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Hygrometer or sling psychrometer: which humidity reading can you trust?

A sling psychrometer does not measure humidity at all. It measures how much a wetted thermometer cools when air moves across it, and humidity falls out of the arithmetic. That is the whole difference, and it decides which instrument you log with and which one you check with.

By the Akermin Editorial Team · 6 min read

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A hygrometer hands you relative humidity directly from a sensor. A sling psychrometer hands you two temperatures and leaves you to calculate humidity from the gap between them. The consequence is that the psychrometer measures a thermodynamic effect, which cannot drift, while the hygrometer measures a material property, which can. So the roles divide cleanly: the digital hygrometer is the monitor, because it logs, alerts and sits on a shelf for months, and the psychrometer is the reference, because you can rebuild its answer from first principles when you no longer trust the monitor.

Two instruments answering the same question by different routes

A consumer hygrometer almost always senses humidity with a thin polymer film held between two electrodes. The film absorbs and releases water vapour until it is in equilibrium with the air, its dielectric constant changes as it does, and the capacitance between the electrodes changes with it. A factory curve converts that capacitance into a percentage. Everything about the long term accuracy of such an instrument is a statement about the polymer: how fast it equilibrates, how cleanly it releases water after a soaking, and how much its response shifts as it ages and collects contaminants.

A sling psychrometer has no humidity sensor in it. Two matched thermometers sit on a frame, one bulb bare and one wrapped in a wick soaked in distilled water, and you whirl the frame so air rushes past both. Water leaves the wick, and the latent heat of that evaporation cools the wet bulb until the heat it loses to evaporation equals the heat it gains from the passing air. Dry air evaporates more water and cools the wick further. The gap between the two thermometers, the wet bulb depression, is therefore a direct measure of how far the air is from saturation. The procedure matters more than the hardware, and how to use a psychrometer covers the handling.

The arithmetic a sling makes you do

The depression becomes humidity through the psychrometric equation. Actual vapour pressure equals the saturation vapour pressure at the wet bulb temperature, minus a coefficient multiplied by atmospheric pressure multiplied by the depression. For a properly ventilated psychrometer at sea level that coefficient is close to 0.000666 per kelvin. Saturation vapour pressure comes from a Magnus style expression: 6.112 hectopascals multiplied by the exponential of 17.62 times the temperature divided by 243.12 plus the temperature, with temperature in degrees Celsius. Relative humidity is then the actual vapour pressure divided by the saturation vapour pressure at the dry bulb temperature.

That chain is worth seeing written out, because it explains why some handheld instruments publish quantities you would otherwise only find on a psychrometric chart. The Triplett MS475 states that its humidity and temperature probe reports relative humidity, air temperature, mixing ratio, vapour pressure and dew point, which are five coordinates on the same chart rather than five independent measurements. The ERAY digital psychrometer states a humidity range of 0 to 100 percent RH together with wet bulb and dew point ranges of minus 50 to 60 degrees Celsius, which is the same arithmetic driven in reverse: humidity measured, wet bulb computed. Whether a wetted wick is involved anywhere in that instrument is Not stated in the listing bullets, and on the evidence of the bullets it is not.

A wet bulb depression worked through at twenty degrees

Hold the dry bulb at 20 degrees Celsius and walk the wet bulb down. Saturation vapour pressure at 20 degrees is 23.3 hectopascals, so every row below divides its vapour pressure by that figure.

Wet bulb (degrees C) Depression (K) Vapour pressure (hPa) Relative humidity Dew point (degrees C)
19.0 1.0 21.2 91 percent 18.5
17.0 3.0 17.3 74 percent 15.3
15.0 5.0 13.7 59 percent 11.6
13.0 7.0 10.2 44 percent 7.3
11.0 9.0 7.0 30 percent 2.0

Two things fall out of the table. The first is the sensitivity: across the middle of that range, one kelvin of extra depression is worth roughly 7.5 percent RH, so one tenth of a kelvin of error in either thermometer is worth about three quarters of a percent RH. The second is that the relationship is not linear, and it gets steeper as the air gets drier, which is why psychrometry is awkward in very dry rooms and why a mis-set thermometer does more damage at 30 percent than at 90 percent.

