A submersible level sensor needs a vent tube so that atmospheric pressure reaches the back of its diaphragm. Air pressure also acts on the liquid surface, so venting cancels it and the sensor measures only the liquid column (ΔP = ρgh). Without a vent, every 1 mbar of weather change reads as about 10 mm of water.
What does the vent tube in a submersible level sensor do?
It gives the sensor a gauge reference: the inside of the probe is connected through a thin tube in the cable to the open air at the top of the installation. The probe at the bottom of the tank then compares the pressure in the liquid with the same atmosphere that presses on the liquid surface.
- Front of the diaphragm. At depth h the liquid presses with P_atm + ρ·g·h — the column of liquid plus the air on top of it.
- Back of the diaphragm. The vent tube connects the inside of the probe to the same atmosphere, so P_atm also acts from behind.
- Result. The diaphragm deflects only by ρ·g·h, so the output follows liquid level alone: h = ΔP ÷ (ρ·g).
This is the same gauge-pressure principle explained in gauge vs absolute vs sealed gauge pressure; the full measuring chain from pressure to 4–20 mA is in how a hydrostatic level sensor works.
How big is the error without a vent tube?
Large on low ranges: every 1 mbar of unvented air-pressure change is about 10.2 mm of water. Weather typically moves barometric pressure by tens of millibar, and altitude shifts it by hundreds.
| Relation | Formula | Meaning |
|---|---|---|
| Vented (gauge) reading | h = (P_liquid − P_atm) ÷ (ρ·g) | Air pressure cancels; only the column is measured |
| Unvented error | Δh = ΔP_atm ÷ (ρ·g) | Change in air pressure since the reference was fixed |
| Water, per millibar | 100 Pa ÷ (1,000 kg/m³ × 9.80665 m/s²) ≈ 10.2 mm | 1 mbar ≈ 10.2 mm H₂O |
Pa, kg/m³ and m/s² are the SI units defined by NIST (SI units) ↗. The table shows what an unvented reading would drift by, as a percentage of full scale (calculated, water, ρ = 1,000 kg/m³):
| Range (mWC) | 10 mbar swing (102 mm) | 30 mbar swing (306 mm) | Sealed at 1,013 mbar, used at 1,500 m (≈ 168 mbar, 1.71 m) |
|---|---|---|---|
| 0–1 | 10.2% FS | 30.6% FS | 171% FS |
| 0–5 | 2.04% FS | 6.12% FS | 34.2% FS |
| 0–10 | 1.02% FS | 3.06% FS | 17.1% FS |
| 0–40 | 0.25% FS | 0.76% FS | 4.28% FS |
| 0–100 | 0.10% FS | 0.31% FS | 1.71% FS |
| 0–200 | 0.05% FS | 0.15% FS | 0.86% FS |
Worked example: 0–10 mWC well sensor without a vent
- Accuracy class ±0.1% FS on 0–10 mWC = ±10 mm.
- A front passes and the barometer falls by 30 mbar: Δh = 3,000 Pa ÷ (1,000 × 9.80665) ≈ 306 mm.
- An unvented sensor would report the level 306 mm lower although the water has not moved — about 30 times its accuracy class.
- In diesel (specific gravity about 0.84 — use the value for your fuel) the same 30 mbar reads as 306 ÷ 0.84 ≈ 364 mm, because a lighter liquid needs a taller column for the same pressure.
All values are calculated, not measured. Convert your own levels and densities with the hydrostatic level calculator and units with the pressure unit converter.
Vented, absolute or sealed: which submersible sensor should I use?
