How to Properly Calibrate a Hygrometer
Follow instructions for the device if specific instructions were provided. Otherwise, most hygrometers can be easily removed from the humidor and will need to be removed to calibrate the hygrometer. Some hygrometers are not designed for removal like some "pre-calibrated" models. Most hygrometers can be removed by gently pressing from the back. Your hygrometer has been sleeping in a closed box for a while and you will wake it up now by placing the hygrometer in a wet cloth for half an hour or until the unit reads 95-100%. Return to its normal place and allow 2 hours to reset. If the hygrometer does not reach 95-100% as described above, you will need to calibrate the hygrometer as detailed below for the salt test.
This test is for analog hygrometers which are not pre-calibrated. The salt test is not recommended for digital/electronic models and performing the salt test on these models will void the warranty.
Like chemistry? Neither do I... so I will keep this chemistry stuff as brief as possible. In nature, when salt and water (NaCl and H2O for you science aficionados), are in a saturated solution at equilibrium, the resultant humidity is 75%. This gives a fantastic reference point to calibrate your hygrometer. Here's the procedure you can use to create this science experiment: you need a Ziploc bag, a beer bottle cap (or other small container) a small amount of salt (regular table salt will work, save the Mediterranean sea salt for cooking), and regular tap water.
Place the salt in the bottle cap (or other small container). Dampen the salt with the water. Do not put so much in that the salt gets "soupy". You want a damp pile of salt in the bottle cap. Place both the hygrometer and the bottle cap full of damp salt in the Ziploc bag and seal it well. (It is important not to let air in or out while the test is going on.)
Keep it like this for at least 8 hours. After 8 hours in the damp salt environment, the actual humidity inside the bag will be 75%. Compare it to your hygrometer; your hygrometer should also read 75% if it is properly calibrated. 99.9% of hygrometers will not read 75 at this point, if yours does, today is your lucky day and we encourage you to buy some lottery tickets (please remember me if you win!) If your hygrometer does not read 75, then you will know exactly how far off your hygrometer is off at this point, note the amount and direction, such as subtract four or add 10. Now if you are tired of reading, you can apply the above adjustment to your hygrometer reading. You can also calibrate your hygrometer to read accurately using the instructions below.
Proper Hygrometer Adjustment
Please read this section carefully as this information will contradict most printed information found elsewhere as well as falsehoods and misconceptions about calibrating a hygrometer. The world is full of self proclaimed experts who will tell you how to calibrate a hygrometer incorrectly. If you choose to follow their advice, remember that we sell replacement hygrometers in all shapes and sizes and we can help you replace the hygrometer you break when you don't follow our instructions.
To calibrate your analog hygrometer after performing the salt test, turn it over to the back. Look in the center. If you see a screw of some sort, examine it more carefully. If it is painted red or if it appears to barely have a slot to accept a screwdriver it is not the type that can be set with a set screw. If it does have a slot or a true calibration screw proceed to "Instructions B", otherwise follow directions listed at "Instructions A."
To calibrate this style hygrometer you will need to remove the rear stainless steel plate. Using a very thin flat blade of a screwdriver circle the edge of the plate till you locate an appropriate location to loosen the plate and pry it from the hygrometer. Beneath the plate you will see one of the configurations below. Some models have a plastic insert, remove this and set aside remembering how it was placed so you may replace later. Inside you will see a coil which should not be touched. You will see either a ring about the size of a nickel or an adjustment tab, turning either of these will adjust the reading of the hygrometer, but don't do that just yet. Now holding your hand over the back of the hygrometer, turn it over and notate the reading and either add or subtract the difference noted from the salt test. This will be the reading you will want your hygrometer to display, this reading will not be 75 as your hygrometer is not in the salt test environment at this time. Turn the hygrometer over again and turn either the adjustment tab or the adjustment ring and at the same time watch the needle at the front till it comes to the setting you have determined above. Your hygrometer is now properly calibrated. Begin to reassemble the parts, replace the plastic insert if there was one and replace the stainless steel back and apply pressure to the back while holding the bezel only, do not apply pressure to the glass lens as the lens will crack if pressure is applied.
For only the hygrometers with an adjustment screw at the rear center (read information above on how to differentiate types before proceeding), get a very thin flat screwdriver ready to make adjustments. Before adjusting anything you must do some math. Look at the front of the hygrometer to determine the humidity reading now add or subtract the amount you calculated after you did the salt test. This result will be what you should set your hygrometer to. Using your screwdriver, rotate the set screw until the hygrometer rests at the humidity reading you determined above. If turning the set screw does nothing and does not change the reading you are using the incorrect instructions and you must follow the instructions listed under instructions A.
At this point if you completed the steps listed above you are done for now, but you should salt test your hygrometer every 6 months or so to be sure of the accuracy.
If all this science and tests reminds you too much of high school and you want nothing to do with it we have two suggestions!
- Use a Boveda Hygrometer Calibration Bag, an easy one step system to calibrate any analog hygrometer. This can be used instead of the salt test, but you still must follow the calibrating instructions.
- Purchase a digital hygrometer as they are accurate and require no calibration.