# 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.

- Canonical: https://genetixbay.com/knowledge/answers/calibrate-a-hygrometer.html
- Topic: Environment · Type: How-to
- Evidence: Established · Status: Draft: awaiting expert review
- Full article: [Humidity by stage: RH targets from clone to cure](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)
- Also asked as: How do I do a salt test on a hygrometer? | Hygrometer calibration for a grow tent | My humidity reading seems wrong | Why do my two hygrometers disagree?
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## 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](https://genetixbay.com/knowledge/glossary/relative-humidity.md)** at room temperature [1]. The difference from 75 is your offset. For a second point, **magnesium [chloride](https://genetixbay.com/knowledge/glossary/chloride-nutrient.md)** 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](https://genetixbay.com/knowledge/glossary/sodium.md) 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](https://genetixbay.com/knowledge/glossary/vpd.md) 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](https://genetixbay.com/knowledge/glossary/botrytis.md) and [powdery mildew](https://genetixbay.com/knowledge/glossary/powdery-mildew.md) [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

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

## Common mistakes

- A wet salt solution with no undissolved [crystals](https://genetixbay.com/knowledge/glossary/trichome.md).
- 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.

## Related

- Full article: [Humidity by stage](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)
- [VPD explained](https://genetixbay.com/knowledge/environment/vpd-explained.md)
- [How do I lower the humidity in my grow tent?](https://genetixbay.com/knowledge/answers/how-to-lower-humidity-in-a-grow-tent.md)
- [What meters do I actually need?](https://genetixbay.com/knowledge/answers/meters-you-need-for-growing.md)
- Terms: [relative humidity](https://genetixbay.com/knowledge/glossary/relative-humidity.md) · [two-point calibration](https://genetixbay.com/knowledge/glossary/two-point-calibration.md)

## 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

1. Greenspan L, 1977. Humidity fixed points of binary saturated aqueous solutions. Journal of Research of the National Bureau of Standards 81A(1):89–96. https://doi.org/10.6028/jres.081A.011
2. 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. https://www.fao.org/4/x0490e/x0490e07.htm
3. Punja ZK, 2021. Emerging diseases of Cannabis sativa and sustainable management. Pest Management Science 77:3857–3870. https://pmc.ncbi.nlm.nih.gov/articles/PMC8451794/