Symptom
The level trend shows steps, spikes, or sudden flips to a fixed value that the wet well or tank could not physically have done, while the pumps and the rest of the process behave normally between the jumps.
The short answer
Level Reading Jumps
A level reading that jumps while the actual level moves smoothly is a measurement artifact, and its shape identifies the cause. A jump to the same fixed value each time is a false echo from an obstruction on a radar or ultrasonic. Spikes that come and go with inflow are turbulence, splashing, or foam. Steps that coincide with a pump start are electrical noise or a ground loop. Dropouts to zero or full scale are a loose connection, a wet cable, or a lost echo. Trend the level at high resolution against the pump status and the pattern names the cause.
Key points
- Trend it at one-second resolution with the pump status. The shape of the jump is the diagnosis.
- A jump to the same fixed value every time is a false echo or a stuck reading, not the water.
- Spikes that follow inflow or pump starts are turbulence, foam, noise, or a ground loop.
- A dropout to 4 mA or 20 mA is a signal loss: connection, cable, echo, or transmitter fault.
- Controller filtering hides the jumps and the pumps cycle on the wrong level. Find the cause instead.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| False echo from an obstruction | Radar or ultrasonic: the reading flips to a fixed distance as the surface passes a pump, rail, cable, or the inflow pipe. Re-run the false echo mapping at low level. |
| Turbulence or splashing at the inflow | Spikes during inflow surges; worse when the well is low and the inflow falls further. Relocate the sensor, add a stilling well or a baffle, or increase damping modestly. |
| Foam or floating debris | Reading rises or becomes erratic when foam is present; ultrasonic and radar read the foam surface. Higher frequency radar, foam control, or a submersible transmitter as a second measurement. |
| Condensation on an ultrasonic transducer | Erratic readings in humid or cold conditions, often in the morning. A sunshade, a heated transducer, or radar. |
| Temperature gradient over an ultrasonic | Slow drift and steps as the air warms; the speed of sound changes. Temperature compensation at the transducer; radar. |
| Electrical noise or ground loop | Steps or spikes exactly at a pump or drive start. Check shield grounding, cable routing, and the loop for a second ground. |
| Loose terminal or damaged cable | Dropouts to zero or full scale, often when the cable moves or in wet weather. Inspect terminations and the cable in the well. |
| Submersible transmitter vent tube blocked | A slow offset that changes with barometric pressure, and steps when the vent clears. Check the vent tube and the desiccant. |
| Submersible transmitter fouled or buried | Reading stops responding or steps when the pump stirs the grit. Clean and raise the transmitter. |
| Controller filtering or scaling | Steps that match the resolution of an integer scaling, or a filter that lets a spike through at the update rate. Check the raw value against the scaled one. |
| Wireless or network link dropping | The reading holds and then jumps to the current value when the link returns. Check the communication status and timestamps. |
Read the trend first
A level measurement in a wet well is one of the noisiest signals in a utility, and the shape of the disturbance says more than any measurement at the panel. Pull a trend at the fastest resolution the historian has, overlay the pump run status and, if available, the inflow or a rain gauge, and look at when the jumps happen and what value they jump to.
| Trend shape | Points toward |
|---|---|
| Flips to the same value every time, then returns | False echo from a fixed obstruction |
| Spikes upward during inflow surges | Splashing, turbulence at the inflow |
| Erratic reading that rises above the true level | Foam or a floating mat on the surface |
| Step or spike exactly at a pump or drive start | Electrical noise or a ground loop |
| Dropout to 4 mA, 20 mA, or the failure current | Signal loss: connection, cable, lost echo, transmitter fault |
| Erratic in the early morning or in humid weather | Condensation on an ultrasonic transducer |
| Slow drift with steps as the day warms | Temperature effect on an ultrasonic; air temperature gradient in the well |
| Holds flat, then jumps to catch up | Communication link dropping; stale value |
| Steps of a fixed size | Integer scaling resolution in the controller |
Diagnostic procedure
- 1
Confirm the water is not doing it
A well with a large inflow surge really does jump a few inches. Compare the jump with what the inflow could produce and with a float or a second measurement if there is one.
