2-wire (loop-powered): Power supply (+) → Transmitter (+), Transmitter (−) → PLC AI (+). PLC AI (−) → Power supply (−). Loop current = 4–20 mA proportional to pressure.
3-wire: Brown = +24 V supply, Blue = 0 V supply, Black/Yellow = current output to PLC AI (+). PLC AI (−) → 0 V. Use a 250 Ω burden resistor if your PLC input requires a 1–5 V voltage input.
4–20 mA calculator · Pressure unit converter (psig / psia / bar) · What is a pressure transducer?
What Is a 4-20mA Signal and Why Does It Matter?
The 4-20mA current loop is the dominant standard for industrial process measurement. Instead of transmitting a voltage (which degrades with cable length), the transmitter regulates the loop current in proportion to the measured pressure. Because current is the same everywhere in a series loop, a 100 m cable introduces no measurement error — only a voltage drop that reduces compliance headroom.
| Signal Type | Noise Immunity | Cable Length | Short Circuit Safe | Live Zero Fault Detection |
|---|---|---|---|---|
| 4-20 mA | Excellent | Up to 1,000 m | Yes (current-limited) | Yes — 0 mA = fault |
| 0-10 V | Moderate | Up to 50 m | No (requires series R) | No — 0 V is valid zero |
| 0-5 V | Moderate | Up to 30 m | No | No |
| HART (on 4-20mA) | Excellent | Up to 3,000 m | Yes | Yes + diagnostics |
The 4 mA live zero is the critical advantage: a genuine zero-pressure reading produces 4 mA, while a broken wire, dead transmitter, or shorted output produces 0 mA. Your PLC can detect the fault condition automatically — something a 0-10 V signal cannot do.
Not sure whether you need a continuous 4-20mA transmitter or a simple on/off device? See pressure switch vs pressure transmitter: what is the difference.
2-Wire Loop-Powered Wiring — Step by Step
A 2-wire transmitter draws its operating power from the same two wires that carry the 4-20mA signal. The transmitter, power supply, and PLC input are all in series. This is the simplest and most common configuration for process pressure transmitters.
- 1Verify transmitter polarity markings. 2-wire transmitters have a (+) terminal and a (−) terminal. The (+) is typically connected to the power supply positive; the output current exits from the (−) terminal.
- 2Connect the power supply (+) to the transmitter (+) terminal. Use shielded twisted-pair cable (0.5 mm² minimum). For runs over 100 m, use 0.75 mm² to limit resistive voltage drop.
- 3Connect the transmitter (−) terminal to the PLC analog input (+) terminal. The loop current flows through the PLC input.
- 4Connect the PLC analog input (−) or AGND terminal back to the power supply (−). This completes the series loop. All three devices — supply, transmitter, PLC — are in one continuous circuit.
- 5Check if your PLC AI card requires an external burden resistor. Many modern PLC AI cards have an internal 250 Ω resistor and accept 4-20mA directly. Older cards or voltage-mode cards require you to wire a 250 Ω resistor in parallel with the input to develop a 1-5 V signal. Check your PLC card manual.
- 6Ground the cable shield at the control panel end only. Leave the field end floating to prevent ground loops. Tape or insulate the field end of the shield.
- 7Power up and verify at 4 mA (0% pressure). Use a multimeter in series (current mode) or measure voltage across the 250 Ω burden: 1.00 V = 4 mA, 5.00 V = 20 mA. Apply known pressure and confirm proportional response.
| Step | Connection | Common mistake to avoid |
|---|---|---|
| 1 | Supply (+) → Transmitter (+) | Reversed polarity — transmitter outputs 0 mA and the PLC reports a wire break. |
| 2 | Transmitter (−) → PLC AI (+) | Landing on a voltage input with no 250 Ω burden — the loop stays open, no reading. |
| 3 | PLC AI (−) → Supply (−) | Return wire left off — the series loop is not closed, no current flows. |
| 4 | Cable shield → panel ground only | Shield grounded at both ends — ground loop, noisy or drifting value. |
| 5 | Verify 4 mA at 0 % (1.00 V across 250 Ω) | Skipping the zero check — a wrong range or polarity goes unnoticed. |
3-Wire Externally-Powered Wiring — Step by Step
A 3-wire transmitter has a dedicated power supply circuit (Brown = +Vcc, Blue = 0 V) separate from the current output wire (Black or Yellow). This allows the transmitter to draw more operating power without affecting the output signal, and is common in transmitters with displays or communication interfaces.
| Step | Connection | Common mistake to avoid |
|---|---|---|
| 1 | Supply +24 V → Transmitter V+ (Brown) | Brown and Blue swapped — transmitter does not power up. |
| 2 | Supply 0 V → Transmitter GND (Blue) and PLC AI (−) | Two different 0 V references — ground loop, reading drifts. |
| 3 | Transmitter OUT (Black) → PLC AI (+) | Output landed on a voltage input without a 250 Ω burden — no reading. |
| 4 | Cable shield → panel ground only | Shield grounded at both ends — ground loop and noise. |
- The transmitter GND (Blue) and PLC AI (−) reference must share the same 0 V potential.
