Commissioning 4-20 mA and 0-10 V monitoring

Most analogue faults found at commissioning are a wrong range at the receiver, a loop that runs out of voltage near full scale, or a broken wire that reads as zero. Each one has a quick test with a meter and a loop calibrator. This guide gives the numbers for each test, with an EpiSensor ZIO (4-20 mA) or ZVO (0-10 V) as the receiver.

Download the acceptance record (.txt). Print it, or fill it in on site.

Define the measurement

Before you connect anything, record the instrument tag, the range and unit, the transmitter model and its calibration certificate. Also record the output characteristic. Most outputs are linear, but a differential-pressure flow transmitter can be set to a square-root output, and some outputs are reversed (20 mA at the low end of the range). The acceptance record at the end of this guide has a field for each item.

Keep the raw current beside the scaled value. In Edge, each ZIO channel reports its loop current in mA as four sensors: Now, Avg, Min and Max. Make the engineering value a linear calculated sensor, y = m × x + c, and keep the mA sensors. For a 0–10 bar transmitter, m = 10 bar ÷ 16 mA = 0.625 bar/mA and c = −4 mA × 0.625 bar/mA = −2.5 bar. A broken loop then shows 0.000 mA and −2.5 bar, a value below full vacuum. A reading of exactly 10 bar beside 20.000 mA can be a saturated input, not a true full-scale pressure.

Wire a 4-20 mA loop

In a two-wire loop the transmitter takes its power from the loop and sets the loop current between 4 and 20 mA. The supply, the transmitter and the receiver are in series, so the same current flows through each of them.

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The ZIO measures the loop current. It does not supply it. The datasheet gives the power source as external and the maximum input voltage as 36 V DC, so a two-wire loop needs its own supply, usually 24 V DC. The datasheet gives the measuring range (0-20 mA) and the resolution (0.001 mA). It does not give the input resistance, the accuracy or the isolation between the two channels of a ZIO-22. Measure the input resistance on site, as shown in the next section, and get the accuracy from EpiSensor before you set a pass limit.

A four-wire transmitter has its own supply and drives the loop current itself. Connect its output directly to the ZIO input, with no loop supply. A loop supply in series with an active output can force the current over range or damage the output stage. A three-wire transmitter shares its supply negative with the signal return. Connect the ZIO input between the signal output and 0 V.

Two loops from one supply on a ZIO-22 are normal if each loop returns through its own channel. If a transmitter is earthed at the process end, or the two loops use supplies at different potentials, fit a loop isolator. Without one, the loops can share a return path and each reading shifts with the other loop's current.

Earth the cable screen at one end only, normally the receiver end. Keep signal cable away from power cable. Before you switch on the supply, check the polarity at the transmitter and at the ZIO with a meter.

Check the loop voltage headroom

At 20 mA, the supply must cover the transmitter's minimum terminal voltage plus the voltage drop across every series resistance, with a margin. List every series element: the receiver input, the cable out and back, any barrier or isolator, a local indicator and surge protection. Loop-powered indicators are usually rated as a voltage drop, not a resistance. Subtract that drop from the supply directly.

For a 24 V supply, a transmitter that needs 10.5 V, a 250 Ω receiver input and 20 Ω of cable loop resistance:

24 V − 10.5 V − 20 mA × 270 Ω = 8.1 V of headroom

The 250 Ω here is a common receiver value. It turns 4-20 mA into 1-5 V. It is not the ZIO's figure. To find the ZIO's input resistance, hold the loop at a steady current, measure the voltage across the ZIO input terminals, and divide by the current. Use that result in the calculation.

The same loop can carry up to 675 Ω in total before the headroom reaches zero: (24 V − 10.5 V) ÷ 20 mA. If the transmitter talks HART, a handheld communicator needs at least 230 Ω of loop resistance, and 250 Ω is the usual value. If the receiver and cable give less, add a series resistor and do the headroom calculation again.

If the transmitter or its cable shorts, the full supply voltage appears across the receiver input. At 24 V this is inside the ZIO's 36 V DC rating. A 48 V supply is not. The loop current is then limited only by the input and cable resistance, so use a loop supply with a current limit or a fuse.

Wire a voltage signal

The ZVO reads 0-10 V DC at 0.001 V resolution. A voltage input measures the signal against a common reference. If the source's common is 0.2 V away from the receiver's common, every reading is 0.2 V wrong. That is 2 % of a 0-10 V span. Before you accept the signal, measure between the two commons with a meter. Then compare the source voltage before and after you connect the ZVO. A drop of more than a few millivolts means the receiver loads the output, or the commons are at different potentials. For a long cable run, or between panels on different earths, use the transmitter's 4-20 mA output or fit a signal isolator.

