Wet Bulb and Dry Bulb Thermometers

Sling Psychrometer

I needed suitable liquid filled thermometers to set up a fixed station wet bulb/dry bulb psychrometer. I searched the usual eBay sources and came up empty. I finally located a new-in-box sling psychrometer in the listings, so I purchased that and planned on using the thermometers for my fixed-station setup.

A sling psychrometer is commonly used to determine the humidity by measuring both the wet bulb and dry bulb thermometers at the same time and then using a chart, the humidity is computed. Both thermometers are mounted onto a frame and this is attached to a small handle. The thermometers are exactly the same, the difference being that one has a small cotton wick attached to the bulb that is wet with water. The other thermometer has a normal dry bulb. After wetting the wet bulb wick with water, the whole assembly is whirled around in a circle a few seconds to create airflow over the thermometer bulbs. Since the wet bulb is evaporating, it is slightly cooler compared to the dry bulb. This evaporative difference in temperatures is what is needed to compute the relative humidity.

My fixed station setup is similar in function. The main difference is that instead of whirling the thermometers around, I have them permanently mounted inside the Cotton Region shelter.

Fixed Wet Dry Bulb Thermometers

Interestingly, I have compared the wet bulb readings between the sling psychrometer and the fixed wet bulb thermometer and found them to be exactly the same. Since my wet bulb is always wet and evaporating, the temperature is always instantly available. Using a sling psychrometer required first wetting the bulb, and then causing evaporation via the whirling motion to get an accurate reading.

When the thermometers arrived in the mail, I removed the wooden handle since it would not be needed. I mounted the thermometer assembly onto a fixed wooden frame inside the CRS. I carefully attached a cotton wick to the wet bulb. To keep the wick moist, I re-purposed a pill bottle by drilling a small hole into the cap for the cotton wick.

Wet Bulb Thermometer

I filled the bottle with distilled water to avoid mineral deposit buildup. The water wicks up the cotton and wets the bulb almost instantly.

Wet Bulb Thermometer

The natural airflow inside the CRS creates the evaporative cooling needed to get an accurate wet bulb reading. Notice the difference in the temperature readings between the wet and dry bulb thermometers.

Wet Bulb Thermometer Wet Bulb Thermometer

The whole setup works very well and my electronic Davis humidity sensor seems to track evenly to the manual readings taken using the wet bulb temp. The pill bottle water reservoir will usually last several weeks before needing refilling. One interesting observation is I can somewhat predict the humidity just by observing how much water has evaporated from the bottle. Right after a strong cold front when the humidity is very low, the water evaporates very quickly and the bottle needs refilling more often. On very humid days, the wick just sets there saturated and the wet bulb temp is almost the same as the dry bulb temp. It can be fascinating to see first-hand how the surrounding air absorbs water.

I do not take the wet bulb temperature readings everyday, as my Davis electronic sensor is very accurate. Still it is convenient to have an accurate manual method to compute the relative humidity and compare and calibrate the electronic sensor.

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