Tech Tip: Level Measurement in a Vented Tank
24 December 2013
Pressure technology is one of the most common ways to measure level in a process. When you’re measuring water level, the setup is simple. One inch H20 (InH20) pressure equals one inch of level in your tank.
However, what do you do when you’re measuring something other than water? If you can fully drain and fill the tank, you can use modes 2&3 in the DSIII pressure transmitter to set the transmitter’s current output. Most of the time, this isn’t logistically possible, so now we have to do some math. Specific gravity (SG) is the ratio of a fluid’s density compared to water. Using water as our pressure reference, we can convert a pressure measurement taken in inches of water to inches of something else. Let’s do the math. For this example, let’s use diesel fuel with an SG of 0.9. Our tank has a height (h1) of 100”. The transmitter is mounted 6” from the bottom of the tank, giving a measurable level of 94” (h3). To calculate your pressure span that corresponds to the height to be measured, simply multiply the height (h3) by the SG of the process fluid. h3 x SG = pressure value (InH20) 94” x 0.9 = 84.6 InH20
In the transmitter we can now set mode 5 (4 mA pressure value) to 0 InH20. Set mode 6 (20 mA pressure value) to 84.6 InH20. Since a pressure unit of inches H20 doesn’t mean much to a technician in the field, change the displayed units to % using mode 13.
For the transmitter to display a level unit (inches) instead of a pressure unit, we need PDM software and a HART modem (or another form of HART communication). Under the local display settings, turn “scaling” on, set the units to “inch”, and the range to 6 to 100. With these settings, when the transmitter is reading zero pressure, the display will show “6 inch”. At full scale, the transmitter will display “100 inch”.
Finish well. To finalize your transmitter setup, we need to correct the offset of the transmitter. If possible, vent the transmitter to the atmosphere. In mode 7, the transmitter should display 0 inH20. If it doesn’t, press and hold the “up” and “down” buttons simultaneously for 2-3 seconds. The display will flash and the reading will go to 0 inH20. If you cannot vent to the atmosphere, we can calculate what the current pressure reading should be, given a secondary level measurement (through a sight glass, with a ruler, etc). (secondary level measurement – h2) x SG = calculated pressure (inH20) (55” – 6”) x 0.9 = 44.1 inH20
Enter this value into mode 7 using the up and down buttons. Your transmitter will now be showing “55 inch” in the display. Configuration is complete!