This tech tip is based on a segment of a recent podcast episode with Joey Henderson, “Using Refrigerant Troubleshooting Charts w/ Joey Henderson.” You can listen to that podcast HERE.

Credit: efficientcomfort.net

If you’ve seen a superheat charging chart (like the one above), you’ll notice that you need to take the dry-bulb (DB) temperature entering the condenser and the wet-bulb (WB) temperature at the evaporator. You will also need to measure the return DB and WB temperatures to figure out your target delta T, which is one of the Five Pillars of HVAC Diagnosis.

Unless the air has reached the dew point (100% relative humidity), you’ll get different wet-bulb and dry-bulb temperature readings in the same location. That’s because WB temperature readings account for humidity, and DB temperatures only account for sensible heat. However, both are important in charging (by superheat) and standard diagnostics.  

The Origin of WB and DB

The story of how wet-bulb and dry-bulb came to be goes all the way back to the days of the mercury thermometer.

We could use the bulb as is—with nothing else on it—to measure the temperature. The reading would be our “dry-bulb” temperature. 

However, we could also put a type of wet cloth over the end of the bulb; covering the bulb in this way and then exposing it to an airstream will saturate the air that comes into contact with the bulb. Unless the air is fully saturated (100% RH), the thermometer will read a lower temperature than if the bulb didn’t have that wet cloth over it; this temperature is the “wet-bulb” temperature. 

Uses for WB and DB Temperature

We take the indoor WB temperature at the coil, and we take outdoor DB temperature at the condenser. These two values can help us find our superheat targets.

Unlike the DB temperature, the WB temperature accounts for latent heat in the air. We take WB temperature readings at the coil because it indicates the greatest amount of heat moving over the coil. (Remember, it takes a lot more energy to move latent BTUs than sensible ones.)

Modern Tools for Measuring WB Temperature

Nowadays, we have psychrometric probes that can measure wet-bulb temperatures. Back in the day, we used to have only sling psychrometers.

You have to be careful when it comes to choosing tools to measure wet-bulb temperature. Inaccurate sensors are more common on cheaper tools. (I’m not saying that you have to buy the most expensive psychrometric probes you see, but the adage “you get what you pay for” holds true.)

Maintenance is critical for psychrometers of any kind. They can easily get dirty with everyday use, and you may have to replace batteries. That’s why it’s a good idea to have multiple and calibrate them to each other to keep track of the accuracy while you’re using one regularly and not using the other.

Conclusion

Dry-bulb and wet-bulb temperatures are really there to give us different measurements of energy. Temperature is average molecular energy, but it doesn’t account for latent heat. Wet-bulb temperature measurements let us account for total enthalpy, not just the heat we can feel.

It’s kind of neat that a wet cloth over a bulb really does make all the difference in the information you can gather from a temperature reading. (Unless you’re at the dew point, that is!)