How does calibrating hygrometers with salt work?

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somitomi
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How does calibrating hygrometers with salt work?

Postby somitomi » Mon Dec 03, 2018 3:03 am UTC

Recently I bought an old hair tension hygrometer at a thrift store which wasn't in agreement with the digital one we already have in the flat, so I looked up how to calibrate such an instrument. Thus, I learned that people most concerned with the accuracy of hygrometers are cigar enthusiasts and the household method all cigar webites recomend is:
  1. put a little salt in a cup and add water to make a "sludge"
  2. put cup and hygrometer in a a zip-loc bag
  3. wait 6-12 hours
  4. humidity inside the bag should be at 75% percent, adjust hygrometer accordingly
I have no doubt this is reasonably accurate considering how many sources agree on it, but I can't help wondering why it works. I assume the air inside the bag reaches some kind of equilibrium, but I'm not sure what role salt plays in all this.
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Re: How does calibrating hygrometers with salt work?

Postby pogrmman » Mon Dec 03, 2018 4:00 am UTC

I think it has something so do with the salt changing the vapor pressure of the water. According to this, it seems like NaCl for whatever reason keeps the humidity about 75% no matter what the temperature. Other salts differ. I wonder if it’s because of the relatively temperature-insensitive solubility compared to other salts...

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Re: How does calibrating hygrometers with salt work?

Postby Tub » Mon Dec 03, 2018 11:08 am UTC

Story time. I once bought a bunch of cheap DHT22 sensors to measure temperature and humidity at home. They're digital, "no calibration needed", yet no two sensors would agree on a value. IIRC the largest span was a difference of 10% humidity, which can be the difference between "you need to humidify the room" and "you need to dehumidify the room".

Trying to calibrate them, I've learned a few things:
a) no two websites will agree on the exact way of conducting the experiment. Do you add just a little bit of salt? Do you over-saturate it? How long do you need to wait? And, most importantly, there was disagreement on the exact humidity the experiment was supposed to produce.
b) I eventually did the experiment to the best of my abilities. I don't remember the exact readings I got, but they were wildly improbable for the postulated humidity.
c) none of the (digital or non-digital) hygrometers at my friends' houses agreed with each other, either.
d) apparently, to calibrate a $5 sensor, you need to buy a $1000 sensor, and even that one is going to be inaccurate compared to a $5000 sensor. Humidity is really complicated.

That project is still on the ever-growing pile of projects I need to finish someday. :roll:

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Re: How does calibrating hygrometers with salt work?

Postby pogrmman » Mon Dec 03, 2018 5:59 pm UTC

If you ever do it again, you should use way more salt than will dissolve in the water. Theoretically, if it’s more humid than what it’s “supposed” to be, there’ll be enough salt to saturate the “extra” water from the air.

I did read one place saying the best way to do it is to make a cone of salt and get it wet, but not so wet that it falls apart. That way, it supposedly fixes certain density issues that can arise that cause the humidity to slowly drift upwards after equilibrium is reached.

The big thing is that I can’t really find anything that suggests how much of the brine you need — if you want to control the humidity in a 1L area, can you get by with 4 or 5 mL of brine or do you need like 100?

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Re: How does calibrating hygrometers with salt work?

Postby somitomi » Mon Dec 03, 2018 9:42 pm UTC

pogrmman wrote:I think it has something so do with the salt changing the vapor pressure of the water. According to this, it seems like NaCl for whatever reason keeps the humidity about 75% no matter what the temperature. Other salts differ. I wonder if it’s because of the relatively temperature-insensitive solubility compared to other salts...

Fascinating, might read it properly once I have the time
Tub wrote:d) apparently, to calibrate a $5 sensor, you need to buy a $1000 sensor, and even that one is going to be inaccurate compared to a $5000 sensor. Humidity is really complicated.

I think that's just metrology in general, but humidity is a particularly tricky thing as far as I know.
pogrmman wrote:If you ever do it again, you should use way more salt than will dissolve in the water. Theoretically, if it’s more humid than what it’s “supposed” to be, there’ll be enough salt to saturate the “extra” water from the air.

I did just that, but since then I learned I can't turn the adjustment screw enough to compensate for the 12 percentage point difference I observed. I'll have to do it again after I figure that out, because now it's just less wrong.
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