# Converting 4-20mA Signal

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#### David

I have two transducers and I want to take the differential between the two and convert that to a water level. This will be done through a PLC that controls a valve on the surface. What is the calculation that converts the 4-20mA signal to a true water level?

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#### Bob Hogg

One transducer will be wet and one dry - right? If there is a space between the two like 3' then water level can be anywhere between. Unless you have a timed deal? If you only want a simple valve controlled level -we can offer a HD machanical float rod - that opens a sturdy valve in either SS or Bronze. 1/2" NPT to 2" NPT. Or, a simple way would be to use our continuous probe that, as well as 4 - 20 has a 1 to 10 VDC resistive output that can be hooked direct to a meter or scale. Thanks Bob Hogg www.almegcontrols.com > I have two transducers and I want to take the differential between the two and convert that to a water level. This will be done through a PLC that controls a valve on the surface. What is the calculation that converts the 4-20mA signal to a true water level? >

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#### jmGiraud

Differential Pressure XTR's are designed for what you are trying to achieve. If your tank is an horizontal one, you need to linearise the reading. This applies also to other tank geometry.

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#### Cahya Hari K

What PLC did you use ? It is simple if you used AB PLC with Scale (SCP) command. Reagrds Cahya Hari K Eng.Dept. Somit Karsa Trinergi, PT

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#### September, Clyde

The easiest, not necessarily the most accurate way to calculate the ratio of the change in diff. signal is the change in water level. Note: You do not specify much - Is the one transducer at the bottom of whatever you trying to measure? Can this vessel be drained? etc Clyde September

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#### PMSC Control System

Dave, Although you did not specify that you were using a pressure probe for the water measurement I'm assuming that is being used. The probe [or transducer] should have the PSI rating on it - if not - the part number and a call to the vendor should give you the rating. You are probably dealing with a 0-5PSI or 0-10 PSI unit. For the 10 PSI unit -- 4ma is the reading for the probe out of water. 20ma for the probe in 23 feet of water. I'll use this as an example. Converting this to a usable value in a PLC or data logger depends on the input scaling of your analog input -and- the dropping resistor used. For a 0 to 5vdc input use a dropping resistor of 250 ohms. 4ma gives you an input of 1.0vdc at the PLC. 20ma gives you 5.0vdc. A total of 4.0 volts for the full range of the probe. If the probe's physical range was 23 feet. Divide the 23 feet by the 4.0 volts. This gives you a scaling factor of 5.75 feet per volt measured. Our reading starts at 1.0 volt to begin with so we have to subtract 5.75 from the value read to establish the depth of the probe. Once you have calculated this value you can establish a reference for the true depth. Your mileage may vary.... Dave Gunderson Hoover Dam Tech Services Group - Special Projects Branch Personal Web Site: http://damguide.com