Overview
A two-component extruder sealant feed line currently uses electrocontact (pressure switch) manometers for pressure supervision. The migration target is a compact PLC (Owen PR200 family) with analog inputs, reading a continuous 4-20 mA pressure transmitter with an upper range limit of 40 MPa (400 bar). This changes the control architecture from discrete threshold contacts to a continuous measured value, so thresholds, alarms and trending move from mechanical contacts into program logic.
Two engineering problems must be solved in order:
- Select a transmitter whose process connection, wetted material, overpressure rating and output type survive a high-viscosity sealant service at 400 bar.
- Convert the 4-20 mA loop current into engineering units (MPa or bar) inside the PLC, then drive display, alarms and interlocks from that value.
40 MPa (megapascal) = 400 bar = 4000 N/cm². Confirm the unit prefix on every datasheet and every PLC scaling constant; a factor-of-10⁹ typo in the scale block produces a plausible-looking but meaningless display.Unit and Range Reference
| Quantity | Value | Note |
|---|---|---|
| Upper range limit (URL) | 40 MPa | Sensor "ВПИ" / span end |
| Same in bar | 400 bar | 1 MPa = 10 bar |
| Same in kgf/cm² (approx.) | ≈ 408 | 1 bar ≈ 1.0197 kgf/cm² |
| Lower range limit | 0 MPa (gauge) | Specify gauge, not absolute, for hydraulic service |
| Output | 4-20 mA, 2-wire | Loop-powered from the same 24 VDC supply |
| Supply | 24 VDC | Matches a 24 VDC PLC variant; avoids a separate 230 VAC feed to the field device |
Sensor Selection Checklist
The field report stopped at "buy a transmitter with URL = 40 MPa". That is necessary but not sufficient. Confirm each item below against the manufacturer datasheet before ordering — do not assume a catalogue default:
| Parameter | What to verify | Why it matters here |
|---|---|---|
| Process connection thread | Exact thread form and size (e.g. G1/2, M20×1.5, 1/2" NPT) on the existing manometer port | Electrocontact manometers are often on a different thread than a transmitter's standard offering; an adapter adds a dead volume that packs with sealant |
| Overpressure / burst rating | Manufacturer-stated overload limit relative to 40 MPa | Extruder pump start-up and nozzle blockage produce pressure spikes above steady-state; select so the spike stays inside the rated overload, not merely inside the span |
| Diaphragm type | Flush (front-face) diaphragm vs. internal cavity | Viscous sealant plugs a recessed cavity and freezes the reading; a flush diaphragm is normally required for polymer/sealant service |
| Wetted material | Stainless steel grade / seal elastomer compatibility with the specific sealant and any purge solvent | Chemical attack on the diaphragm is a silent drift failure |
| Process temperature | Medium temperature at the tap vs. sensor rating, plus temperature error coefficient | Heated sealant lines can exceed a standard transmitter's electronics limit; a cooling element or capillary seal may be needed |
| Accuracy / total error | % of span, and whether it includes non-linearity, hysteresis, repeatability | 0.5 % of 40 MPa is ±0.2 MPa (±2 bar) — verify that this resolution supports the process tolerance |
| Response time | Sensor time constant + PLC scan + any software filter | Determines whether a blockage spike is captured or averaged away |
| Enclosure / vibration | IP rating, connector type (DIN 43650, M12), cable gland | Extruder frames vibrate; a loose 4-20 mA connector reads as an intermittent 0 mA fault |
Wiring the 4-20 mA Loop
A 2-wire transmitter is powered by the loop. With a 24 VDC PLC, the same supply typically feeds both. Basic loop:
+24 VDC ----> Transmitter (+)
Transmitter (-) ----> PLC analog input (+) / I-input terminal
PLC analog input common ----> 0 V of the 24 VDC supplyPoints to check:
- Confirm from the PLC manual whether the analog input terminal is the current sink (input measures current returning to 0 V) or requires an external sense arrangement, and whether the input must be jumpered/configured for current rather than voltage.
- Verify the loop voltage budget: available supply (24 VDC nominal) minus the transmitter's minimum operating voltage must exceed the drop across the PLC input impedance plus cable resistance at 20 mA.
- Use shielded twisted pair for the loop and ground the shield at one end only — the panel side. Route it away from the extruder drive and any heater power cabling.
- Keep the transmitter case bonded to the machine ground; a floating case near a VFD is a noise injector.
Configuring the Analog Input and Scaling
On PR200 modifications with analog inputs, the inputs are configured by default for 4-20 mA current signals, so no hardware jumper change is typically needed — but confirm the input mode in the project configuration screen of Owen Logic before writing logic.
The conversion is a straight linear map. Working directly in current:
P [MPa] = (I_mA - 4.0) / (20.0 - 4.0) * 40.0
P [bar] = (I_mA - 4.0) / 16.0 * 400.0| Loop current | Pressure (MPa) | Pressure (bar) | Interpretation |
|---|---|---|---|
| < 3.6 mA | — | — | Break / sensor fault (typical NAMUR-style low limit — confirm the transmitter's declared fault current) |
| 4.0 mA | 0.0 | 0 | Zero |
| 8.0 mA | 10.0 | 100 | 25 % of span |
| 12.0 mA | 20.0 | 200 | 50 % of span |
| 16.0 mA | 30.0 | 300 | 75 % of span |
| 20.0 mA | 40.0 | 400 | Full scale |
| > 21 mA | — | — | Over-range / short — confirm the transmitter's upper saturation value |
Using the Scale macro in Owen Logic
- Install Owen Logic and create a project for the exact PR200 modification and supply voltage (24 VDC variant) you ordered. The device model in the project must match the hardware or the input list will not correspond.
