Convert a 4-20 mA loop current to engineering units and get the multiplier and offset for an Edge Calculated device.
4 mA reads 0 and 20 mA reads 10
In the 4 to 20 mA measuring span
Edge Linear operation, for a sensor that reports milliamps
Measured value
5bar
Check the values to see a result.
Use m and c in Edge only when its source sensor reports the raw current in mA. Check the source unit first.
Value = low + (I − 4) × (high − low) ÷ 16I = 4 + 16 × (value − low) ÷ (high − low)
12 mA on a 4-20 mA range of 0 to 10 bar reads 5 bar.
Tip: Transmitters that follow NAMUR NE 43 signal a fault at 3.6 mA or less, or 21 mA or more. A reading there is a fault, not a measurement.
A 4-20 mA transmitter sends 4 mA at the bottom of its range and 20 mA at the top, with a straight line between. To convert a current to a reading, find how far it is along the 16 mA span and apply that share to the engineering range: 12 mA is halfway, so on a 0 to 10 bar transmitter it reads 5 bar.
If Edge receives the raw current in mA, use the multiplier (m) and offset (c) below to create a Calculated device with the Linear operation. The source unit matters: values already scaled to engineering units, recorded in amperes, or stored as ADC counts need a different conversion.
value = low + (I − 4) × (high − low) ÷ 16
low and high are the values at 4 mA and 20 mA. The range can start below zero, as it does for a −50 to 150 °C temperature transmitter.
I = 4 + 16 × (value − low) ÷ (high − low)
Use this to check a transmitter: apply a known pressure or temperature and compare the current you measure with the one expected.
% = (I − 4) ÷ 16 × 100
The same for every range: 8 mA is 25%, 12 mA is 50% and 16 mA is 75%.
output = m × input + c; m = (high − low) ÷ 16; c = low − 4m
In Edge, open Devices, add a Calculated device, name the output sensor and set its engineering unit. Under Calculations choose Linear, select the exact raw current sensor as the single input, then enter this calculator’s m and c. Use Manual test values to check 4, 12 and 20 mA against the low, midpoint and high values before saving. Verify live input units and output timestamps after saving.
value = low + (high − low) × √((I − 4) ÷ 16)
A differential-pressure transmitter across an orifice plate or venturi gives a current linear in pressure, and flow rises with the square root of pressure. Choose “Square root” under More options when the receiver has to extract the root.
8 mA is 4 mA above the live zero, a quarter of the 16 mA span. A quarter of 10 bar is 2.5 bar.
For an Edge source reporting mA, Linear needs m = 0.625 and c = −2.5. This gives 0 bar at 4 mA, 5 bar at 12 mA and 10 bar at 20 mA.
Measured value 2.5 bar Open in the calculator
On a −50 to 150 °C range the span is 200 °C. At 10.4 mA the signal is 40% of the way along, so the reading is −50 + 0.4 × 200 = 30 °C.
Measured value 30 °C Open in the calculator
A DP flow transmitter ranged 0 to 50 m³/h sends 8 mA, 25% of its span. Flow goes with the square root of pressure, so the flow is √0.25 = 50% of the range: 25 m³/h. Read linearly, the same signal would suggest 12.5 m³/h.
Extract the root in one place only. If the transmitter already outputs linear flow, leave the calculator on Linear.
Measured value 25 m³/h Open in the calculator
Each whole milliamp as a share of the span, as a reading on a 0 to 10 bar range, and as flow on a square-root range.
| Current (mA) | Share of span (%) | 0 to 10 bar (bar) | Square-root flow (%) |
|---|---|---|---|
| 4 | 0 | 0 | 0 |
| 5 | 6.25 | 0.625 | 25 |
| 6 | 12.5 | 1.25 | 35.4 |
| 7 | 18.75 | 1.875 | 43.3 |
| 8 | 25 | 2.5 | 50 |
| 9 | 31.25 | 3.125 | 55.9 |
| 10 | 37.5 | 3.75 | 61.2 |
| 11 | 43.75 | 4.375 | 66.1 |
| 12 | 50 | 5 | 70.7 |
| 13 | 56.25 | 5.625 | 75 |
| 14 | 62.5 | 6.25 | 79.1 |
| 15 | 68.75 | 6.875 | 82.9 |
| 16 | 75 | 7.5 | 86.6 |
| 17 | 81.25 | 8.125 | 90.1 |
| 18 | 87.5 | 8.75 | 93.5 |
| 19 | 93.75 | 9.375 | 96.8 |
| 20 | 100 | 10 | 100 |
Subtract 4, divide by 16 and multiply by 100. 12 mA is (12 − 4) ÷ 16 × 100 = 50%.
The 4 mA live zero separates a real minimum reading from a broken wire or dead transmitter near 0 mA; a 0-20 mA loop cannot make that distinction at its zero reading. The live current can also power a two-wire transmitter. A current signal holds up well over long, electrically noisy cable runs.
A little either side is normal over-range. The NAMUR NE 43 recommendation uses 3.8 to 20.5 mA for measurement and treats 3.6 mA or less, or 21 mA or more, as a fault signal. Check which convention your transmitter follows.
Only for flow measured by differential pressure, and only if the transmitter does not already do it. Most modern DP transmitters can output linear flow. Extracting the root twice gives a reading that is badly wrong at low flow.
It depends on the supply voltage, the transmitter’s minimum voltage and the resistance of the receiver and cable. The 4-20 mA loop voltage calculator works it out.
A ZIO reads a 4-20 mA transmitter to 0.001 mA and sends the reading to the Gateway over the wireless mesh, with no signal cable back to a panel.