Why a wick drifts and a polymer film drifts differently

Capacitive sensors ship with a tolerance band rather than a calibration certificate. The Govee H5179 states humidity accuracy of plus or minus 3 percent RH and temperature accuracy of plus or minus 0.3 degrees Celsius, with a refresh every two seconds. Read the first figure as a window, not a wobble: a sensor sitting inside its tolerance can read 62 when the air is at 65 and never move, which is enough to turn a decision about a damp wall into a coin toss. Ageing, dust, condensation events and strong solvents all shift that window further, and the shift is not a simple offset. There is a structured way to find out where you stand, set out in how to calibrate a hygrometer, and a worked example of what a three percent band permits in practice in the JEDEW hygrometer review.

A psychrometer has no equivalent failure, but it has its own list, and usefully the errors nearly all point the same way. A crusted or dirty wick evaporates less water, so the wet bulb sits warmer, the depression comes out smaller and the humidity reads high. Too little air speed does the same thing. Radiant heat from a window or a lamp on the wet bulb does the same thing again. Warm water in the reservoir does the same thing one more time. If you are getting psychrometer answers that look suspiciously humid, almost every plausible cause is on that list.

Where the sling stops being practical

  • It produces one reading at the moment you stop swinging. There is no overnight record, and humidity problems are almost always overnight problems.
  • It needs distilled water, a clean wick and roughly a metre of clear space to swing in, none of which a crawl space or a packed airing cupboard offers.
  • Below zero the wick freezes and the standard coefficient no longer applies.
  • It cannot raise an alarm, so nothing tells you that a cellar went past 70 percent while you were out.
  • It needs a patient operator. Two people slinging the same instrument in the same room routinely disagree by more than the tolerance of a cheap digital sensor.

Those limits are why continuous monitoring has moved to networked sensors. The GoveeLife mini sensor three pack states that a gateway pairs up to ten sensors, holds twenty days of trend data for viewing and exports two years of it, with a stated two year battery life per sensor. That is a different job from the one a sling does, and a fair comparison has to say so. A probe on a lead is a third option again, and the ALLOSUN thermometer hygrometer review looks at what a short lead and an unstated tolerance mean together.

Choosing the method for the room in front of you

Decide by the question, not by the instrument. If the question is whether a room spends its nights above a threshold, you need a logger and the psychrometer is irrelevant. If the question is whether the logger is telling the truth, a slung psychrometer handled carefully is a reference you can audit line by line, which no sealed sensor offers. If the question involves condensation on a cold surface, you want dew point, and you can get it from either route provided you know which route you took. The one position that does not survive contact with the arithmetic is treating a single digital percentage as a measurement rather than as a number inside a band.

Frequently Asked Questions

Does a sling psychrometer need calibration?

The humidity figure does not, because it comes from arithmetic rather than from a sensor curve. The two thermometers do. If either thermometer reads a few tenths of a kelvin out, the derived humidity is wrong even though nothing about the method has drifted.

Why do you have to swing it?

The standard psychrometer coefficient assumes air moves across the wet wick at roughly 3 to 5 metres per second. Below that the wick sits in a pocket of air it has already humidified, the depression comes out too small, and the calculated humidity reads too high.

Is a digital psychrometer the same thing?

Not in the way that matters. Most handheld units labelled psychrometer measure relative humidity with a capacitive sensor and then compute wet bulb and dew point from it. That is the same psychrometric arithmetic run backwards, so it inherits the sensor tolerance instead of escaping it.

What happens below freezing?

The wick freezes and you are measuring an ice bulb, not a wet bulb, which needs a different coefficient and a different table. Cold store and cold loft work is one of the clearest cases for a capacitive instrument instead.

Can I use tap water in the wick?

Use distilled water. Dissolved minerals lower the vapour pressure above the wick slightly and then build up as a crust that slows evaporation. Both errors push the wet bulb reading up, which makes the room look more humid than it is.