Hygrometer, Whirling

Quick Answer: The Hygrometer, Whirling is a hand-operated humidity instrument carrying a dry-bulb and a wet-bulb thermometer on a frame that is swung round on its handle. Swinging forces air across the wet wick, and the difference between the two readings gives relative humidity from a table.

Why the Hygrometer Is Whirled

A wet-bulb thermometer only reaches its true reading when air moves steadily over the wick. A static instrument relies on whatever draught there happens to be, which varies from room to room. A whirling hygrometer removes that uncertainty: turning the frame at a steady pace drives air across the wet bulb fast enough for full evaporative cooling, so readings taken in a still store room and on a windy playground are made under the same conditions.

The usual routine is to wet the wick with distilled water, whirl for a minute or so, read the wet bulb straight away and then the dry bulb, and repeat until the wet-bulb reading stops falling. The lowest steady wet-bulb value and the dry-bulb value are then used in the hygrometric table. Readings should be taken in shade, holding the instrument at arm’s length and away from the body.

Because it is portable and needs no power, the whirling type is also a common check on digital thermo-hygrometers and wall-mounted instruments. LabEquip also lists a sling psychrometer in its biology range, which works on the same principle. The thermometer type, scale range and table for this item are confirmed on request.

Specifications

Applications

• Quick humidity surveys of classrooms, stores and greenhouses
• Outdoor weather observations and microclimate fieldwork
• Checking the accuracy of digital thermo-hygrometers and wall hygrometers
• Physics practicals on evaporation, latent heat and humidity

Care & Handling

• Check the swing path is clear of people and objects before whirling.
• Wet the wick with distilled water only, and replace it when it is dirty or hard.
• Whirl at a steady pace, stopping to read, until the wet-bulb reading no longer falls.
• Keep the whirling hygrometer in its case to protect the thermometers from knocks.

Why Choose LabEquip

Geography and science departments running fieldwork, and labs that need to check their electronic humidity meters, find a whirling hygrometer the simplest dependable instrument to own. LabEquip lists it in its General Lab Products range; contact LabEquip about thermometer type and range.

Frequently Asked Questions

How long should the hygrometer be whirled?

Whirl it for about a minute, read the wet bulb, then whirl again and repeat until the wet-bulb reading stops falling. Use that lowest steady value with the dry-bulb reading.

Why read the wet bulb first?

The wet bulb starts warming as soon as whirling stops, because evaporation slows. Reading it first, quickly, gives the value closest to the true wet-bulb temperature.

Is a whirling hygrometer the same as a sling psychrometer?

Yes, the names describe the same principle: a dry and a wet bulb thermometer swung through the air. Designs differ in the frame and handle but are used in the same way.

Why is it more reliable than a static wet and dry bulb hygrometer?

Whirling ensures fast, steady air flow over the wet bulb, so it cools fully. A static instrument depends on natural draughts, which in still air can make humidity read too high.

Where should readings be taken?

In shade, away from walls and the body, with the instrument held at arm’s length. Sunlight, body heat and breath all affect the thermometers.

Can it be used to check a digital hygrometer?

Yes. Take a whirling reading next to the digital unit and compare the relative humidity values. A consistent large difference suggests the electronic sensor needs attention.

Last Updated: September 2026