sparkytools

Fourth year Instrumentation · tool

4–20 mA scaling calculator

Set the transmitter’s range (the values at 4 and at 20 mA), then enter either the loop current or a process value. You get the other, the percent of span, and the voltage across the sense resistor.

I have the…
  • Process value—
  • Loop current—
  • Percent of span—
  • Voltage across the resistor—
  • Span—

Show the working

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    How it works

    A 4–20 mA signal is a straight line: 4 mA is the bottom of the range (the lower range value, LRV), 20 mA is the top (the upper range value, URV), and the 16 mA between them is the span. Every milliamp above 4 is one-sixteenth of the way up.

    • PV = LRV + (mA − 4) ÷ 16 × spancurrent to value
    • mA = 4 + 16 × (PV − LRV) ÷ spanvalue to current
    • % = (mA − 4) ÷ 16 × 100percent of span
    • V = mA ÷ 1000 × Racross a sense resistor

    Worked: a temperature transmitter ranged 32–392 °F (0–200 °C)

    1. Span = 392 − 32 = 360 °F (200 °C).
    2. The loop reads 12 mA: (12 − 4) ÷ 16 = 0.5 — halfway up the span.
    3. PV = 32 + 0.5 × 360 = 212 °F — which is 0 + 0.5 × 200 = 100 °C.
    4. Across 250 Ω: 0.012 × 250 = 3 V.

    Questions people ask

    Why does the signal start at 4 mA and not zero?

    "Live zero." If 0% were 0 mA, a broken wire, a dead transmitter and a real reading of zero would all look the same. Starting at 4 mA means a healthy loop always has current flowing — enough to power a two-wire transmitter — and anything well below 4 mA is a fault, not a reading.

    What does a reading under 4 mA or over 20 mA mean?

    Usually a problem. Many transmitters deliberately drive the loop below about 3.6 mA or above about 21 mA to flag a failure (the NAMUR NE 43 convention), and a reading of 0 mA is typically an open loop or no supply. A little over 20 mA can simply be the process past the top of the range. The calculator still scales it so you can see what it would mean.

    Why a 250 Ω resistor?

    Many controllers read a voltage, not a current. 4–20 mA through 250 Ω is 1–5 V across it (V = I × R), so a 250 Ω resistor turns the loop into a 1–5 V signal. Other values work; enter yours to see the voltage range.

    How do I measure the loop without breaking it?

    Many transmitters have test terminals across a diode in the loop — put a meter on mA there and the current goes through the meter. Otherwise use a milliamp clamp meter, or break the loop and put the meter in series (knowing the controller will see the loop open while you do).

    Related

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