- Connecting the transmitter to a field-mounted power supply and the PLC to a panel-mounted supply with different GND potentials creates a ground loop that corrupts the 4-20mA reading.
- Solution: use one isolated 24 V supply, or use a loop-isolating signal conditioner between the transmitter output and the PLC input.
Burden Resistance Calculation
Burden resistance (Rburden) is the total series resistance the transmitter must drive while still maintaining its output current. If the burden exceeds the transmitter's maximum, the output will saturate before reaching 20 mA at full-scale pressure.
Most industrial installations are well within limits with 24 V DC supply and a 250 Ω PLC input. Issues arise when daisy-chaining multiple devices (HART communicators, barriers, isolators) in the same loop — always sum all series resistances.
HART superimposes a ±0.5 mA digital signal on the 4-20mA current. The loop must contain at least 230 Ω (250 Ω is the usual PLC burden) or the digital signal is shorted out by the low-impedance supply. Connect the communicator or HART modem in parallel with the burden resistor — never in series — so that the analog reading is undisturbed while you read the process value, range and diagnostic flags.
PLC Signal Scaling: From Raw Counts to Engineering Units
Once the 4-20mA signal reaches the PLC analog input card, it must be scaled to engineering units (bar, PSI, kPa, etc.). With a 250 Ω burden resistor the voltage presented to the AI card is 1–5 V (4 mA × 250 Ω = 1 V; 20 mA × 250 Ω = 5 V).
| Loop Current | Voltage at 250 Ω | % of Range | 14-bit Raw Count (0–16383) | Meaning |
|---|---|---|---|---|
| 0 mA | 0 V | — | 0 | Fault / broken wire |
| 4 mA | 1.00 V | 0% | 3277 | Range minimum (0 bar) |
| 12 mA | 3.00 V | 50% | 9830 | Range midpoint |
| 20 mA | 5.00 V | 100% | 16383 | Range maximum (full scale) |
| > 20.5 mA | > 5.125 V | >100% | >16647 | Over-range or fault |
4-20mA to Pressure Conversion Table (0–10 bar and 0–400 bar Examples)
To convert a measured loop current into pressure, use P = (I − 4 mA) / 16 mA × (Pmax − Pmin) + Pmin. The 16 mA span is fixed, so the only thing that changes between transmitters is the pressure range. The tables below are the two cases our customers ask about most: a low-pressure 0–10 bar transmitter (pneumatics, water, HVAC) and a 0–400 bar transmitter (hydraulic power units and presses) — the top of the SEGMENsensor range of 0–0.1 bar to 0–400 bar.
| Loop current | % of span | Voltage at 250 Ω | 0–10 bar transmitter | 0–400 bar transmitter |
|---|---|---|---|---|
| 4.0 mA | 0% | 1.00 V | 0 bar (0 psi) | 0 bar (0 psi) |
| 8.0 mA | 25% | 2.00 V | 2.5 bar (36.3 psi) | 100 bar (1,450 psi) |
| 12.0 mA | 50% | 3.00 V | 5.0 bar (72.5 psi) | 200 bar (2,901 psi) |
| 16.0 mA | 75% | 4.00 V | 7.5 bar (108.8 psi) | 300 bar (4,351 psi) |
| 20.0 mA | 100% | 5.00 V | 10 bar (145.0 psi) | 400 bar (5,801 psi) |
| Sensitivity | — | 0.25 V per mA | 1.6 mA per bar | 0.04 mA per bar |
The last row explains why range selection matters for wiring and PLC resolution. On a 0–400 bar transmitter, 1 bar is only 0.04 mA, so a ±0.25% FS accuracy class equals ±1 bar (±0.04 mA) and the ±0.1% FS high-accuracy class equals ±0.4 bar. On a 0–10 bar transmitter the same ±0.25% FS is ±0.025 bar. Choose the smallest range that still covers the maximum working pressure, and make sure the PLC input card resolves well below the accuracy band — a 14-bit card gives roughly 13,100 counts over the 4–20 mA span. Bar-to-psi factors follow NIST SP 811, Appendix B (1 bar = 14.5038 psi).
