An ultrasonic level transmitter’s 4–20 mA signal remains near 10 mA, then intermittently jumps to 20 mA. The evidence does not establish whether the fault originates in the transmitter, field wiring, electrical interference, input hardware, or sampling configuration. Isolate each part of the loop before changing multiple variables.
Define the failure boundary
Record the transmitter output and receiving input during an event. Determine whether the current physically changes from 10 mA to 20 mA or only the displayed or converted value changes. This distinction separates a field-loop problem from an input conversion or processing problem.
| Observation | Supported interpretation | Next check |
|---|---|---|
| Measured loop current jumps to 20 mA | The cause is upstream of, or within, the current loop | Inspect the sensor, cable routing, shielding, and nearby inverter-driven motors |
| Loop current stays near 10 mA while the converted value jumps | The receiver or its processing path is suspect | Inject a known 4–20 mA signal into the receiver |
| Replacement sensor removes the jump | The original sensor is suspect | Evaluate or replace the original sensor |
Check interference and grounding
Inspect the signal-cable route for nearby motors operated by an inverter. If such equipment is present, test greater cable separation and evaluate whether reducing the inverter carrier frequency changes the symptom. Treat interference as a hypothesis until the jump correlates with inverter or motor operation.
Inspect the analog shield and grounding arrangement. The evidence specifically recommends avoiding shield grounding at both ends and considering separate analog grounding. Change only one grounding or routing condition at a time, then reproduce the operating conditions and compare the result.
Isolate the sensor from the input
- Disconnect the ultrasonic sensor from the receiving converter or analog input.
- Apply a known 4–20 mA signal to that input.
- Verify that the converted value follows the injected current accurately and does not jump.
- If the input passes, repeat the test with another sensor. If the replacement is stable under the same wiring and process conditions, investigate the original sensor.
This procedure distinguishes sensor failure from receiving-input behavior without assuming that every sudden jump is electrical noise.
Evaluate sampling only after hardware checks
If the measured current contains brief disturbances, increasing the sampling interval or applicable input filtering may reduce visible hunting. Consult the receiving device manual for its buffer-memory settings. Do not use sampling changes to conceal a sustained transition to 20 mA; first confirm the physical loop current and isolate its source.
FAQ
Why does a 4–20 mA ultrasonic level signal jump from 10 mA to 20 mA?
The evidence supports several possibilities: interference from an inverter-driven motor, grounding or shield problems, a defective sensor, an input-conversion fault, or sampling behavior. Measure the loop current during the jump to determine whether the change is physical or exists only in the converted value.
How do I test whether the 4–20 mA sensor or analog input is faulty?
Inject a known 4–20 mA signal directly into the converter or analog input and verify its response. If that path is stable, test another sensor under the same conditions.
Can a nearby inverter-driven motor cause 4–20 mA hunting?
It is a supported hypothesis when the signal cable runs near the motor or inverter wiring. Test additional separation, inspect shield grounding, and determine whether changing the inverter carrier frequency changes the symptom.
