but hey... maybe i should get a smaller device and buy the bag! without a solid point of reference, i can't say it would be better or worse that what i'm using.


at this point, the charts on sausageboy's links seem to give me the confidence of reliability that i was looking for.

[Yannis]

I have tried both boveda kit and a cup of salt and the difference is negligible, about 1-2%.
The tricky point is to keep temperature stable within a few degrees during calibration so equilibrium can be reached. If temperature varies a lot then humidity will also vary and it takes time to settle. For my controller I put a cup with salt inside chamber (where temperature is stable within 1 °C) and I started calibration after a few hours when everything was at the same temperature.

edit
Ofcourse sensor and cup were placed inside a ziplock bag when calibration started.