A pressure transmitter has a range of -1 to 3 Bar and an output of 4 to 20 mA. Calculate the required pressure input and expected mA output to calibrate the device at 10%, 20% and 85% of it's range.

Question

Also, if the same pressure transmitter was vented to the atmosphere, having the general atmospheric pressure with no other pressure applied, what would the mA output be?

Answer 1

One could assume this is a linear relationship between the input pressure and output current in the form y=m*x+b where x is the input pressure in bars, and y is the output current in mA. Note: m=the slope=(20-4)/(3- -1)=16/4=4 mA/bar. Next, write the equation as point-slope form in which is: y-20=4*(x-3). Finally, after doing some algebraic manipulation, the linear equation is:

y=4x+8.

The range would be{y | 4mA<=y<=20mA} Hence, the range is 20-4=16 mA. If the output were to be incremented by 10%, then it would be incremented by 1.6 mA from the initial current. This implies if the output is at 10% of the range, then the output would read 4+1.6=5.6 mA. Therefore, find the percent value of the given range, then add it to the initial current, 4mA.

Now, to answer your question on what would be the reading on the transmitter if it was exposed to the outside atmosphere, then consider the atmospheric pressure at sea level is 1 bar. Hence, the reading would be y=4*(1)+4=8 mA. Moreover, the percent range would be at 4/16*100%=25%.