A key part of work (and your PE controls exam!) is understanding ranges of equipment. Let's imagine the following tank full of liquid and try to figure out both the span and the zero, given the following information.
What is the span? This is the difference between the minimum and maximum values of the device. The zero is the value at the minimum range of the sensor.
Consider the following differential pressure sensor used to determine level inside of a tank. It is 1,000 mm tall (H1) and the differential pressure sensor is located 250 mm (H2) below it. The current tank level varies, we can use H for the time being. The fluid inside the tank has a specific gravity of 1 (SG1). The fluid on the tube on the side is a silicon derivate and have a specific gravity of 0.95 (SG2).
An easy way to mess up is to apply bad assumptions. Please verify your assumptions in the field, if you have a real system... But we need assumptions to make reasonable estimates. Let's assume...
Negligible pressure losses- reasonable if your system has a short run of pipe
System maintained- aka clean tube, no leakage
The math for pressure differential (as mm of H2O) is...
Pressure differential = (SG1 * H) + (SG1 * H2) - (SG2 * (H1+H2))
Pressure differential @ MAX = (1*1000) + (1*250) - (0.95*1250) = 62.5
Pressure differential @ MIN = (1*0) + (1*250) - (0.95*1250) = -937.5 mm H2O
Span = PD(MAX) - PD (MIN) = 62.5 - - 937.5 = 1000 mm.
Obviously, this is a trivial result for a trivial problem... but you would be surprised how many people scale dP transmitters for level and miss the fluid leg's impact.
Try different sizes or specific gravities! Try scaling these to different signals! Consider what values you would get if either side of the transmitter's tube were clogged...
I made a spreadsheet for you to play with. See what you can do with it or see if you can find any mistakes I may have left!
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