SOP: Functional Checking of Ratiometric Output Pressure Transmitter

1. Purpose

To define the standard procedure for checking and verifying the output of a ratiometric pressure transmitter by applying known pressure values and measuring the corresponding output voltage.

2. Scope

This procedure applies to pressure transmitters with ratiometric voltage output, where the output voltage is proportional to the supply voltage.

3. Principle of Operation (Ratiometric Output)

• The output voltage of the transmitter is a fixed percentage of the supply voltage.

• If the supply voltage changes, the output voltage also changes proportionally.

• Therefore:

  Knowing and measuring the supply voltage is mandatory before checking output accuracy.

4. Instruments & Tools Required

• Regulated DC power supply (as per transmitter specification)

• Digital multimeter (for voltage measurement)

• Pressure source / pressure calibrator (–1 to +5 bar)

• Test leads and suitable fittings

5. Preconditions

• Transmitter must be properly mounted or securely connected.

• Correct supply voltage must be applied as per datasheet.

• Pressure source must be accurate and leak-free.

• Ambient temperature conditions should be stable.

6. Test Setup

• Connect the supply voltage to the pressure transmitter.

• Connect the multimeter to the output terminals in voltage measurement mode.

• Connect the pressure source to the pressure port of the transmitter.

7. Test Procedure

Step 1: Power Supply Check

1. Switch ON the power supply.

2. Measure and record the actual supply voltage using a multimeter.

3. Ensure the supply voltage is within the specified range.

Step 2: Zero / Ambient Check

1. Keep the transmitter at ambient pressure condition (no applied pressure).

2. Measure and record the output voltage.

3. Verify that the output is reasonable as per the ratiometric range.

Step 3: Pressure Application Test

1. Apply the following pressures one by one:

   • –1 bar

   • 0 bar

   • 1 bar

   • 2 bar

   • 3 bar

   • 4 bar

   • 5 bar

2. At each pressure point:

   • Allow the signal to stabilize.

   • Measure and record the output voltage on the multimeter.

8. Expected Output (Ratiometric)

For a transmitter with range –1 to +5 bar, the output should be approximately:

9. Calculation Example

If supply voltage = 10.00 V

Then:

• At –1 bar → Output ≈ 1.00 V

• At 2 bar → Output ≈ 5.00 V

• At 5 bar → Output ≈ 9.00 V

10. Acceptance Criteria

• Output voltage should:

  • Follow the ratiometric percentage values

  • Be linear

  • Be stable at each pressure point

• Deviation should be within specified tolerance (as per datasheet).

11. Important Notes

• ⚠️ Always measure supply voltage first.

• ⚠️ Do NOT compare output to fixed voltage values unless supply is perfectly regulated.

• ⚠️ Output will change if supply voltage changes — this is normal behavior for ratiometric sensors.

12. Troubleshooting Hints

• If output is shifted → Check supply voltage first.

• If output is unstable → Check grounding and shielding.

• If output is non-linear → Sensor may be damaged or out of calibration.

13. Conclusion

This procedure ensures that the pressure transmitter is functioning correctly and producing accurate ratiometric output proportional to applied pressure and supply voltage.