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It's time to turn on the humidifier(s) here in St. Charles.

Common Salt Solutions for DIY Calibration/ verification:
Different salts naturally maintain specific humidity levels when mixed with water and sealed in a container. Here are some examples:
Salt Approx. RH at 25°C
Table salt (NaCl) 75%
Potassium carbonate (K₂CO₃) 43%
Magnesium chloride (MgCl₂) 33%
Potassium sulfate (K₂SO₄) 97%
Lithium chloride (LiCl) 11%

SOURCE:
How to Accurately Calibrate a Hygrometer Humidity Sensor | Fluke Calibration | Fluke:
https://www.fluke.com/en-us/learn/blog/ ... hygrometer

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Measure Twice,

Karl Borum

Seems like the potassium carbonate and the magnesium chloride would be the ones to test in the range we are generally looking for…I’m not sure I need my hygrometer to be accurate at the extreme ranges.

Handy post!

Potassium carbonate is the Goldy Locks salt for builders.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

The salt only applies to analog/ mechanical hygrometers…..

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Measure Twice,

Karl Borum

Good to know…

I don't really agree with this personally - I've used Potassium carbonate for years with digital and analog hydrometers with no problems.
Just be beware that direct contact between the salt solution and the hygrometer can damage any sensor.
I used a larges plastic jar or bag, small dish with saturated salt (24 hrs to stabilise) and suspend the instrument above it on a perforated platform.
The vapor pressure of potassium carbonate is negligible at room temperature, so no damaging salts touch the sensor.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

The salt applies to everything. When you create a chamber that reaches an equilibrium of ~43% humidity, it doesn't matter which instrument you're using to check it; the chamber remains constant, indicating how far off your instruments are. Most (all?) digital hygrometers can't be calibrated, but their margin of error can be noted.

I like to use an old Abbeon and Potassium Carbonate as a slurry in a small dish inside a clear container, and leave it for 24 hours, then calibrate. Then I just check the digitals based on that. Basically the same thing as Colin.

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I just use a sling psychrometer to calibrate the digitals. Close enough for me.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

There was a series of posts a while back on this topic, as well as a powered wet bulb system (semi-automated wet-bulb/dry-bulb system)... worth reading.

https://luthiersforum.com/forum/viewtopic.php?f=10101&t=51980&p=683812&hilit=hygrometer#p683812

These days, Boveda makes a nice little kit which contains a salt mix calibrated for 49%. All of my hygrometers go into a see-through polycarbonate plastic dry box with a couple of the 8 gram packets and after 24 hours, are adjusted (analog) or labeled for +/- value and calibration temperature/percent.

10-Pack of 8 gram, 49% RH packets:

https://www.amazon.com/Boveda-49-Percent-2-way-Humidity-Control/dp/B00JV27MCM


4-pack of 40 gram, 49% RH packets:

https://www.amazon.com/Boveda-Bass-Guitar-Hardware-B49HA-40-4P/dp/B09GRP7H9Z

On digital hygrometers - the inexpensive ones are likely not billed as adjustable, but I have found at least one low cost unit ($10 for 6 units) that had a factory use calibration pot on the board... all that was required was willingness to smash open the glued-together case to get at it. Higher-end industrial or laboratory hygrometers will have calibration capability - usually a +/- pot for the set point and a +/- slope adjustment for use within the stated accuracy range. My Extech 445815 (same as the boys use) had slowly drifted from 4% low reading (70 deg F/49% RH) to 8% (70 deg F/49% RH) over the last five years, but is still usable (the calibration pot only does slope, and that is fairly accurate, so I do my little mental math when I read the displayed RH value).

I keep a supply of sandwich baggies (also used for return of dead frets, removed parts, and - of course - keeping the odd sandwich fresh), cheap cellulose household sponges, and $2 TASOGEN digital hygrometers around for customers that blithely go through life trusting that humidification is only necessary for other people's instruments. After fixing their cracks, separated bridges, or other RH-related damage, I send them home with a temporary RH maintenance kit and a request to sin no more with regard to proper humidity control. Cost of this kit is just over $2 per instrument plus the time to run the 12 hygrometers through an quick accuracy check. It also eliminates the excuse of lack of time or resources should the instrument come back in for re-repair of further RH damage. The boys have used these Tasogen units for the past 4-5 years in their 3D printer material feed unit to monitor filament RH, where they have proven to be quite reliable and a good bargain.

Tasogen Digital Hygrometer 12-pack ($1.67/each):

https://www.amazon.com/Hygrometer-Thermometer-Temperature-Humidity-Fahrenheit/dp/B09TY3WHDR

48-pack of household cellulose sponges ($0.33/each):

https://www.amazon.com/YoleShy-Compressed-Cellulose-Cleaning-Reusable/dp/B0C1476QR6

Some good links there, thanks!