How to Calibrate Humidity Sensors: A Step-by-Step Guide
TL;DR: How to calibrate humidity sensors, in one line: seal the sensor in a jar with a saturated salt paste that holds a known relative humidity, let it settle, and compare the reading. Common table salt (sodium chloride) gives about 75% RH — the reference used by professional labs under the ASTM E104 method. This guide walks through the check step by step and explains why a trustworthy RH reading matters when your environmental records have to satisfy a loans officer or a grant panel.
Relative humidity sensors drift. Dust, condensation, contamination and simple age all nudge the reading over months and years, and unlike temperature you cannot sanity-check RH against a boiling kettle or a bowl of ice. If you look after paper records, textiles or a small display collection, an RH figure you cannot trust is worse than none — because you will make storage decisions on it. The good news is that you can verify a humidity sensor at home with store-cupboard materials.
Why the saturated salt method works
When a salt is mixed with a little distilled water into a paste — wet enough that undissolved crystals remain — the air sealed above it settles at a fixed relative humidity that depends only on the salt and the temperature. Saturated sodium chloride holds roughly 75% RH (about 75.3% at 25°C). It is stable, cheap and the same principle national metrology labs use, which is what makes it a genuine reference rather than a guess.
Step by step: the 75% RH salt check
- Make the paste. Put a few tablespoons of pure salt (no iodine or anti-caking additives) in a small cup and add distilled water a little at a time until it looks like wet sand, with crystals still visible.
- Seal it with the sensor. Place the cup and your humidity logger together in an airtight container — a food tub or zip bag works — making sure the sensor does not touch the paste.
- Wait for equilibrium. Leave it somewhere at a steady temperature for at least a few hours; overnight is better. RH references settle slowly and hate temperature swings.
- Read and compare. The logger should read close to 75% RH. Note the difference — that is your sensor's error at this point.
For a second reference point, saturated lithium chloride gives about 11% RH, letting you check the dry end as well. Two points tell you far more than one.
What counts as a healthy result
Judge the result against the sensor's own specification, not perfection. The Elitech RC-51H, for example, is specified at about ±3%RH in its normal range, so a reading of roughly 72–78% against the salt reference is behaving as designed. A larger gap tells you the unit needs attention.
An honest caveat: on many loggers this salt test is a verification, not a field adjustment. If the RC-51H reads well outside spec, the right step is recalibration rather than forcing an offset by hand. That is where the factory calibration certificate in the box matters — it is your documented starting point, and it tells you when a professional recalibration is due.
Why trustworthy RH matters for collections
For a parish records store, a small museum or a display case with no building-wide climate control, relative humidity is the quiet risk. Central heating switching on and off swings RH hard through the winter season, and it is those swings — not a single reading — that foxes paper and textiles. When you apply to borrow an item or bid for conservation funding, the panel often wants to see twelve months of environmental data. A pointer hygrometer on the wall shows the moment but leaves no record; a logger with a calibrated sensor produces the continuous, credible trace those applications live or die on. Checking it against the salt reference now and again is how you keep that evidence defensible without a lab budget.
For a small collection: a logger you can verify with a salt jar, plus its factory certificate, gives environmental records a funder will trust.
Related reading
Want the wider picture first? See our guide to the temperature and humidity data logger. Running several units and wondering when to send them for recalibration? Read data logger maintenance.
Frequently Asked Questions
What is the saturated salt method for calibrating a humidity sensor?
You seal the sensor in an airtight container with a paste of salt and distilled water. The air above a saturated salt settles at a known relative humidity — about 75% for common table salt — giving you a stable reference to compare the sensor against.
How often should I calibrate a humidity data logger?
An annual check suits most non-critical uses, with a quick salt-jar verification in between. Send a unit for professional recalibration if a check shows it is outside its stated accuracy, or if your records need a fresh certificate.
Can I calibrate a humidity sensor at home without lab equipment?
You can verify one at home with pure salt, distilled water and an airtight tub. Treat it as a check: if the reading is well outside specification, arrange a proper recalibration rather than forcing a manual offset.
Why do museums and archives need calibrated humidity readings?
Because storage decisions and funding applications rest on the data. A calibrated logger produces the continuous, credible relative humidity record a loans officer or grant panel expects, which a wall hygrometer cannot provide.