- 2
Match the pattern
From the table. Most jumps fit one row, and the row says where to go.
- 3
For a fixed-value flip, open the transmitter echo curve
Radar and ultrasonic transmitters show their echo profile in the configuration tool. The false echo is visible as a peak at the distance the reading flips to. Re-map at low level, and check the mounting for a new obstruction: a cable, a rag on a rail, a float.
- 4
For inflow spikes, look at the sensor position
A sensor over the inflow stream or in the pump discharge turbulence reads the disturbance. Move it, add a stilling well or a baffle, or accept a modest increase in damping.
- 5
For pump-start steps, check the electrical path
Shield grounded at one end, cable separated from motor and drive conductors, a second ground on the transmitter or the shield. The ground loop diagnosis page covers the measurements.
- 6
For dropouts, check the connections and the cable
Terminations in the panel and the junction box, the cable where it enters the well and where it hangs, the transmitter connector. Flex the cable while watching the reading.
- 7
For a submersible, check the vent and the sensor
The vent tube must be open to the atmosphere through a dry desiccant; the sensor face must be clear of grit and rags and above the sludge.
- 8
For a held value that jumps, check communications
The communication status and the timestamp. A radio or network link that drops and recovers produces exactly this shape, and the fix is on the link.
- 9
Fix and verify
Correct the cause and watch the trend through several pump cycles, including an inflow event, before calling it done.
Choosing a better measurement
Some wells defeat a single sensor. Foam, grease mats, splashing inflow, and confined geometry can make radar, ultrasonic, and submersible pressure each unreliable in turn. The robust answer at such a station is two measurements of different types, a radar and a submersible transmitter, compared in the controller, with the floats as the third layer. The controller uses the one that is behaving, alarms the disagreement, and never lets one bad sensor run the pumps alone.
Frequently asked questions
- Why does the level jump only when the well is nearly empty?
- The obstruction or the disturbance is low in the well: a pump body, the inflow falling further when the level is low, or a submersible transmitter being stirred by the pump intake. Re-map at low level, move the sensor, or raise the stop setpoint slightly.
- The radar was fine for years and now jumps. What changed?
- Something new in the beam: a float cable, a rag caught on a rail, a pump replaced with a different profile, a new ladder. Look in the well from the hatch. The echo curve in the configuration tool shows the new reflection.
- Can I just add deadband to the pump setpoints?
- Deadband stops the pump from cycling on noise at the setpoint, and it is part of a good sequence. It does not fix a measurement that jumps a foot, and a deadband wide enough to cover that changes the station operation. Fix the measurement.
- Should I replace the ultrasonic with a radar?
- If the causes are condensation, temperature gradients, or vapor, yes; radar is largely immune to all three and has become the default for wet wells. If the cause is a false echo from an obstruction, a radar has the same problem and needs the same mapping.
Related topics
- Wet Well Level MeasurementHow lift station wet well level is measured, which technology suits which well, the failure modes that flood a station, and why the high level float must stay hardwired.
- Radar Level MeasurementNon-contact and guided wave radar, why it handles a wet well better than most alternatives, and the installation details that decide whether it works.
- Ultrasonic Level MeasurementNon-contact level by time of flight: how the sensor works, the blanking zone and beam angle, temperature compensation, the surfaces and vapors that defeat it, mounting rules, and what to do when it loses its echo.
- Hydrostatic Level MeasurementMeasuring level from the pressure of the liquid above a sensor: submersible transducers, bubblers, and base-mounted transmitters, the density assumption behind all of them, venting, installation, and what makes them drift.
- How to Configure a Radar Level TransmitterCommission a non-contact radar on a wet well or tank: mount it, set the reference and range from measured elevations, map false echoes with the vessel empty, set damping and the output, and verify against a tape at two levels before the controller uses it.
- 4-20 mA Signal Is UnstableAn analog reading that will not settle. How to separate a genuine process swing from induced noise, a ground loop, or a loop running out of voltage.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.