Hygrometer, Wet & Dry Bulb

Quick Answer: The Hygrometer, Wet & Dry Bulb (also called a Mason’s hygrometer) is a humidity instrument with two thermometers mounted side by side: one reads the air temperature, while the other’s bulb is covered by a wick drawing water from a small reservoir. Relative humidity is found from the difference between the two readings using a table.

How the Stationary Wet and Dry Bulb Instrument Works

Water evaporating from the wick draws heat from the wet bulb, so it reads lower than the dry bulb. How much lower depends on how dry the air is: in dry air the water evaporates quickly and the gap is wide, while in humid air evaporation slows and the readings draw together. That gap, called the wet-bulb depression, is read off a hygrometric table alongside the dry-bulb temperature; the table is often printed on the instrument’s backboard or supplied with it.

Being static is what distinguishes this item from the whirling hygrometer. A wet and dry bulb hygrometer hangs on a wall or stands in a weather screen and is read in place, so it can be checked several times a day without any handling. The trade-off is that evaporation depends on the air moving past the wick; in very still air the wet bulb does not cool fully and humidity reads too high. The whirling type forces air past the bulb and gives more consistent readings, which is why it is used to check static instruments.

Accuracy depends mostly on the wick. It should be clean muslin or cotton, fully wetted, and fed with distilled or clean rainwater; hard tap water leaves mineral deposits that stop it drawing water. The thermometer type, scale range and whether the table is included are not stated in the title and are confirmed on request.

Applications

• Daily humidity readings at school weather stations
• Monitoring humidity in greenhouses, stores and textile or paper rooms
• Checking egg incubators, where a wet-bulb reading is the usual humidity guide
• Physics lessons on evaporation and latent heat

Care & Handling

• Refill the reservoir with distilled or clean rainwater, not hard tap water.
• Replace the wick when it becomes stiff, discoloured or slow to wet.
• Mount the instrument in shade with free air movement, away from heaters and direct sun.
• Check the wick is wet, then read the wet bulb and the dry bulb at eye level.

Specifications

Why Choose LabEquip

Schools with weather stations, horticulture and poultry science departments, and storerooms where humidity matters use static hygrometers because they can be read at a glance all day. LabEquip lists this instrument in its General Lab Products range; contact LabEquip for the thermometer type and scale range.

Frequently Asked Questions

Why does the wet bulb read lower than the dry bulb?

Water evaporating from the wick takes heat from the bulb and cools it. Dry air speeds up evaporation and widens the gap between the readings; moist air slows it and narrows the gap.

How do I find the relative humidity?

Take the dry-bulb temperature minus the wet-bulb temperature, find that difference in the hygrometric table on the row for the dry-bulb temperature, and read off the relative humidity as a percentage.

What water should be used in the reservoir?

Distilled water or clean rainwater. Hard tap water leaves minerals in the wick, which slows evaporation and makes humidity read too high.

How is it different from a whirling hygrometer?

The wet and dry bulb type is stationary and read in place, so it is convenient for repeated readings. A whirling hygrometer is swung to force air past the wet bulb, which gives more consistent results in still air.

Why might it read too humid indoors?

In still indoor air the wet bulb may not cool fully because evaporation is slow. Place the instrument where air moves freely, or check it with a whirling hygrometer.

When should the wick be changed?

When it becomes stiff, discoloured or slow to draw water. A clean, fully wetted wick is essential; a dirty wick is the most common cause of wrong readings.

Last Updated: September 2026