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Short answer: Hygrometer calibration means comparing an instrument’s relative humidity (%RH) reading with a reference of known value. In laboratories the reference is often a saturated salt solution in a sealed, temperature-stable vessel; OIML R 121 defines eleven such humidity fixed points between about 2 %RH and 98.5 %RH. A reading is only as trustworthy as the reference’s stated uncertainty and the stability of its temperature.
Definition and scope
A hygrometer measures the humidity of air. Relative humidity, as OIML R 121 defines it, is the ratio (as a percentage) of the water vapour mole fraction in moist air to the mole fraction the air would hold if saturated with respect to water at the same pressure and temperature. Calibration here means checking the hygrometer against a reference value and recording the difference, together with the uncertainty of that reference. It is a narrower activity than the general practice described in what is calibration, which covers the concept for any instrument.
This page covers the saturated salt (humidity fixed point) method because two openly published documents describe it in detail: OIML R 121 and Greenspan’s 1977 NBS paper. Other reference approaches exist, but they are not described here.
How the fixed-point method works
OIML R 121 describes the method as convenient, inexpensive and reproducible, and notes it is often used for calibrating and checking instruments that measure the relative humidity of air in industry and research laboratories. It also appears in ISO 483, IEC 260 and ASTM E104-85, as cited in R 121’s reference list.
A hygrostatic solution is a saturated solution of a specified salt, in thermodynamic equilibrium in a three-phase system of solid, solution and gas, at a specific temperature and pressure. The air above it reproduces a known relative humidity, provided three conditions hold (R 121, section 4.1):
- the total pressure of the air and water vapour mixture is near standard pressure (101 325 Pa);
- solid, solution and gas are in thermodynamic equilibrium; and
- the solution is used at a suitable temperature.
The hygrometer under examination sits in a sealed vessel (a hygrostat) above the solution. R 121 requires the vessel to be made of a corrosion-resistant, non-hygroscopic material that is easy to clean, with the lowest practical volume-to-surface ratio and the largest practical free solution surface. Without air circulation the container height should not exceed the smallest dimension of the free surface of the solution.
Which salts, and what they reproduce
OIML R 121 builds its scale from 11 humidity fixed points (HFP). Each is numbered by its approximate relative humidity at 20 °C. Table 1 lists them with the temperature range over which R 121 gives values.
| Fixed point | Salt | R 121 temperature range |
|---|---|---|
| HFP 4 | Caesium fluoride | 15 to 80 °C |
| HFP 7 | Lithium bromide | 5 to 80 °C |
| HFP 12 | Lithium chloride | 5 to 80 °C |
| HFP 23 | Potassium acetate | 10 to 30 °C |
| HFP 33 | Magnesium chloride | 5 to 80 °C |
| HFP 43 | Potassium carbonate | 5 to 30 °C |
| HFP 59 | Sodium bromide | 5 to 80 °C |
| HFP 70 | Potassium iodide | 5 to 80 °C |
| HFP 75 | Sodium chloride | 5 to 80 °C |
| HFP 85 | Potassium chloride | 5 to 80 °C |
| HFP 98 | Potassium sulfate | 5 to 50 °C |
Table 1. Source: OIML R 121 (1996), section 5.1.
The reproduced humidity changes with temperature, so the same salt gives a different %RH value at each temperature. Table 2 shows published equilibrium values at 25 °C for six of the salts, from Greenspan (1977), who fitted data from 21 investigations to 28 binary saturated solutions.
| Salt | Equilibrium %RH at 25 °C |
|---|---|
| Lithium bromide | 6.37 ± 0.52 |
| Lithium chloride | 11.30 ± 0.27 |
| Potassium acetate | 22.51 ± 0.32 |
| Magnesium chloride | 32.78 ± 0.16 |
| Potassium carbonate | 43.16 ± 0.39 |
| Sodium chloride | 75.29 ± 0.12 |
Table 2. Source: Greenspan, J. Res. NBS 81A(1):89-96 (1977), Table 2. Greenspan assigned each value an uncertainty equal to three times the standard deviation of the predicted value. R 121 treats its own tabulated uncertainties as equivalent to expanded uncertainties with k = 2, so the two documents’ figures are not interchangeable without checking which convention applies.
Where it matters for labs, clinics and supply decisions
The reason a fixed-point check carries weight is the stated uncertainty. R 121 says that under ideal conditions the accuracy of the humidity sources generated by the practice equals the uncertainty figures it tabulates for each salt. Those figures describe the reference, not your hygrometer or your setup. Where humidity affects a result, for example in laboratory environmental monitoring or water activity testing, the calibration record should therefore state the reference used, its temperature and its uncertainty.
Whether to calibrate in-house, send the instrument out or use an on-site service is a separate question, covered in on-site vs send-out vs in-house calibration. Where calibration sits relative to qualification and validation is explained in equipment qualification vs validation vs calibration.
What to record
Each item follows from the conditions R 121 sets; the list is a record-keeping aid, not a procedure from the standard:
- the salt and its fixed point (for example HFP 75, sodium chloride);
- the temperature of the air and the solution, and how it was measured;
- the reference %RH at that temperature and its stated uncertainty, and which document it came from;
- the hygrometer’s reading before and after any adjustment;
- the date and the identity of the instrument.
Common misunderstandings
“Any salt in a jar is a calibration”
R 121 sets conditions: equilibrium among solid, solution and gas, near-standard pressure, and a stable temperature. For commercial hygrostats it recommends forced air circulation in a chamber controlled to ± 0.1 °C or ± 0.2 °C, or a thermally insulated, high-inertia vessel in a room whose temperature varies preferably by less than 1 °C. A jar on a bench does not meet these, so the result is best treated as an informal check rather than a calibration with a known uncertainty.
“The salt always gives the same humidity”
The value depends on temperature (Table 2 is for 25 °C only). R 121 also notes that above 40 °C there is a risk of salt crystals settling on the surface rather than staying in the solution, and that potassium iodide solution must be free of potassium iodate and stored in the dark.
“More dissolved salt is better”
The solution must be saturated with solid salt present. R 121 specifies analytical-reagent salt with distilled or deionised water, with the salt in excess so that 30 to 90 percent of the weighed sample remains undissolved.
FAQ
How do you calibrate a hygrometer with salt?
The method places the hygrometer in a sealed vessel above a saturated salt solution held at a stable temperature, then compares its reading with the reference value for that salt and temperature. OIML R 121 specifies the vessel, salt purity, temperature control and tabulated reference values; follow that document rather than a shortcut.
What humidity does a sodium chloride (table salt) solution give?
Greenspan’s table gives 75.29 ± 0.12 %RH at 25 °C, with 75.65 at 5 °C and 75.09 at 30 °C. Sodium chloride is HFP 75 in R 121.
How often should a hygrometer be calibrated?
Neither OIML R 121 nor Greenspan specifies a calibration interval, so this page does not give one. Intervals come from your quality system, the instrument maker’s documentation or the accreditation standard that applies to your lab.
Sources
- OIML R 121 (1996), The scale of relative humidity of air certified against saturated salt solutions. oiml.org/en/files/pdf_r/r121-e96.pdf (read 2026-11-07).
- Greenspan L., “Humidity Fixed Points of Binary Saturated Aqueous Solutions,” Journal of Research of the National Bureau of Standards 81A(1):89-96 (1977). nvlpubs.nist.gov (read 2026-11-07).