Wiring to Active vs Passive PLC Analog Inputs
Most "no signal" calls after commissioning are not transmitter faults: the loop was wired for the wrong type of PLC analog input. Check the input card manual for whether the channel supplies loop power (active, sometimes labelled "2-wire transmitter" mode) or only measures current (passive, "4-wire transmitter" mode), then wire as follows.
| Transmitter | PLC input type | How to wire | Typical mistake |
|---|---|---|---|
| 2-wire (loop-powered) | Passive (measure only) | External 24 V DC (+) → transmitter (+); transmitter (−) → AI (+); AI (−) → 24 V (0 V). This is Fig. 1. | No external supply in the loop — reads 0 mA. |
| 2-wire (loop-powered) | Active (card supplies loop power) | Card loop-supply terminal → transmitter (+); transmitter (−) → AI (+). No external supply. | Adding a second 24 V supply in series — output saturates or the channel's protection trips. |
| 3-wire | Passive (measure only) | 24 V → Brown (V+); 0 V → Blue and AI (−); Black (OUT) → AI (+). This is Fig. 2. | AI (−) returned to a different 0 V than the transmitter — ground loop, drifting reading. |
| 3-wire | Active | Use the card's passive / "4-wire" terminals for the signal and power the transmitter from the panel 24 V; do not use the card's loop-supply terminal for the output. | Output wired to the loop-supply terminal — reading pinned at full scale or 0 mA. |
| 4-wire (isolated output) | Passive | Power pair to PWR+ / PWR−; OUT+ → AI (+); OUT− → AI (−). This is Fig. 3. | Signal pair connected to an active channel — two sources fighting in one loop. |
Before the first power-up, measure the open-circuit voltage at the transmitter end of the cable: you should see the supply voltage (typically 24 V DC) across the (+) and (−) conductors, with the correct polarity. Then connect the transmitter and confirm 4.0 mA at atmospheric pressure. For choosing the transmitter itself — range, overpressure, process thread and output — see the 4-20mA pressure transmitter product page and the full pressure sensor range.
Wiring a new loop? Get a transmitter matched to your PLC input
Tell our engineers the pressure range, process connection, supply voltage and PLC input type (active or passive), and we will recommend a matching model and wiring. OEM volume supply · samples of 1–5 pcs ship in 5–7 days · production lead time confirmed with the quotation · reply within 24 hours.
2-Wire vs 3-Wire: Which Configuration to Choose?
Choose 2-Wire When:
- Simple pressure/level/flow measurement
- Cable runs are long (> 30 m) — fewer wires
- Intrinsically safe (ATEX/IECEx) installations — 2-wire IS barriers are simpler
- Existing 2-conductor cable infrastructure
- Transmitter power consumption < 25 mW at 4 mA
- No local display required
Choose 3-Wire When:
- Transmitter has a built-in LCD display (needs more power)
- HART or Modbus RTU communication required
- High-power signal conditioning electronics
- Transmitter datasheet specifies external power supply
- Multiple output signals (4-20mA + switching output)
- Existing 3-conductor cable infrastructure
A third option, the 4-wire transmitter, keeps the power supply pair and the 4-20mA signal pair fully separate. It is used when the output must be galvanically isolated from the supply — for example when the transmitter and the PLC sit on different ground potentials. All three arrangements are compared side by side in 2-wire vs 3-wire vs 4-wire transmitters.
Wiring is only half the job — the transmitter's pressure range, accuracy class (±0.25% FS standard or ±0.1% FS high-accuracy), process connection and wetted materials must also match the application. Our 4-20 mA pressure transmitter selection guide covers range selection, over-pressure margin, accuracy and OEM ordering options step by step.
| Step | Connection | Common mistake to avoid |
|---|---|---|
| 1 | Supply (+) → PWR+, Supply (−) → PWR− | Power wired into the signal terminals — no output, possible damage. |
| 2 | OUT+ → PLC AI (+) | A power wire mixed into the signal pair — offset or noisy reading. |
| 3 | OUT− → PLC AI (−) | Signal return left open — no current, PLC reads 0 mA. |
| 4 | Confirm the PLC input has (or add) a 250 Ω burden | Missing burden resistor — open loop, no reading. |
| 5 | Cable shield → panel ground only | Shield grounded at both ends — ground loop. |
What is the difference between a 2-wire, 3-wire and 4-wire 4-20 mA wiring diagram?
