Wet Bulb and Dry Bulb Calculations in CBAS

I was asked recently how to setup a Wet Bulb calculation in CBAS. Any Analog Input Calculation point can be setup as Wet Bulb. But first, a little info on Wet Bulb and Dry Bulb Temps.

Definition: A Wet bulb measures the temperature read by a thermometer covered in a wet cloth. As water evaporates from the cloth, evaporation cools the thermometer. This mirrors how the human body cools itself with sweat.   

Definition: The Dry Bulb temperature, usually referred to as air temperature, is the air property that is most commonly used. When people refer to the temperature of the air, they are normally referring to its dry bulb temperature.

The Dry Bulb Temperature refers basically to the ambient air temperature. It is called “Dry Bulb” because the air temperature is indicated by a thermometer not affected by the moisture of the air.

The Dry Bulb and Wet Bulb Calculation is available in CBAS Logic. It was added in CBAS 19.

There are many preset calculations in Logic that only require 2 existing Points to be associated.

To program an Analog Input Software Point to calculate one of the functions listed above, go to the Program screen of the Input. Click Program Calculation, then click the first line of the Logic section or click Insert Line.

Click Set Value of Software Point which is the last line.

Click the Calculation button.

Let’s choose “Wet Bulb Of”.

The above window asks for a Dry Bulb Temperature. Choose the temperature in the space.

Next, choose the Relative Humidity Point for the same space.

Then choose End Equation and Finished. The calculation is done in the background when CBAS is in Real Mode.

When you do the Dry Bulb Calculation on another Analog Input Software Point, the first Point you will be asked for is the Dewpoint. So, a Dew Point point has to be set up first. Just make a software Analog Input Calculation for that and set up the calculation by selecting “Dew Point Of” and adding the points as prompted.

To add Dry Bulb, do the same procedure, but choose “Dry Bulb Of”,

You can also do this on an Analog Output Setpoint. Click Program Logic and click the first line.

From the choices, choose Command and choose the Setpoint you are on. Choose Calculation and the next screen gives