Seems like a waste of money?? Being that we dont use 75% bags…couldnt I just get the 58% bags that I use anyways and put it in a bag and throw the hygrometer in it? Not sure whats special about the 75% bags.
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I just use the cheapo humidity sensors. I think they were 5 for $20 on amazon. They all read within 1% of each other so that’s pretty good. No calibration required. I use the 62% bovedas and the sensors usually read 60-61 which is good enough for me.
All the best
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I have 2 of the caliber IV which can be calibrated. One is reading 46% and the other 42%. Big difference.
I just didnt know if the calibration kit bags were made different.
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The chemistry in the calibration version for saturated salt solutions makes them more consistent across temperature.
For instance (this is from here):
The 75% is Sodium Chloride, so you could make you own. Here it’s listed as Natrium chloride (NaCl). It’s one out of two chemistries shown above that maintains accuracy at tenths of a decimal place across temperature.
Whether it’s worth accuracy down to tenths across temperature when the off the shelf sensors are typically spec’ed around +/-2% is the question. Errors are additive, so if you are trying to get real precise, it’s not terribly expensive to buy a calibration pack. Though, I’d say the majority of the time we’d be ‘close enough’ without calibrating.
But, sensors that detect humidity do age and drift. There are capacitive, resistive, thermal conductivity, etc technologies. All of which have a variety of problems and can vary depending on the age, environment, and so forth. So it’s worth checking them for calibration occasionally.
And, yes, you could use the other non-cal packs for precise calibration if you know the chemistry of the solution, have a relatively stable temperature over the calibration period (something like 24+ hours), and can find the reference table. The problem with some packs is that we don’t necessarily know the chemistry and it is likely there is a mixture of different salts in there (shown above are for single salts).
Even so, you’ll probably get it close enough for the purpose even if you experience minor temperature drift using the standard packs. Very much dependent on the level of confidence you’d like to achieve.
edit: And another ‘but’ to add into consideration, measuring temperature is also surprisingly difficult and as such it should be considered relative as well. As an example, if a temperature sensor is ‘off’ by five degrees, a humidity calibration using magnesium nitrate equates to around 1-2% error in the humidity measurement due to the temperature error. Then add the intrinsic error of the sensor itself. If you have two temperatures across two sensors, the additive error can become large. So, there’s a bunch of stuff to consider when trying to get independent sensors to match each other.
My suggestions to get sensors as close to each other as possible but relative to the ‘real’ value: 1) use a single temperature probe (or the temperature from one of the sensors as the reference temperature) 2) plan to calibrate the sensor temperature along with the humidity 3) select a solution that minimizes drift or that you know the temperature drift data for 4) use the same calibration pack for calibration of all of the sensors 5) minimize temperature fluctuations 6) allow the humidity to stabilize in a closed container over a day or so 7) calibrate both the temperature and the humidity at the time of the measurement.
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In the cigar world, we use wet salt to calibrate hygrometers to 75%. It works perfectly.
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This one may also work … 
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I had heard it wasnt too accurate. Probably be close enough though
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I’m not sure. I’ve calibrated a lot of hygrometers and it’s always been dead on at 75%. If a hygrometer has read differently, it has always been the hygrometer. I tend to calibrate several at a time (I run 4 in my grow room and 3 in my humidor). I run several for accuracy because I didn’t want to fork out the hundreds of dollars for a super accurate one, so I just bought several of the low/mid-range ones and take the average. With that method, I think I can say it’s always within +/- 1% of 75% for me and not be misstating fact, but that’s a math problem. I know I can say it’s definitely close enough for 99% of applications.
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So just regular table salt?
Yeah, just regular, non-iodized table salt is what I use. Not sure if iodine would effect the outcome. Maybe? I’m not to fancy when I do it. I put a lot of saturated salt in Tupperware, put a platform inside to put the hygrometers on and shut the lid. It takes a few minutes, but it should read 75%.
Oh, and use distilled water. That’s what I was always told.
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So u add the distilled water to the salt to mske a slurry?
Exactly! It’s a wet slurry, super-saturated, but not so much that water is running out of it in the tupperware.