Product review (5 stars): Is your guitar case humidified? Find out
Works fine. I can check the humidity in the guitar case to make sure it's where I want it to be. Or in a room. These small instruments are not calibrated, so you have to check their accuracy. One way I know of is to use another hygrometer you trust and compare the results.

Since your guitar case may not be humidified enough, the hygrometer lets you check the environment of the case before you put in your guitar and humidity device. No use putting the instrument in a dry case and try to keep it at 40% humidity. The water in the humidity pack will simply equilibrate with the dry environment of the case and you will dry out your humidity pack while not getting moisture in the air around the guitar. So first, use the hygrometer to check the case's moisture, then humidify your case if you need to.

Unfortunately, small measurement devices are prone to being uncalibrated. So you have to calibrate it to see how far off it is. You can do with with the salt test and then make a note on your reading versus reality: this little unit is not able to be adjusted.

A salt test is an easy test that basically puts your hygrometer into an isolated 75% RH (relative humidity) environment, in which you can then determine how inaccurate the reading is and adjust accordingly.

How to do the salt test:
 You will need:
Sandwich sized ziploc bag
 Bottle cap from 2 liter soda bottle
 Table salt  (do not use fancy grey salt: plain white salt)
 Your hygrometer

STEP 1:
Fill cap 3/4 of the way up with  table salt

STEP 2:

Add filtered or distilled water (not tap) to the bottle cap and  saturate the salt. Make a slurry (slushy mix) No water should be floating on top of the salt slurry.

STEP 3:

Place the hygrometer and cap with salt/water mixture  inside of the ziploc bag.

STEP 4:
Let cap and hygrometer sit in closed bag for 6 hours. It should read 75% after this time.

If your hygrometer reads 80%, subtract five points from any reading. If it reads 70%, add five points.

If you want to be fancy, you can buy a test kit:&nbsp;<a data-hook="product-link-linked" class="a-link-normal" href="/Boveda-One-Step-Hygrometer-Calibration-Kit-75-RH-Standard-Small/dp/B000A3UBLA/ref=cm_cr_arp_d_rvw_txt?ie=UTF8">Boveda One-Step Hygrometer Calibration Kit - 75% RH - Standard Small</a>&nbsp;The advantage of this is you can do a 32% test, because the inaccuracy of the hygrometer may not be linear and if you are reading for near 40%, it may be better to test at the lower portion of the scale.

I keep the hygrometer, which is no bigger than several sticks of gum, in the small compartment in the case--under the neck. It's there to make sure the case is always at the right level of humidification.