Geothermal and current loops… it all comes around

These days, quite a few of you are installing closed loop geothermal systems. These systems use long loops of flexible pipe installed underground or underwater to heat or cool a building or residence. Of course, a pump keeps the fluid moving around the loop.

Now jump to variable speed, constant pressure water systems. If you are involved with these systems, especially larger ones, you’ve probably seen or heard the term “4 to 20 milliamp pressure transducer” or “4 to 20 milliamp current loop”. These are loops as well, but loops of electrical current instead of water. And once again, it’s terminology that gets thrown out there without much explanation. So, let’s explain.

Many variable frequency drives (VFDs), especially larger units such as Franklin Electric’s HPX, utilize these 4-20 milliamp loops in conjunction with a pressure transducer. “Transducer” is just a general term for a device that converts a mechanical measurement into an electrical signal. In our case, that parameter is going to be pressure, And, keep in mind that you’ll hear the terms transducer and sensor used interchangeably in our industry.

A small power supply in the drive sends out a low DC voltage to the transducer. In the case of the HPX, its 24 volts DC. These are the “4-20 mA” terminals on the HPX. In our geothermal system, this would be the pump. Two wires connect the power supply to the transducer. This makes “the loop” or the flexible pipe. The transducer then limits the amount of current passing through it based on the amount of water pressure it is experiencing. For example, the 4-20 mA transducer used with the HPX will allow 4 milliamps to flow if the pressure is 0 psi. The upper limit of pressure can be programmed into the HPX, and at this pressure, say 80 psi, the pressure transducer will allow 20 milliamps to flow. Hence the name 4 to 20 milliamp current loop. The VFD controller then knows exactly how much pressure is out there by the amount of current “in the loop”.

But here’s a question … why the lower limit of 4 milliamps? Why not make 0 psi equal to 0 milliamps? There’s actually a very good reason. A lower limit of 4 milliamps allows the VFD to always know if the transducer is really connected. If the lower limit for the current was 0 milliamps, the pressure could either be 0 psi or the cable could be broken. Having 0 psi equate to 4 milliamps means the VFD always knows it’s in contact with the pressure transducer, even when there’s no pressure.

So now, the next time your’re explaining to your customer how a VFD works, you can also explain a 4 to 20 milliamp current loop. Once again, it all comes around.

2 thoughts on “Geothermal and current loops… it all comes around

  1. The 4mA lower limit allows the “transducer” to include electronic amplifiers, filters, and other signal conditioners.

    No simple tranducer “naturally” outputs 0-20 or 4-20mA; some conversion is needed.

    And of course if current does go to zero, you know right-off that the wire broke and the reading can’t be trusted.

    (Similar on my old Chevy. A thermistor tells the ECU how cold the engine is, to guide choke and idle speed. The ECU is smart enough that if this wire reads “infinity”, it should be ignored. My problem was that the thermistor failed “almost infinite”, the ECU believed the engine was near -60 degrees, and poured in gas to run the “really cold” engine.)

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