The short answer
Hydrostatic Level Measurement
A hydrostatic level instrument measures the pressure at a point below the surface and converts it to the height of liquid above that point, using the liquid density. A submersible transducer hangs in the liquid; a bubbler pushes air through a tube and measures the back pressure; a base-mounted transmitter reads through a tap in the tank. All three depend on the density being what the calibration assumed, on the reference pressure being atmospheric, and on the sensor staying where it was installed.
Key points
- Level equals pressure divided by density times gravity; a change in density is read as a change in level.
- A gauge sensor needs a vent to atmosphere, usually a tube in the cable, and a blocked or wet vent produces a slow drift.
- The reading is the height above the sensor, so the sensor elevation is part of the calibration and must be recorded.
- Bubblers keep the electronics out of the liquid at the cost of an air supply and a tube that can plug.
- Fouling, cable damage, and a shifted sensor are the failures; comparison with a tape or a float is the check.
The principle
The pressure at a depth in a liquid is the weight of the column above it. For water, every foot of depth adds about 0.433 psi, or every 2.31 feet adds one psi. A sensor at the bottom of a tank measuring 4.33 psi is under ten feet of water. The instrument does that arithmetic with a density it was told at calibration, and it is right as long as the liquid is what it was told.
Level = Pressure / (Density x g)
- Level is the height of liquid above the sensor
- Pressure is the gauge pressure at the sensor, referenced to atmosphere
- Density is the liquid density, about 62.4 lb per cubic foot for water, and g is gravity
- For water in US units, level in feet is pressure in psi times 2.31
Three ways to do it
- Submersible transducer
- A sealed pressure sensor on a cable, lowered into the liquid and held at a fixed elevation. The cable carries the signal and a vent tube. It is the default in wet wells, tanks, and wells because it is cheap, accurate, and needs no tap in the vessel. Its weaknesses are that it is in the liquid, with everything that means for fouling, and that the cable is vulnerable.
- Bubbler
- A tube runs to a point near the bottom, and a small compressor or a plant air supply pushes air down it at a low, regulated flow. The pressure needed to push bubbles out of the end equals the head above it, and a pressure transmitter at the top of the tube, in the dry, reads it. Nothing electrical is in the liquid, which suits corrosive, hot, or classified locations. The costs are an air supply that must not fail and a tube that can plug or freeze.
- Base-mounted transmitter
- A pressure transmitter on a tap at the bottom of the tank or on the inlet piping, reading the static head. Accessible for calibration without draining and no cable in the liquid. It needs a tap, it reads velocity effects if the tap is on a live line, and it needs a diaphragm seal on liquids that would plug or attack it.
Venting and the reference
A gauge pressure sensor compares the process pressure with the atmosphere, and a submersible unit reaches the atmosphere through a small tube in its cable that ends in the junction box. If the tube is pinched, blocked with dirt, or has taken on water from condensation, the reference is no longer atmospheric, and the reading drifts with the weather and the temperature. The junction box gets a vented cap with a desiccant or a breather, the cable is not tie-wrapped so tightly that the tube closes, and a slow unexplained drift is checked at the vent first.
An absolute sensor avoids the vent by measuring against a sealed vacuum, and then needs a separate barometric measurement subtracted from it, either in the transmitter or in the controller. That trades a vent tube for a second instrument.
Installation
- 1
Fix the sensor elevation.
The instrument reads height above itself. Hang it on a stainless cable or a rigid pipe to a known elevation, record that elevation against the tank floor on the drawing, and enter it in the scaling so the reading is referenced to the floor, not to the sensor.
- 2
Keep it still.
A transducer that swings in the inflow or is dragged by the pump reads a moving elevation. A stilling tube or a weighted bracket holds it.
- 3
Make it retrievable.
It will need cleaning. Hang it from the hatch on a cable and a clip so that it can be lifted without anyone entering the well.
- 4
Protect the cable.
Away from the pump intakes, away from the ladder, in a conduit or a tube through the wall, with strain relief at the top and a drip loop at the junction box.
- 5
Locate the tap for a base-mounted unit on a dead leg.
Static head only. A tap on the inlet or discharge piping reads flow effects as level.
Failures and checks
| Symptom | Likely cause | Check |
|---|---|---|
| Reads high, slowly worsening | Grease or solids on the diaphragm | Lift and clean; compare with a tape |
| Slow drift with the weather | Vent tube blocked or wet | Inspect the vent and the junction box |
| Sudden offset | Sensor moved or cable slipped | Check the hanging point and the elevation |
| Reads low or zero | Cable damaged or sensor pulled out of the liquid | Inspect the cable; measure the loop |
| Correct at one level, wrong at another | Density or span wrong | Recalibrate against a tape at two levels |
| Reads high while a pump runs | Tap on a live line or sensor in the intake flow | Relocate to a dead leg or a stilling tube |
Frequently asked questions
- How does a submersible level transducer work?
- It measures the pressure of the liquid above it, which is proportional to depth for a known density, and outputs 4 to 20 mA over a vented cable. Level above the sensor is the pressure divided by the liquid density.
- Why does my level transducer drift with the weather?
- The vent tube in the cable is blocked or wet, so the reference side is no longer at atmospheric pressure and the reading follows the barometer and the temperature. Inspect the vent and the junction box breather.
- When should I use a bubbler instead of a transducer?
- When nothing electrical should be in the liquid: corrosive chemicals, high temperature, a classified area, or a liquid that fouls diaphragms badly. Accept the air supply and the purge tube as the maintenance items in exchange.
- Does the instrument need recalibrating for a different liquid?
- Yes, if the density differs from what it was calibrated for. A specific gravity of 1.1 reads ten percent low on a water calibration. Enter the actual specific gravity, or calibrate against a measured level in the actual liquid.
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.
- 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.
- Float SwitchesThe oldest level instrument and still the one that saves a station: how tilt and stem floats work, the wiring conventions, mounting and spacing in a wet well, classified-area ratings, and how to use them alongside a continuous instrument.
- Tank Level ControlHow a storage tank is filled and drawn on level setpoints, how the setpoints are chosen to keep the water turning over, and what the station does when the tank stops talking.
- Pressure TransmittersGauge, absolute, and differential pressure transmitters: how they sense, how range, turndown, and accuracy specifications work, the process connection and its accessories, and the installation errors that show up as calibration problems.
- 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.
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.