The difference is how many conductors carry power and how many carry the signal. A 2-wire loop uses one pair for both power and the 4–20 mA signal; a 3-wire transmitter adds a separate supply conductor and shares 0 V with the signal; a 4-wire transmitter keeps a power pair and a signal pair fully separate. The table compares the three diagrams side by side.
| Item | 2-wire (loop-powered) | 3-wire | 4-wire |
|---|---|---|---|
| Conductors at the transmitter | 2: loop (+) and loop (−) | 3: V+ (Brown), 0 V (Blue), OUT (Black) | 4: PWR+, PWR−, OUT+, OUT− |
| Where the power comes from | The 4–20 mA loop itself (24 V DC supply or an active PLC input) | Separate 24 V DC supply | Separate supply, often 24 V DC |
| Signal reference | Series loop: supply (+) → transmitter → PLC input → supply (−) | OUT → PLC AI (+); AI (−) returns to the shared 0 V | OUT+ → PLC AI (+), OUT− → PLC AI (−); isolated from power |
| Burden check | Total loop resistance ≤ (Vsupply − Vmin) / 0.020 A, e.g. ≤ 600 Ω at 24 V with a 12 V minimum | Output compliance from the datasheet; supply wiring does not add to the signal burden | Output compliance from the datasheet; signal pair is separate |
| Typical use | Pressure and level transmitters, long cable runs, intrinsically safe loops | Transmitters with displays or extra electronics, sensors on machine harnesses | Outputs that must be galvanically isolated, different ground potentials |
| Most common wiring mistake | Adding a second supply in series with an active PLC input | AI (−) not tied to the transmitter 0 V — signal floats | Power wired into the signal terminals |
The 4–20 mA analog current signal for process instruments is defined in IEC 60381-1. Figures 1–3 above show each diagram with terminal labels; SEGMENsensor pressure transmitters are supplied in the standard 2-wire 4–20 mA version, and the wire colours and terminal assignment for each model are printed on the pressure transmitter datasheet (PDF).
How long can a 4-20 mA cable run be? A worked loop-resistance example
A 4–20 mA cable can run until the total loop resistance reaches the limit set by the supply voltage. The current itself does not weaken with distance; the cable only adds resistance. The worked example below uses a 24 V DC supply, a transmitter that needs 12 V at its terminals and a PLC analog input with a 250 Ω burden.
- Maximum loop resistance: Rmax = (24 V − 12 V) / 0.020 A = 600 Ω.
- Subtract the PLC input: 600 Ω − 250 Ω = 350 Ω left for cable and other devices.
- Subtract any barrier or isolator: with an intrinsically safe barrier of 280 Ω (example value — take yours from the barrier datasheet), 70 Ω is left for the cable.
- Divide by the cable resistance: copper at 20 °C has 0.0172 Ω·mm²/m, so a 0.5 mm² pair adds about 6.9 Ω per 100 m of run (both conductors). The 4–20 mA loop resistance calculator does this for any wire size, in mm² or AWG.
| Conductor size | Loop resistance per 100 m of run (both conductors, 20 °C) | Theoretical max. run, 350 Ω budget (no barrier) | Theoretical max. run, 70 Ω budget (280 Ω barrier) |
|---|---|---|---|
| 0.25 mm² (≈ 24 AWG) | ≈ 13.8 Ω | ≈ 2,500 m | ≈ 500 m |
| 0.5 mm² (≈ 20 AWG) | ≈ 6.9 Ω | ≈ 5,000 m | ≈ 1,000 m |
These are theoretical limits. Copper resistance rises by about 0.4 % per °C, and long runs pick up more electrical noise, so leave a margin of at least 20 % and keep the shield grounded at the panel end only. To check your own loop, enter the supply voltage, burden and cable data in the 4-20 mA loop calculator.
Send us your supply voltage, PLC input type, barrier model and cable length through the contact form. A SEGMENsensor engineer replies within 24 hours with a matched 4-20 mA pressure transmitter — OEM volume supply, samples in 5–7 days.
How do you test a 4-20 mA pressure transmitter with a multimeter?
Measure three things in order: the supply voltage at the transmitter terminals, the loop current in series with the transmitter, and the current at known test pressures. A healthy transmitter reads 4.00 mA at zero pressure, 12.00 mA at half scale and 20.00 mA at full scale, while its terminal voltage stays above the datasheet minimum at 20 mA. For a full 5-point check against a reference gauge, see how to calibrate a pressure transducer.
