Analog and digital hygrometers: calibration and the salt test

Analog and digital hygrometers measure relative humidity, but neither display format guarantees accuracy. A salt test checks the reading against the atmosphere above a saturated salt solution in a sealed container. Sodium chloride provides an approximate reference near 75% RH at ordinary room temperatures. This is a useful check, not proof of accuracy across the instrument's entire operating range.

How the two types work

An analog hygrometer translates a humidity-sensitive element's physical response into movement of a pointer. The mechanism varies between instruments. A digital hygrometer commonly uses an electronic sensor whose electrical properties change with moisture, then converts that response into a numerical reading. Both depend on the condition of the sensing element and the quality of the instrument.

A clear display can make a digital model easier to read, but more digits do not necessarily mean less error. Resolution is the smallest change shown on the display. Accuracy describes how closely a measurement agrees with the reference under stated conditions. A display showing tenths of a percent can still have an error of several percentage points.

Checking, calibrating and adjusting

A check asks whether the instrument agrees adequately with a known reference. Calibration establishes the relationship between its readings and reference values under specified conditions. Adjustment changes the instrument so its indication agrees more closely with a reference. Everyday instructions often use “calibrate” for all three, which can obscure what was actually done.

Some analog instruments have an adjustment screw. Some digital instruments accept an offset; others cannot be adjusted by the user. Consult the model's instructions before changing anything. A recorded correction can help interpret a consistent error, but it does not repair unstable behaviour or establish accuracy at other humidity levels.

Why a saturated salt solution works

When solid sodium chloride and its saturated aqueous solution coexist in a sealed space, the air above them approaches a characteristic equilibrium humidity. The value depends on temperature. Vaisala's explanation of saturated-salt calibration describes the approximate 75% reference and the importance of temperature stability.

Saturated means that the liquid has dissolved as much salt as it can at that temperature. Some undissolved solid should remain. A container of dry salt alone, or an arbitrary dilute saltwater mixture, is not the same reference. The sensor measures the air above the mixture and must not touch the liquid or the salt.

Carrying out an approximate home check

Use a clean, small open cup inside a larger container that seals well and allows the display to be read without opening it. Put sodium chloride in the cup and add enough distilled water to produce a wet mixture while leaving visible undissolved crystals. Keep the cup stable so the liquid cannot spill onto the instrument.

Place the hygrometer beside the cup, close the outer container and keep the assembly at a steady room temperature, away from sunlight and heat sources. Give the atmosphere and sensor time to equilibrate. Follow the instrument or reference procedure for the waiting period; a reading taken immediately after sealing is not the reference value.

Observe the reading through the closed container and record the temperature, time and indicated RH. Opening the lid changes the environment. If adjustment requires opening the container, make the adjustment as the instrument manufacturer directs, then reseal and repeat the check after equilibration. Do not assume the reading remains valid while the container is open.

Interpreting a difference

Suppose an approximate reference is 75% RH and the stable reading is 72% RH. The observed difference is three percentage points. It is not a three-percent relative error, and it is not a reason to assume the same correction applies everywhere. This example illustrates the arithmetic; it does not assign a certified value to a home-made mixture.

A home setup introduces uncertainty from salt purity, temperature variation, leakage and incomplete equilibration. For a more defensible result, use a reference with a stated humidity, temperature range and uncertainty, or an appropriate calibration service. Vaisala's salt-properties documentation illustrates why references are specified rather than treated as exact universal numbers.

Reading conditions in an enclosure

Once returned to service, the hygrometer measures its immediate surroundings. A position directly beside a wet reservoir may not represent the rest of the enclosure. Give it time to respond after relocation. Comparing two instruments is useful for detecting disagreement, but agreement between them is not independent proof that either is correct.

Keep a dated record of checks, adjustments and unusual behaviour. Recheck after suspected wetting or inconsistent readings, following the manufacturer's guidance. The aim is to understand the measurement's limits. A trustworthy record of an approximate measurement is more informative than an apparently precise number whose reference and conditions are unknown.