- In the device configuration, verify the analog input is set to the 4-20 mA current type and note the variable name assigned to it.
- Place the
Scalemacro on the circuit sheet. Wire the analog input variable to its input. - Set the macro's input range to the raw span produced by the analog input for 4-20 mA, and the output range to
0 ... 40for MPa or0 ... 400for bar. Check in the macro's help/description whether it expects the input in mA, in percent, or in raw ADC counts — the constants you enter depend on that, and this is the single most common configuration error. - Assign the macro output to a REAL variable, e.g.
Pressure_MPa. - Drive the display from that variable with an explicit unit label and a fixed number of decimals (0.1 MPa / 1 bar resolution is usually adequate).
Recreating the electrocontact manometer function
The old manometers gave two discrete contacts. Reproduce them in logic with comparators plus hysteresis so the outputs do not chatter around the setpoint:
HighAlarm := (Pressure_MPa >= HI_SP) ; reset when Pressure_MPa < (HI_SP - HYST)
LowAlarm := (Pressure_MPa <= LO_SP) ; reset when Pressure_MPa > (LO_SP + HYST)
SensorFault := (I_mA < 3.6) OR (I_mA > 21.0)Add a start-up inhibit timer on the low alarm so the pump can build pressure without tripping, and route SensorFault into the same interlock chain as the high alarm — a broken loop must not read as "zero pressure, keep pumping".
Verification and Commissioning
-
Offline simulation. Owen Logic includes an emulation mode. Force the analog input variable across its range and confirm
Pressure_MPafollows the table above. This validates the scale constants before the hardware exists. Note that display behaviour is not fully reproduced in emulation — verify screens on the physical device. - Loop injection. With the transmitter disconnected, inject 4, 8, 12, 16 and 20 mA from a loop calibrator into the PLC input. Record the displayed value at each point and confirm the error is within the analog input's declared accuracy. Any offset here is a PLC-side scaling or wiring problem, not a sensor problem.
- Zero check. With the line depressurized and vented to atmosphere, the transmitter should read approximately 4 mA / 0 MPa. A standing offset indicates a plugged diaphragm, trapped sealant, or a shifted sensor zero.
- Cross-check against a reference. Compare the PLC reading with a calibrated test gauge on the same manifold at two or three operating pressures.
- Noise check. Run the extruder and drives at full load and watch for reading jitter. If present, verify shield grounding and cable routing before adding a software filter — filtering hides the symptom and slows the trip response.
-
Alarm proving. Temporarily lower the high setpoint into the normal operating band and confirm the interlock acts, then restore it. Separately, open one loop conductor and confirm
SensorFaulttrips the interlock.
Common Configuration Errors
| Symptom | Likely cause | Fix |
|---|---|---|
| Reading fixed at 0 or full scale | Input configured for voltage instead of current, or Scale input range set in the wrong units | Recheck input mode and the units the Scale macro expects |
| Display reads 25 % high at zero pressure | Scaling written as 0-20 mA instead of 4-20 mA | Set the input low limit to the 4 mA point, not 0 |
| Value off by exactly 10× | MPa/bar mix-up in the output range | 1 MPa = 10 bar; pick one unit and label it everywhere |
| Reading dead / never changes with process | Sealant packed into a recessed process connection or adapter | Move to a flush-diaphragm transmitter mounted without adapters |
| Intermittent zero reading | Loose field connector or broken shield under vibration | Re-terminate; add strain relief; enable the <3.6 mA fault detection |
| Sluggish response to blockage | Excessive software filter time constant | Reduce the filter and fix the noise source instead |
If the transmitter selection remains open — particularly thread form, diaphragm style and material compatibility with the specific sealant — supply the medium, temperature, thread and required overload margin to the manufacturer's technical support (Owen: [email protected]) and have them confirm the part number against those constraints in writing.
FAQ
How do I convert 4-20 mA to 0-400 bar in a PLC?
Use a linear map: P[bar] = (I_mA - 4) / 16 * 400. So 4 mA = 0 bar, 12 mA = 200 bar, 20 mA = 400 bar. In Owen Logic, the Scale macro does this — set the input range to the 4-20 mA span and the output range to 0-400.
Is 40 MPa the same as 400 bar?
Yes. 1 MPa = 10 bar, so 40 MPa = 400 bar ≈ 408 kgf/cm². Watch the prefix case: "mPa" is millipascal and is not a hydraulic pressure unit.
Do I need to change a jumper for a 4-20 mA input on a PR200?
On PR200 modifications with analog inputs, the inputs are configured for 4-20 mA current signals by default, so typically nothing needs changing. Confirm the input mode in the device configuration of your Owen Logic project before writing logic.
Why does my pressure reading stay frozen on a sealant extruder?
Viscous sealant packs into a recessed process connection or adapter and stops transmitting pressure to the diaphragm. Use a flush-diaphragm transmitter mounted directly on the process tap without adapters.
How do I detect a broken 4-20 mA sensor loop in software?
Flag a fault when the measured current drops below roughly 3.6 mA or exceeds about 21 mA, and route that flag into the same interlock as the high-pressure alarm. Confirm the exact fault-current limits in your transmitter's datasheet.
Can I test the scaling without the PLC hardware?
Yes. Owen Logic's emulation mode lets you force the analog input variable and verify the scaled output at 4, 8, 12, 16 and 20 mA equivalents. Display screens still need verification on the physical device.
