NC Cooperative Extension

How to Make a Wet Bulb Thermometer

How to Make a Wet-Bulb Thermometer

Properly managing the curing process and maintaining the barn can reduce fuel use drastically. No matter what type of bulk barn is used, fuel consumption can be reduced by tightening up the barn and using a wet-bulb thermometer to gauge ventilation. This wet-bulb thermometer can be made for a small amount of money . The homemade wet-bulb thermometer shown has been used successfully as an indicator of wet-bulb temperature in bulk barns.

Homemade Wet Bulb 1


2 – pieces 1″ PVC pipe, 7″ long (one piece with ¼” hole drilled 2″ from one end)   

3- piece 1″ PVC pipe, 2″ long

2 – 90 degree 1″ PVC elbows

1 – 1″ PVC “T” connection

3 – end caps, 1″ PVC

1 can – All purpuse PVC glue

1 – piece athletic shoestring, 7″ long (wick)

1 – Tobacco curing thermometer with bulb guards cut out

2 – wide rubber band

1 – twist-tie to tie wick to thermometer bulb

Instructions for Construction, Filling and Using    Homemade Wet Bulb 2

1. Glue all PVC pipe together as shown in figure except for the one end cap at the top so you can take it off to refill it with water.

2. Carefully remove the plastic or metal around the bulb of the thermometer so you can attach the wick to it.  (you may have to remove the bulb from the back plate to do this).

3. If you did remove the bulb replace it and calibrate according to another thermometer.

4. Secure thermometer to PVC pipe with rubber bands or light gauge wire.

5. Fill with water and replace the cap at the top.

6. Insert wick into the hole you maded in the PVC. A small nail, large toothpick, or kitchen match stick may be helpful.

7. Place in curing barn where air flow is strong. In updraft barns (e.g. Roanoke, Long, Powell) lay wet-bulb device flat on its back on the perforated floor near the loading doors. In down-draft barns (e.g. bulk tobacco), lay wet-bulb device flat on its back near the loading doors where air flow will strike the wet bulb of the thermometer.

8. Open fresh air vents only enough to maintain a wet-bulb temperature of 100°F to 105°F during leaf-drying and 105°F to 110°F during stem-drying.  The dry bulb temperature will be between 100°F to 135°F during leaf-drying and 135°F to 165°F during stem-drying.  The greater the difference between the wet bulb and the dry bulb, the lower the humidity levels in in the barn and the less of a chance of scald.  


The information on this page came from The Flue-cured Tobacco, Postharvest & System Efficiency Guide by Dr. Michael Boyette and Dr. Grant Ellington of NCSU and the Tobacco Barn Retrofit Information webpage

Webpage and pictures were created by Jacob Searcy: Bertie Extension Agent – Agriculture

Was the information on this page helpful? Yes check No close