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Sure. You can preform a salt test. All you need is:
some table salt
a small plastic lid from an empty milk jug, soda bottle, or water bottle
a zip-lock bag
some distilled water
Some patience
The science behind this test is that a wet sodium chloride (table salt) mixture will establish a relative humidity in an enclosed space of precisely 75%. By creating that environment inside a plastic bag, you can check to see how far off from 75% your hygrometer is, and then calibrate it accordingly. Here’s how you do it:
Step 1: Fill the empty bottle cap about 3/4 full of salt.
Step 2: Pour a very small amount of distilled water into the cap, to create a slurry. You don’t want all the salt to dissolve. You want it to appear grainy and gooey, like wet sand at the beach just after the surf has uncovered it. You can pour off excess water if you put too much in, or play with the water/salt mixture to get it right.
Step 3: Place the lid inside the zip-lock bag, then place the hygrometer inside the bag, display side up, near to the bottle cap. The technically “ideal” location for measurement is suspended 3 inches above the salt mixture, but that level of accuracy isn’t necessary for a $25 device. Placing it right next to the bottle cap is fine.
Step 4: Seal the bag, but don’t push any air out of it. Whatever normal amount of air is naturally inside the bag as you put stuff inside is fine. Place the bag somewhere that’s not too warm — ideally around 70F — that won’t fluctuate much in temperature.
Step 5: Leave the bag overnight. At a minimum, the bag should stay sealed for 6-8 hours. Ideally, 12-24 hours would be great. You want to make sure the humidity inside the bag has stabilized at 75%.
Step 6: Read the relative humidity level on your hygrometer. If it reads exactly 75% — you’re done! If it’s too high or too low, take it out of the bag and use whatever adjustment method is available (buttons for digital, adjustment screw for analog) to set it to exactly 75%. You now have a perfectly calibrated hygrometer!
You can actually use this method to calibrate multiple hygrometers at once (as many as you can fit in the bag). Experts suggest that you re-calibrate a hygrometer every 6 months, but for my application I’ll probably only do it every year or two.