A 4–20 mA pressure device with a 0–6 bar range provides a continuous pressure signal, not a discrete pump command. Scale the analog input once, apply separate pump-on and pump-off thresholds, and investigate signal instability before enabling any averaging or filtering.
Scale the 4–20 mA Signal to Pressure
Configure the analog amplifier for a 4–20 mA sensor, a measurement minimum of 0, and a maximum of 6000 when the displayed value uses three decimal places. In that representation, 6000 corresponds to 6.000 bar. The analog amplifier then calculates its gain and offset automatically.
| Input current | Engineering value | Three-decimal representation |
|---|---|---|
| 4 mA | 0 bar | 0 |
| 20 mA | 6 bar | 6000 |
For a measured loop current I, the supported linear conversion is:
Pressure_bar = (I_mA - 4 mA) × (6 bar / 16 mA)Configure Low-Pressure Pump Control
Use separate adjustable setpoints: start the secondary pump when pressure drops to the pump-on threshold, then stop it only after pressure rises to the pump-off threshold. Keeping these setpoints separate prevents a fluctuating value near one threshold from repeatedly changing the command.
- Pass the analog input through the analog amplifier configured for 4–20 mA and 0–6000.
- Feed the already-scaled value into the analog threshold trigger.
- Set the threshold trigger gain to 1 and offset to 0; scaling it again would alter the engineering value.
- Enter the required pump-on and pump-off setpoints through the display.
The evidence does not establish the origin of a 0–1000 threshold-trigger range. Do not assume that range matches the scaled pressure value. Confirm that the threshold block receives the same 0–6000 representation, or reconcile its range before commissioning.
Diagnose and Smooth an Unstable Signal
First determine whether the restless value reflects real pressure changes or a measurement error. If the behavior is erroneous, correct the measurement problem instead of masking it on the HMI. If the movement is valid but the display needs smoothing, process the scaled signal with an Average Value or Analog Filter block.
Apply smoothing deliberately because it changes how quickly the displayed or controlled value responds. The evidence supplies no filter setting or averaging interval, so select those values from observed process behavior and verify the result rather than assigning an unsupported default.
Verify Scaling and Switching
Confirm the low and high endpoints first: 4 mA must produce 0, and 20 mA must produce 6000. Then move the pressure below the configured pump-on setpoint and verify that the secondary-pump command turns on. Raise the pressure through the pump-off setpoint and verify that the command turns off. Finally, observe the scaled value around both thresholds to confirm that signal movement does not cause unwanted switching.
FAQ
How do I scale a 4–20 mA pressure signal for 0–6 bar?
Configure the analog amplifier for 4–20 mA with a minimum of 0 and a maximum of 6000 when using three decimal places. This maps 4 mA to 0 bar and 20 mA to 6.000 bar.
What gain and offset should the analog threshold trigger use?
Use gain 1 and offset 0 when the analog amplifier has already scaled the input. This passes the 0–6000 engineering value into the threshold logic without scaling it again.
How can I stop a 4–20 mA pressure value from jumping on the HMI?
First determine whether the movement is a real process change or a measurement error. If it is valid movement, use an Average Value or Analog Filter block and verify that the resulting response remains suitable for pump switching.
