It is not uncommon for the humidity measurements to differ when comparing the Kestrel Meter to a sling psychrometer (or belt kit). Typically, you might see that the sling gives a reading 5-10
% higher than the Kestrel Meter. Slings are susceptible to the following errors: - If the sock on the wet bulb thermometer is not clean, the humidity measured from a sling will err on the high side. - If the water used for the wet bulb thermometer is not clean, the humidity measured from a sling will err on the high side. - If the sling is not swung around long enough, the 
humidity measured from a sling will err on the high side. - If the thermometer measurements are not read quickly enough after swinging the sling, the humidity measured from a sling will err on the high side. Unfortunately, all of the common errors with a sling psychrometer will yield humidities which err on the high side. It is also important to use the Kestrel Meter correctly in order to measure an accurate humidity. Ideally, the instrument should be left in the environment for at least 15 minutes. This allows the entire unit to equilibrate to the surrounding conditions. If this is not possible, especially if the Kestrel Meter is being moved to significantly different conditions, then it is best to hold the instrument into an air flow of at least 3mph for 15-30 seconds. If there is no wind, the unit can be waved back and forth to create air flow past the sensors for the same amount of time. With a properly calibrated Kestrel Meter and sling, and with proper use, the humidity measurements are likely to both be within the manufacturer's specification. See Kestrel Fire Weather Meters.