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Scaling 4–20 mA and 0–10 V signals
One formula covers every linear transmitter. Here it is, with worked examples, a calculator and the mix-up that causes most scaling problems.
The one formula
A linear transmitter spreads its range evenly across its signal. To turn a signal into the value the BAS should show:
Value = low + (mA − 4) ÷ 16 × (high − low)low and high are the transmitter range from the nameplate
The 4 is where the signal starts and the 16 is its width (20 − 4). For voltage signals, swap those two numbers:
| Signal | Subtract | Divide by |
|---|---|---|
| 4–20 mA | 4 | 16 |
| 0–10 V | 0 | 10 |
| 2–10 V | 2 | 8 |
| 0–5 V | 0 | 5 |
| 1–5 V | 1 | 4 |
Worked example
Duct static transmitter, 0 to 5 in. w.c., 4–20 mA
Your meter reads 12 mA in series with the loop.
0 + (12 − 4) ÷ 16 × (5 − 0) = 2.5 in. w.c.
If the BAS shows something else, the range or signal type in the point setup doesn't match the transmitter.
Range that doesn't start at zero: −20 to 120 °F, 4–20 mA
Your meter reads 8 mA.
−20 + (8 − 4) ÷ 16 × (120 − (−20)) = 15 °F
Scaling calculator
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Quick check points
| Percent of range | 4–20 mA | 0–10 V | 2–10 V |
|---|---|---|---|
| 0% | 4 mA | 0 V | 2 V |
| 25% | 8 mA | 2.5 V | 4 V |
| 50% | 12 mA | 5 V | 6 V |
| 75% | 16 mA | 7.5 V | 8 V |
| 100% | 20 mA | 10 V | 10 V |
The 0–10 V and 2–10 V mix-up
These two look alike on a meter and cause a lot of wrong readings and half-stroked actuators. The same 5 V means two different things:
- On a 0–10 V signal, 5 V is 50% of the range.
- On a 2–10 V signal, 5 V is (5 − 2) ÷ 8 = 37.5% of the range.
If a value or a valve position is right at the top of the range and wrong everywhere else, check that the device and the BAS point agree on the signal type.
Working backward
To find the signal you should measure for a known value:
mA = 4 + (value − low) ÷ (high − low) × 16
This is handy when you have a trusted reference reading and want to know whether the transmitter or the BAS scaling is the part that's off.
Field notes
- Measure current in series with the loop, or use a clamp meter made for milliamp loops.
- Many controllers read a current loop as a voltage across a resistor. 250 ohms turns 4–20 mA into 1–5 V, and 500 ohms turns it into 2–10 V.
- A working 4–20 mA transmitter doesn't go below 4 mA at the bottom of its range. A reading near zero usually points to an open loop, lost power or reversed wiring.
- Read the range from the transmitter itself. Some have switches or jumpers that select the range, so the submittal may not match what is installed.
Pass criteria. The workbook's Test Guide passes a generic 4–20 mA point when the BAS value matches the calculated value within ±2% of span. That figure is field practice, not a published standard. Use the project specification when it gives a number.
Log the check while you are at the panel
The free Panelside app has this calculator built in and records each point as pass or fail. The checkout workbook turns those results into a sheet you can hand in.
Coming soon. The app and the store are not open yet.