What reading should you expect at each test point?
| Step | Meter setting and connection | Expected reading | If the reading is wrong |
|---|---|---|---|
| 1. Supply at source | DC V across the power supply output, loop connected | Nominal supply, e.g. 24 V DC | Supply, fuse or breaker problem — fix before testing the transmitter |
| 2. Terminal voltage (A) | DC V across transmitter (+) and (−) | At or above the datasheet minimum (12 V in the examples on this page), also at 20 mA | Too much loop resistance or a weak supply — see the burden calculation above |
| 3. Loop current (B) | DC mA, meter in series: open the loop at one terminal and close it through the meter leads | 4.00 mA at zero (vented) pressure | 0 mA = open loop or reversed polarity; below 3.6 mA or above 21 mA = transmitter fault band |
| 4. Non-intrusive check (C) | DC V across the 250 Ω input resistor | 1.00 V at 4 mA, 5.00 V at 20 mA (I = V ÷ 250 Ω) | Correct mA at B but wrong volts at C = resistor value or PLC input type is wrong |
| 5. Span check | Apply 0 %, 50 % and 100 % of range with a hand pump and a reference gauge | 4.00 / 12.00 / 20.00 mA (I = 4 + 16 × P ÷ FS) | Error in % of span = (Imeasured − Iexpected) ÷ 16 mA × 100 |
| 6. Compare with the PLC | Read the PLC value while the meter shows a known current | PLC value matches the meter within its own input accuracy | Meter right, PLC wrong = scaling, input range or channel configuration error |
To judge step 5, convert the accuracy class into current: 16 mA of span × 0.25 % = ±0.04 mA for a ±0.25% FS transmitter, and 16 mA × 0.1 % = ±0.016 mA for the ±0.1% FS high-accuracy class (calculated values). A 4½-digit meter reading 0.001 mA is needed to see the high-accuracy class clearly; a 3½-digit meter is enough for the ±0.25% FS class. The 4–20 mA calculator gives the expected current for any pressure in your range, and accuracy in % FS vs % of reading explains the error budget.
What does each wrong reading mean?
| What the meter shows | Most likely cause | What to do |
|---|---|---|
| 0 mA and 0 V at the transmitter terminals | Open loop upstream: broken wire, open fuse, loose terminal | Walk the loop with the voltmeter from the supply toward the transmitter until the voltage disappears |
| 0 mA but full supply voltage at the terminals | Reversed polarity or the open point is downstream of the transmitter | Check (+)/(−) at the transmitter, then continuity to the PLC input and back to 0 V |
| Current rises with pressure but stops before 20 mA | Terminal voltage falls below the minimum at high current | Measure step 2 at full pressure; reduce loop resistance or raise the supply |
| Zero reads above 4.00 mA with the port vented | Pressure trapped in the impulse line, blocked vent on a gauge model, or mounting stress on low ranges | Vent the line, check the reference vent, then re-zero only if the model provides zero adjustment |
| Reading jumps by tenths of a mA | Pressure pulsation, or a ground loop from a shield earthed at both ends | Earth the shield at the panel end only; damp pulsation with a pressure snubber |
| Steady 21–22 mA or below 3.6 mA | Transmitter reporting over-range or an internal fault | Check process pressure against the range, then read diagnostics with a HART communicator if the model supports HART |
Tell us the range, output, supply voltage and process connection through the contact form. A SEGMENsensor engineer replies within 24 hours; qualification samples of 1–5 pcs ship in 5–7 days for OEM project validation, with production planned around your annual volume. The pressure transmitter datasheet (PDF) lists every range and wiring option.
Common Wiring Faults & How to Diagnose Them
These are the four most common 4-20mA installation problems, with root causes and systematic test procedures.
