Custom equations for the 4-20 mA input

Please read pages 6-12 and 6-13 in the Reference Guide of the “TrendReader for Windows”. You will find the formula and an example of a 4-20 mA custom equation. If you do not have the manual then please look at the following example:

Example: Suppose you want to monitor a pressure transducer that has an input range of 0-100 KiloPascal (Kpa) and an output signal of 4 to 20 milliamperes (mA) using a 0-25 mA data logger channel (SR7 or SRP7 or OWL500 data loggers). To write the equation line, follow these steps:

Identify the variables: From the above information we have:

Xmin = The transducer’s minimum output (for this example it is 4 mA)

Xmax = The transducer’s maximum output (the signal from the transducer to your data logger). For this example it is 20mA

Ymin = The transducer’s minimum measurement (for this example it is 0 Kpa)

Ymax = The transducer’s maximum measurement (for this example it is 100Kpa)

Hence: Xmin=4 mA, Xmax=20 mA, Ymin=0 Kpa, Ymax=100 KPa

Plug the variables into the following equation: (standard equation for linear transducers)

Answer0={N} * {(Ymax-Ymin)/(Xmax-Mmin)} – {Xmax * {(Ymax-Ymin)/(Xmax-Mmin)} –Ymax}

For our example it is:

Answer0={N} * {(100-0)/(20-4)} – {20 * {(100-0)/(20-4)} –100}

Which simplifies to: Answer0=N*6.25-25

Now insert the value of N which is:

N=Source0/255*25 for SR7 and SRP7 loggers

N=Source0/248*(full scale of range selected) for OWL loggers

Therefore, if you use SR7 or SRP7 the equation for our example comes out as:

Answer0=(Source0/255*25)*6.26-25

And this is the requested custom equation which should be used.

Now do the following: (in TrendReader for Windows)

From the Main menu bar choose EQUATION, NEW EQUATION . The following equation window opens. Enter your above calculated custom equation on that window, as is shown below:

After writing the equation, select FILE, SAVE and make a note of the equation number.

Important Note: When typing the equation, be sure not to leave any spaces between characters.

Make sure that you type “0” which is “Zero”, not the character o.

Now communicate with your logger (open the Com port so that you see the table having the information about the channels of the logger) and as for the equation number on your 0-25mA channel insert the equation number of your custom equation (in our example it was equation number 109).

As soon as you change the equation number you will see that the real time values change to your requested units.

Save this new setup to your logger.