Kestrel Meter vs. Sling Psychrometer

March 22, 2017 Monica Devlin Comments

Is your sling psychrometer reading different from your Kestrel? It's not uncommon. Typically, you might see that the sling gives a reading 5-10% higher than the Kestrel Meter. There are some real common errors seen with Slings. The Humidity on a Sling will err on the high side if:

The sock on the wet bulb or the water used for the wet bulb thermometer is not clean.
The sling is not swung around long enough.
The thermometer measurements are not read quickly enough after swinging the sling.

These are all of the common errors with a sling psychrometer causing humidities which err on the high side.

Using a Kestrel Meter correctly to get accurate humidity readings:

Best scenario, the meter should be in the environment for at least 15 minutes. This allows the entire unit to equilibrate to the surrounding conditions.
If this is not possible, the meter needs airflow in the environment, especially if it is being moved to significantly different conditions. Hold the meter into an air flow of at least 3mph for 15-30 seconds.
If there is no wind, you can swing the meter on the lanyard to create air flow past the sensors for the same amount of time.

With a properly calibrated Kestrel Weather & Environmental Meter and sling, and with proper use, the humidity measurements are likely to both be within the manufacturer's specification.