| Symptom | Likely Cause | Diagnostic Step | Fix |
|---|---|---|---|
| 0 mA / no signal at PLC | Broken wire, blown fuse, reversed polarity, transmitter not powered | Measure voltage at transmitter terminals: should be 12–36 V. Measure current with clamp meter in the loop. | Restore wire continuity; check fuse; verify polarity (+/−) |
| Signal stuck at 3.6–4 mA, no response to pressure | Process tap blocked, transmitter electronics fault, isolation valve closed | Apply known pressure at transmitter (use hand pump). Check process connection is open. | Open isolation valve; clean impulse line; replace transmitter if electronics failed |
| Noisy/fluctuating reading in PLC | Ground loop; cable runs near VFDs/motors; shield not grounded correctly | Temporarily shorten cable; measure signal quality near transmitter vs. at panel. Check if noise correlates with motor operation. | Ground shield at panel end only; route cable away from power cables; add galvanic isolator |
| Reading offset — always high or low by fixed amount | Incorrect PLC scaling; wrong burden resistance; transmitter not at true zero | Measure actual current at 0% pressure. Compare with expected 4 mA. Check scaling parameters in PLC AI block. | Re-zero transmitter; correct PLC scaling constants; verify burden resistance |
| Output saturates at 20 mA below full-scale pressure | Burden resistance exceeds transmitter maximum compliance | Calculate total loop resistance (all series devices). Compare with transmitter datasheet maximum burden at 24 V. | Increase supply voltage (to 36 V if transmitter supports it); reduce burden (remove isolators, use lower-resistance AI card) |
| Reversed polarity (+ and − swapped) | Supply (+) landed on the transmitter (−) terminal, or the cable pair colours were assumed rather than checked | Measure voltage at the transmitter terminals with the meter polarity as marked: a negative reading confirms the swap. Most 2-wire transmitters have reverse-polarity protection and simply output 0 mA. | Swap the two conductors at the transmitter; label + and − on both ends; re-check 4 mA at zero pressure |
| Reading constant at 21–22 mA (or 3.6 mA) regardless of pressure | Transmitter is signalling an internal fault or over-range using the NAMUR NE 43 fault band (< 3.6 mA or > 21 mA) | Vent the process connection to atmosphere: a healthy transmitter returns to 4 mA. If it stays in the fault band, read the diagnostic code via HART or check for a ruptured diaphragm / over-pressure event. | Confirm the applied pressure is inside the calibrated range; re-zero; replace the transmitter if the diaphragm or electronics are damaged |
- To measure loop current without breaking the circuit, use a clamp meter around one conductor of the twisted pair. Most industrial clamp meters accurately measure DC currents in the 4-20mA range.
- Alternatively, measure voltage across a known burden resistor: V = I × R. At 250 Ω: 1.000 V = 4 mA, 5.000 V = 20 mA. This is non-invasive and very precise.
- A HART communicator connected across the loop (minimum 230 Ω in the loop) can read the process value digitally and show diagnostic flags, often revealing the fault source without any disassembly.
Which SEGMENsensor Pressure Transmitter Fits Each Wiring Case?
Every standard SEGMENsensor 4–20 mA pressure transmitter is a 2-wire, loop-powered device, so the 2-wire diagram in Fig. 1 applies to all three series below. Choose the series by pressure range and process medium; choose a switch instead when the PLC only needs an on/off signal. Ranges, outputs and ratings are taken from the 4–20 mA pressure transmitter page, the pressure sensor overview and the datasheet; the exact option set for one part is confirmed in the ordering code.
| Wiring case in this guide | SEGMENsensor model | Range | Output and wiring | Key rating | Product page |
|---|---|---|---|---|---|
| 2-wire loop to a PLC analog input on compressed air, pneumatics, water treatment or HVAC | PT-C series (ceramic sensing element) | 0–0.1 bar to 0–40 bar | 4–20 mA 2-wire loop-powered; M12×1 4-pin connector, G1/4" male port | ±0.25% FS (±0.1% FS high-accuracy version); max 600 Ω loop resistance at 24 V | 4–20 mA transmitter |
| 2-wire loop on hydraulic systems, presses or injection moulding machines | PT-P series (316L piezoresistive diaphragm) | 0–60 bar to 0–400 bar | 4–20 mA 2-wire loop-powered; G1/2" or high-pressure fitting options | Burst pressure 3× full scale (standard ranges); vibration 10 g, 10–2000 Hz | 4–20 mA transmitter |
| Loop on acids, alkalis, food-grade or high-temperature steam lines | PT-S series (special media, flush diaphragm) | Specified per medium at order | 4–20 mA 2-wire loop-powered; hygienic tri-clamp or flush-diaphragm connection | PVDF or Hastelloy C diaphragm; wetted material chosen for the process medium | 4–20 mA transmitter |
| PLC with a 0–10 V voltage input, or an RS485 bus instead of an analog card | 4–20 mA transmitter with 0–10 V or RS485 output option | 0–0.1 bar to 0–400 bar | 0–10 V or RS485; terminal assignment confirmed with the ordering code | ±0.25% FS (±0.1% FS option); G1/4, G1/2, 1/4 NPT, 1/2 NPT | Pressure sensors |
| PLC digital input or pump/compressor control that only needs a set point | SYK102 mechanical pressure switch | 0.5–8.5 bar (adjustable) | SPDT relay contact, 16 A / 250 V AC | Set point ±2%; IP65 (IP67 option) | Pressure sensors |
| Vacuum on/off signal to a PLC input | Vacuum pressure switch series | −1 to 0 bar | NPN / PNP / relay / IO-Link; 10–30 V DC | ±2% FS; IP67 | Pressure sensors |
Send these details for a quote: pressure range and type (gauge or absolute), process medium and temperature, process connection, output (4–20 mA 2-wire, 0–10 V or RS485), PLC input type and supply voltage, electrical connection and cable length, application and annual volume. Samples are 1–5 pcs in 5–7 working days; production lead time and MOQ are confirmed with the quotation.
