How do I check if my hygrometer is accurate?

Q: How do I check if my hygrometer is accurate?

Check a hygrometer with a salt test: seal it for 12–24 hours in a bag or jar with a damp slurry of table salt, which holds air at about 75 % RH at room temperature. Note how far it reads off and correct for that. For a second point, magnesium chloride gives about 33 %. Cheap sensors drift 3–5 % RH, enough to mislead VPD and bud-rot decisions.

Short answer

Do a salt test: seal the hygrometer for 12–24 hours with a damp slurry of table salt, which holds the air at about 75 % RH at room temperature ^[1]. The difference from 75 is your offset. For a second point, magnesium chloride gives about 33 % ^[1]. Established physics.

Why

A saturated salt solution in a sealed space settles at a fixed relative humidity set by the salt and the temperature. These "humidity fixed points" are a standard reference for calibrating humidity sensors ^[1]. Sodium chloride sits near 75 % across normal room temperatures, which makes table salt a cheap, reliable check.

Cheap RH sensors drift 3–5 % RH. Every VPD calculation uses RH ^[2], so a sensor reading 5 % low can have you running a late-flower room far more humid than you think.

That matters because humidity is an established driver of cannabis bud rot and powdery mildew ^[3]. A wrong number is worse than no number if you trust it.

The numbers

Reference	RH at about 25 °C (77 °F)
Saturated sodium chloride (table salt)	~75 % ^[1]
Saturated magnesium chloride	~33 % ^[1]
Typical cheap sensor drift	3–5 % RH
Acceptable offset to just correct for	within about ±3 %
Replace if	off by more than ~5 % or changing between tests

What to do

Put a quarter cup of plain salt in a cap or dish; add water until it's a damp slurry, with undissolved salt visible.
Seal it and the hygrometer in a zip bag or jar. Keep the slurry off the sensor.
Leave it 12–24 hours at a steady room temperature, out of sun.
Read it. Note the offset (for example, reads 71 % = 4 % low).
Adjust the meter if it allows; otherwise add the offset mentally and label it.
Optional: repeat with magnesium chloride for a low point.
Retest every few months, and after any humid spell.

Common mistakes

A wet salt solution with no undissolved crystals.
Reading after an hour instead of 12–24.
Testing in a room with big temperature swings.
Trusting a sensor near the light or in still air.
Correcting a drifting sensor forever instead of replacing it.

Full article: Humidity by stage
VPD explained
How do I lower the humidity in my grow tent?
What meters do I actually need?
Terms: relative humidity · two-point calibration

Follow-up questions

How do I do a hygrometer salt test?

To do a hygrometer salt test, mix about a quarter cup of table salt with just enough water to make a damp slurry, not a solution, in a jar cap or small dish. Seal it with the hygrometer in a zip bag or jar, out of sunlight at a steady room temperature, for 12–24 hours. The air settles near 75 % RH; the difference from 75 is your offset.

How accurate are cheap hygrometers?

Cheap digital hygrometers are typically within a few percent RH when new and often drift 3–5 % RH over time, more at the high end of the scale and after exposure to very humid air. That is enough to move a calculated VPD by 0.1–0.2 kPa or hide a lights-off humidity spike. Check them with a salt test every few months and replace one that keeps drifting.

Why do my two hygrometers show different readings?

Two hygrometers in the same grow tent often disagree because of sensor drift, different placement and temperature: a probe nearer the light or in still air reads a different microclimate. Put both together in a salt test to find each one's offset. Then place the trusted one at canopy height, shaded from the light and in moving air, as your reference.

Sources

Greenspan L, 1977. Humidity fixed points of binary saturated aqueous solutions. Journal of Research of the National Bureau of Standards 81A(1):89–96. doi.org/10.6028/jres.081A.011

Allen RG, Pereira LS, Raes D, Smith M, 1998. Crop evapotranspiration: guidelines for computing crop water requirements (FAO Irrigation and Drainage Paper 56), Chapter 3. FAO. fao.org/4/x0490e/x0490e07.htm

Punja ZK, 2021. Emerging diseases of Cannabis sativa and sustainable management. Pest Management Science 77:3857–3870. pmc.ncbi.nlm.nih.gov/articles/PMC8451794/