A hygrometer measures the humidity of the air, and it is one of the more useful instruments to have around the home, whether you are managing the conditions for a musical instrument, a wine store, a terrarium, a humidor or simply your own comfort. Unfortunately, hygrometers are also notoriously unreliable straight out of the box, and they drift over time. Many inexpensive models are simply inaccurate from the start, and even good ones wander as they age. The reassuring news is that you can check and correct a hygrometer at home with nothing more than salt, water, a container and a little patience, using a classic technique known as the salt test.
To understand why the salt test works, you need one elegant piece of science. When you dissolve ordinary table salt in water to make a thick, wet paste — not a solution, but a slush of salt barely moistened with water — the air in a sealed space above that paste will reliably stabilise at a known relative humidity. Specifically, saturated salt solution maintains the air above it at very close to seventy-five percent relative humidity, at normal room temperatures. This is a physical property of salt and water, dependable and repeatable, which makes it a perfect natural reference standard. You do not need any calibrated equipment; the salt itself provides the known value. If you seal a hygrometer in a container with salt paste and wait, the humidity inside will settle at seventy-five percent, and you can then see how far off the hygrometer reads and correct it.
The procedure is simple and requires only common items. Take a small quantity of table salt in an open cup or bottle cap — enough to make a shallow layer. Add a little water, just enough to dampen the salt into a thick, wet slush; you want it soggy but not swimming, with no standing water above the salt. Place this cup, along with your hygrometer, inside a sealable clear container such as a zip-lock bag or a plastic box with a tight lid. The two should share the same air but not touch, so the salt paste cannot splash or contact the instrument. Seal the container thoroughly, and set it somewhere at a stable room temperature, out of direct sunlight and away from draughts or heat sources, because temperature swings will disturb the result. Now wait. The air inside needs time to reach equilibrium with the salt, and this takes several hours; leaving it for around six to eight hours, or overnight, gives a dependable result. Try not to disturb or open the container during this time.
After the waiting period, read the hygrometer through the clear container without opening it, so you do not let in outside air. Compare its reading to the reference value of seventy-five percent. If it reads exactly seventy-five, your hygrometer is accurate and needs no correction. If it reads differently, you have just measured its error precisely. Suppose it reads seventy percent; then it is reading five points low, and you know that in normal use it will understate the true humidity by about five points. A hygrometer reading eighty would be running five points high. This error figure is the valuable result of the test.
What you do with that error depends on your instrument. Some digital hygrometers have a calibration function that lets you enter an offset or adjust the reading directly; in that case, set it so the instrument reads seventy-five in the test conditions, and it will thereafter read correctly. Some analogue hygrometers have a small adjustment screw, often on the back, which you can turn to move the needle to seventy-five while it sits in the test environment; this physically recalibrates the instrument. And for hygrometers with no adjustment at all, which includes many cheap models, you simply apply the known offset in your head from then on: if you measured it reading five points low, add five points to whatever it shows in normal use, and you have an accurate figure despite the instrument's flaw. Knowing the error is almost as good as being able to remove it.
A few practical points improve the reliability of the test. Temperature stability matters, because the seventy-five percent reference holds at normal room temperature, and large temperature changes during the test will throw it off; a spot with steady temperature is essential. Give it enough time, since rushing the equilibrium is the commonest cause of a misleading result; overnight is safest. Make sure the container is genuinely sealed, as any leak lets room air interfere. And repeat the test occasionally over the life of the instrument, because hygrometers drift with age, and a device that was accurate a year ago may have wandered; re-checking every year or so keeps you honest.
With a hygrometer checked and corrected by the salt test, you can trust the humidity readings it gives you, which matters more than people often realise. Whether you are protecting a guitar from cracking in dry winter air, keeping cigars or a wine collection in their ideal range, maintaining the right conditions for reptiles or plants, or simply managing the comfort and health of your home, the value of the reading depends entirely on its accuracy. An uncalibrated hygrometer that reads ten points off is worse than useless, because it gives false confidence. The salt test, costing nothing but a pinch of salt and a night's patience, turns an unreliable gadget into a genuinely trustworthy instrument, and it is one of the most satisfying pieces of practical calibration anyone can do at home.