Download the one-page pressure transmitter datasheet (PDF)
The datasheet lists all ranges from 0–0.1 bar to 0–400 bar, both accuracy classes (±0.25% FS standard, ±0.1% FS high-accuracy), 2-wire 4-20mA and 0–10 V outputs, supply voltage and maximum burden, process connections, wetted materials, temperature limits, IP rating, dimensions and the ordering code. No registration required.
Wiring a pressure transmitter into a battery-storage or data center cooling loop? The coolant loop alarm point and inspection checklist shows where supply and return pressure transmitters sit next to the reservoir level alarms.
Still choosing range, output and accuracy? Use the 4-20 mA pressure transmitter selection guide. Sourcing for an Australian site? See the pressure transducer manufacturer guide for Australia.
Frequently Asked Questions
4-20mA is a current loop standard where the sensor output is represented as a current between 4 mA (0% of range) and 20 mA (100% of range). Current is used instead of voltage because it is unaffected by cable resistance — only a voltage drop occurs, which does not alter the current measurement. The 4 mA live-zero means a dead transmitter or broken wire produces 0 mA, distinguishable from a valid zero-pressure reading.
A 2-wire transmitter draws its power from the same loop as the signal — supply, transmitter, and load are in series, and only two wires are needed. A 3-wire transmitter has a dedicated power supply pair (Brown = +Vcc, Blue = 0V) and a separate output wire (Black) for the current signal. 2-wire is simpler and dominant in process instruments; 3-wire is used when the transmitter needs more power than a 4-20mA loop can supply.
Maximum burden: R_max = (V_supply − V_transmitter_min) / I_max. With a 24 V supply and 12 V transmitter minimum: R_max = (24 − 12) / 0.020 = 600 Ω. Add up all series resistances: PLC AI card (~250 Ω), cable (~7 Ω per 100 m for 0.5 mm²), barriers, isolators. Total must stay below R_max. Exceeding it causes the output to saturate before reaching 20 mA.
Most pressure transmitters require 12–36 V DC; the industrial standard is 24 V DC regulated. Minimum supply voltage must satisfy: V_supply ≥ V_transmitter_min + (I_max × R_burden). At 20 mA with 250 Ω PLC input (5 V drop) and 12 V transmitter minimum: V_min = 12 + 5 = 17 V. A 24 V supply provides comfortable margin. Use a galvanically isolated, regulated power supply to avoid ground loops.
With a 250 Ω burden, 4 mA = 1 V, 20 mA = 5 V. Formula: Eng_value = (raw − count_at_4mA) / (count_at_20mA − count_at_4mA) × (range_max − range_min) + range_min. For a 14-bit card: count_at_4mA = 3277, count_at_20mA = 16383. Siemens users: FC105 SCALE block. Rockwell users: SCL instruction. Most PLCs provide built-in scaling blocks — check your programming manual for the analog input function block.
0 mA indicates: broken loop wire, reversed polarity, blown fuse, or transmitter with no power. Check: (1) verify supply voltage at transmitter terminals (12–36 V DC); (2) measure loop current with a clamp meter; (3) check polarity — (+) terminal to supply (+). A signal stuck at 3.6–4 mA with no pressure response indicates the process connection is blocked or the transmitter electronics have failed.
Ground loops occur when transmitter and PLC have different earth potentials, causing circulating current that corrupts the 4-20mA signal. Prevention: (1) use a single isolated 24 V DC supply per loop or per instrument group; (2) use PLC AI cards with galvanic isolation; (3) ground cable shields at the panel end only — leave field end unconnected; (4) if ground loops persist, insert a loop-isolating signal conditioner between transmitter output and PLC AI.
Yes, provided the supply has enough current capacity (typically 500 mA to 2 A) and each transmitter has its own return wire to the supply. Do not share a common return between transmitters unless the PLC analog input card explicitly supports it. Treat every loop independently: each transmitter gets its own + and − conductors in the multi-core cable and its own AI channel on the PLC.