Plain 0-10 V cannot tell a broken wire from a zero reading, because both read 0 V. Where that difference matters, set the transmitter to 2-10 V (live zero). Then a reading near 0 V is always a fault. For example, 6 V on a 2-10 V output ranged −20 to 80 °C reads 30 °C:

Prove scaling at known points

Apply known currents with a loop calibrator. In source mode, disconnect the transmitter and connect the calibrator to the ZIO input. The calibrator then supplies the current. In simulate mode, the calibrator takes the transmitter's place and regulates the current from the loop supply, so this test also proves the supply and the headroom. Many smart transmitters also have a loop-test function that sets a fixed output current. To check a running loop without breaking it, use a milliamp clamp meter.

Apply at least 4, 12 and 20 mA. The 4 mA and 20 mA points catch a wrong range or a reversed map. The 12 mA point catches a wrong characteristic. If a flow transmitter sends differential pressure and the value is flow, 12 mA is 50 % of the differential pressure but √0.5 = 70.7 % of full flow. Only a midpoint shows that difference. For a linear 0–10 bar pressure transmitter, 12 mA is 5 bar:

At each point, record the applied current, the raw mA in Edge, the scaled value and the time. Set the pass limit before the test. Add the calibrator's uncertainty and the input accuracy to the transmitter's stated accuracy. A transmitter at ±0.1 % of span on 0–10 bar contributes ±0.01 bar before the other two terms. Then apply 3.8 mA and 20.5 mA and record whether the value is clamped, extrapolated or flagged. An Edge linear calculated sensor extrapolates: 20.5 mA on 0–10 bar reads 10.31 bar.

This test proves the data path from the ZIO terminals to the value in Edge. The transmitter's calibration certificate is the evidence for its accuracy in the process.

Temperature transmitters with RTD sensors

Many temperature points reach a ZIO through a transmitter that reads a Pt100 or Pt1000 sensor. The ZIO reads the transmitter's 4-20 mA output, not the RTD. Check the sensor at the transmitter against the IEC 60751 curve. A Pt100 reads 109.73 Ω at 25 °C:

On a two-wire RTD connection the lead resistance adds to the sensor resistance and reads as a higher temperature. Two leads of 2 Ω each add 4 Ω. At 25 °C a Pt100 changes by about 0.39 Ω/°C, so the reading is about 10.3 °C high:

Use a three-wire or four-wire connection between the sensor and the transmitter. A three-wire connection cancels the lead resistance when the leads match, and a four-wire connection removes it. If a two-wire sensor is already installed, short the sensor terminals at the head and measure the resistance of the two leads. Enter that value as lead compensation in the transmitter, if the transmitter has this setting.

Fault states and data quality

NAMUR NE 43 separates measurement from failure in a 4-20 mA signal. The measurement range is 3.8 to 20.5 mA. A transmitter signals a fault at 3.6 mA or below, or at 21 mA or above. Check that the transmitter is set to NE 43 failure signalling, and record which direction it fails: downscale (3.6 mA or below) or upscale (21 mA or above). Then force a sensor fault, for example by disconnecting the RTD at the transmitter, and read the current.

The ZIO's datasheet range is 0-20 mA. Before you rely on an upscale fault, apply 20.5 mA and 21 mA and confirm that Edge shows those currents. If the input stops at 20 mA, an upscale fault reads as full scale. In that case, set the transmitter to fail downscale.

Put the fault alerts on the raw mA sensor, not the scaled value. A threshold on a scaled value moves when someone re-ranges the calculation.

Sampling and reporting

In Edge, each ZIO or ZVO has an averaging time of 1 to 255 s. Each sensor reports on an interval of 1 to 1440 minutes, when the value changes by more than a set delta, or on both. The delta is in the sensor's own unit, mA for a ZIO. Live Stream reports once a second on node firmware 3.22 or later, and it adds a large amount of network and export traffic.

A one-minute mean cannot show a one-second pressure excursion. When short excursions matter, log Min and Max as well as Avg, or add a delta trigger. Record the averaging time, the reporting mode, the interval and the delta for each signal.

Acceptance record

Keep one record for each signal. The downloadable template covers:

Repeat the tests after an instrument replacement, a re-range, or a change to the wiring or the input. Continue with industrial process instrumentation, or build a system from the instrument list.