4-20ma

Wonk alert.  After three weeks of mind numbing legal observation, I thought I’d throw in something from the trial with a different twist.

This is a story about how a roomful of lawyers and engineers can be clueless about a key piece of the technology, and how one of the lesser witnesses can say set them straight and it goes right over their heads (or maybe they ducked).

One of the big controversies being argued in the case is exactly how much effluent was pumped to the RIBs, particularly in April of 2009.  There are no less than three devices with the potential to answer the question.  Problem is that they all give different answers.

So begins the battle of the minds to show which is the more accurate.  Today, the theory was advanced that a pump flow chart recorder, which is required by NHDES and closest physically to the flow sensor, is the gold standard.  The other two, it was explained, are “downstream”.

Two weeks ago Russ Howe, our plant operator, was on the stand and explained that the flow sensor is what is called a 4-20 milliamp sensor.  It’s output is a current loop that varies from 4 to 20 milliamps in proportion to the sensed flow within it’s calibration range, and he said all three devices were connected to the same current loop.  Russ has a background in electronics.

This is an industry standard way to connect sensors to instrumentation because:

  • Any number of instruments can be connected to the sensor
  • The instrument(s) can be any distance from the sensor
  • Every instrument connected in the current loop receives the exact same data

That last point is basic physics.  In any given circuit, the current is constant everywhere.  It cannot be otherwise.  It is exactly the same as it passes through every instrument in the loop.

So if there is any variation, it is in calibrating each instrument to the sensor range, or as was the case in the PLC error, an internal error in the time constant used in the program that accumulated total flow.

The truth is that there has been no credible evidence to demonstrate that any of the recording devices is any more accurate than another, except for the program error described in the PLC controller.  That error, by the way, was algorithmic and is constant at something close to 10% as I recall.

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