Use shielded twisted-pair (STP) cable, 0.5 mm² (20 AWG) minimum conductors. In environments near VFDs or motors, shielded cable is mandatory. Ground the shield at the control panel end only. For cable runs over 200 m, check that loop resistance (both conductors round-trip) stays within your burden budget. Maintain minimum 150 mm physical separation from 230 V AC power cables, run in separate conduit or tray.
Use P = (I − 4 mA) / 16 mA × (Pmax − Pmin) + Pmin. For a 0–10 bar transmitter, 12 mA = 5 bar and 8 mA = 2.5 bar; for a 0–400 bar transmitter, 12 mA = 200 bar and each bar equals 0.04 mA. With a 250 Ω burden, the same values read as 1–5 V at the PLC input.
Connect +24 V DC to the Brown (V+) wire and 0 V to the Blue wire, run the Black output wire to the PLC analog input (+), and return the analog input (−) to the same 0 V rail as the transmitter. On a card that supplies loop power, use its passive or 4-wire terminals for the signal, not the loop-supply terminal.
Yes. If the analog input channel is active (it supplies 24 V to the loop), wire the card's loop-supply terminal to the transmitter (+) and the transmitter (−) to the input (+), with no external power supply. Adding a second supply in series is a common cause of saturated or tripped channels.
It shows which terminals carry supply power and which carry the 4–20 mA signal, and how they connect to the power supply and the PLC analog input. A 2-wire diagram has one series loop (supply +, transmitter, PLC input, supply −); a 3-wire diagram adds a separate supply wire and shares 0 V; a 4-wire diagram shows two independent pairs, one for power and one for the signal.
Distance is limited by loop resistance, not by the signal itself. With a 24 V DC supply, a 12 V transmitter minimum and a 250 Ω PLC input, about 350 Ω is left for cable and barriers. A 0.5 mm² copper pair adds about 6.9 Ω per 100 m of run (both conductors), so the theoretical limit is roughly 5 km without a barrier and about 1 km with a 280 Ω barrier in the loop.
Not in general. A 2-wire transmitter needs the fewest conductors and is the default for pressure measurement. A 4-wire transmitter is chosen when the electronics need more power than the loop can supply, or when the 4–20 mA output must be galvanically isolated from the supply. Choose the wiring the transmitter datasheet specifies and the PLC input card supports.
Set the meter to DC mA and connect it in series with the loop, then vent the pressure port: a healthy transmitter reads 4.00 mA. Apply known pressures with a hand pump and reference gauge and check for 12.00 mA at half scale and 20.00 mA at full scale. Also measure the DC voltage across the transmitter terminals at 20 mA; it must stay at or above the datasheet minimum.
4.00 mA with the process port vented to atmosphere. For a ±0.25% FS transmitter the allowed error is about ±0.04 mA of the 16 mA span (calculated value), so roughly 3.96–4.04 mA. A reading of 0 mA means an open loop or reversed polarity, and a steady reading below 3.6 mA or above 21 mA usually means the transmitter is signalling a fault.
Yes. Measure the DC voltage across the PLC input resistor and divide by its value: with 250 Ω, 1.00 V equals 4 mA and 5.00 V equals 20 mA. A milliamp clamp meter also reads loop current without opening the circuit. Breaking the loop with the meter in series is the most direct method when the resistor value is unknown.
For a 2-wire 4–20 mA loop up to 40 bar on air, water or HVAC, order the PT-C ceramic series; for hydraulics from 60 to 400 bar, the PT-P 316L piezoresistive series; for aggressive, food-grade or hot media, the PT-S flush-diaphragm series. All are rated ±0.25% FS (±0.1% FS option) and allow up to 600 Ω loop resistance at 24 V. For a quote, send range, medium, temperature, process connection, output, PLC input type and annual volume.
Last updated: October 29, 2026 — added a model-fit table that maps each wiring case to a SEGMENsensor pressure transmitter or switch series, the details needed for a quote and one FAQ. Previous update October 16, 2026: added a multimeter test procedure with expected readings at each test point, a reading–cause–action table and three FAQs. Previous update October 6, 2026: added a 2-wire vs 3-wire vs 4-wire wiring diagram comparison table, a worked cable-length and loop-resistance example and three FAQs. Previous update September 29, 2026: added 4-20mA-to-pressure conversion table (0–10 bar and 0–400 bar), active vs passive PLC input wiring table, three FAQs and an in-article quote section. Previous update September 22, 2026: HART loop diagram, fault-band troubleshooting rows and datasheet download.