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Myco Langelo
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Calibrate your Hygrometer
    #10281539 - 05/04/09 02:14 AM (12 years, 8 months ago)

Thought this may be useful to some.  It helped me re-calibrate a 30yr old hygrometer I had laying around and make it useful again.  I'm not sure if this has been previously talked about, I didn't bother searching.

From http://www.cmhc-schl.gc.ca/en/co/maho/yohoyohe/momo/momo_002.cfm


    * 125 ml (about 1/2 cup) table salt
    * 50 ml (about 1/4 cup) tap water


    * hygrometer
    * a coffee cup
    * a 5 L (1.1 gal.) Ziploc™ bag or a well-sealed pressure cooker

Step 1 — Get to know your hygrometer
If your hygrometer has a pointer, look for screws or knobs on it that will allow you to move the pointer. If there are none, or if you have an electronic hygrometer, physical adjustment will not be possible, but you can still calibrate.

Step 2 — Prepare the mixture
Place the tap water and the table salt in the coffee cup and stir for a several seconds.

Step 3 — Set up
Put the coffee cup and your hygrometer inside the plastic bag or pressure cooker, and seal tightly. Note that salty water can damage your hygrometer if it comes in direct contact with it. Put the bag or pressure cooker in a draft-free place and out of direct sunlight, where the room temperature is likely to remain even.

Step 4 — Check your RH reading
After 8 to 12 hours, note your hygrometer’s RH reading. Your hygrometer should read about 75 per cent. If it does, you do not need to adjust it. If it does not read close to 75 per cent, note the difference between your hygrometer reading and 75 per cent.

Step 5 — Correct to the standard
If your hygrometer is adjustable, immediately adjust the reading to 75 per cent. If your hygrometer cannot be adjusted, record the difference you noted in Step 4. In the future, each time you take a reading from your hygrometer, you will need to add or subtract that difference.

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Registered: 06/08/02
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Re: Calibrate your Hygrometer [Re: notagoodusername]
    #10282726 - 05/04/09 10:32 AM (12 years, 8 months ago)

From what I understand, the salt method is a good method to calibrate them. However, hygrometers are fickle little things and are most accurate when calibrated to the range that they will be used in. For our purposes, that means the 90-100% range.

Since theres a big difference between 75% and 99% humidity, it makes sense to calibrate to as close to 100% as possible, which can be done simply by wrapping it in a damp wash cloth for 30 minutes, and then adjusting it as quickly as possible to read 98% (since RH readings will drop rapidly after pulling it out of the washcloth).

Two ways to skin a cat though; both methods certainly work well  :wink:

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Re: Calibrate your Hygrometer [Re: notagoodusername]
    #10283145 - 05/04/09 01:12 PM (12 years, 8 months ago)

This thread has been closed.


notagoodusername said:
I'm not sure if this has been previously talked about, I didn't bother searching.

If you HAD searched, you would see the salt method is completely inappropriate for humidity above 75%. 

Hygrometers are only accurate through a 20% range, so you MUST calibrate at the humidity level you're going to operate at.  That's why we use the damp towel method for mycology.  The salt calibration method is for cigar humidors that operate at 70%.

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