For open tanks, wells and sumps on ranges up to tens of metres, use a vented sensor. Use absolute plus a separate barometer only where a vent tube cannot be kept dry; sealed gauge suits only ranges where the offset is small against the accuracy you need.
| Reference | How air pressure is handled | Typical fit | Watch out for |
|---|---|---|---|
| Vented gauge (vent tube) | Cancelled continuously inside the sensor | Open tanks, wells, sumps, reservoirs; 4–20 mA straight to PLC or SCADA | Vent end must stay open, dry and above flood level |
| Absolute + barometer | Subtracted later: h = (P_abs − P_baro) ÷ (ρ·g) | Data-logger monitoring where the cable end may flood | Second sensor, time-matched records, post-processing |
| Sealed gauge | Not compensated; fixed offset from weather and altitude | High ranges where the offset is negligible | On 0–10 mWC a 30 mbar swing is already 3.06% FS |
The U.S. Geological Survey manual Use of submersible pressure transducers in water-resources investigations (TWRI 08-A3) ↗ describes both approaches: vented transducers need a vent tube to the atmosphere, while absolute transducers need a second transducer used as a barometer.
How do you install and protect the vent tube?
Keep the tube open from probe to air and keep water out of it. Most vented-cable problems come from the top end of the cable, not from the probe.
- Order the cable to length. Cable length = installation depth + run to the junction box. A field cut must leave the vent tube open; never seal it with potting or heat-shrink.
- Terminate in a dry junction box above the highest level — and above any flood level at a wellhead — so liquid cannot enter the open end.
- Fit a desiccant or breather element at the vent end. Humid air cooling in the tube forms condensate, and water in the tube blocks the reference.
- Do not crush the cable. Tight cable glands, clamps or sharp bends can pinch the tube shut; a pinched tube behaves like a sealed sensor.
- Keep the box at local atmosphere. If the vent ends inside a sealed or pressurised cabinet, the cabinet pressure becomes the reference.
The probe body and cable jacket are sealed for immersion — IP68 under IEC 60529 (IP Code) ↗ means protection against continuous immersion under conditions stated by the manufacturer. Only the vent tube is meant to be open, and only at its dry end.
Why does my submersible level reading drift? Vent-tube troubleshooting
| Symptom | Likely cause | What to do |
|---|---|---|
| Level rises and falls with the weather while the tank is still | Vent blocked, pinched or sealed; or the sensor is absolute / sealed gauge | Check the vent end is open; loosen tight glands and clamps on the cable; confirm the sensor type |
| Slow drift, then erratic readings | Condensate or water in the vent tube | Replace saturated desiccant; move the box to a dry spot above the highest level |
| Constant offset after installation | Vent ends in a sealed or pressurised enclosure, or density not set | Vent the box to atmosphere; enter the liquid's specific gravity in the PLC scaling |
| Wrong reading after a cable repair | Splice closed the vent tube | Use a vented junction box instead of an inline splice, or replace the cable |
| Reading includes extra pressure in a closed tank | Head-space pressure above the liquid is not atmospheric | Use a differential measurement that references the head space |
To check the 4–20 mA side of the loop, compare the current with the expected level using the 4–20 mA calculator.
SEGMENsensor submersible level transmitters: key data
| Parameter | Value |
|---|---|
| Measuring range | 0–0.5 to 0–200 mWC, specified at order |
| Accuracy | ±0.1% FS standard; ±0.05% FS high-accuracy version |
| Output | 4–20 mA 2-wire or 0–10 V |
| Supply | 12–36 V DC |
| Process temperature | −20 to +80 °C |
| Probe and protection | 316L stainless steel, IP68 |
| Cable | Vented PVC, 5 m standard, custom lengths to installation depth; vented PTFE for chemical media |
| Samples | From 1 pc for qualification; volume per project |
See the submersible level transmitter page for wetted materials and the selection table, all liquid level sensors, and how to choose a liquid level sensor if hydrostatic is not the only option.
Specifying submersible level sensors for an OEM project?
Send the liquid, maximum depth, cable length to the junction box, output and whether the cable end can flood. Our engineers will confirm vented or absolute and the range. Qualification samples in 5–7 working days · production 3–5 weeks · FOB China · reply within 24 hours.
Frequently Asked Questions
No. Vented (gauge) sensors have one; absolute and sealed gauge sensors do not. An absolute sensor needs a separate barometer to correct its reading, and a sealed sensor keeps an offset that changes with weather and altitude.
Water or condensate blocks the air path, so the reference no longer follows the atmosphere and the reading drifts or becomes erratic. Moisture can also reach the electronics. Keep the vent end in a dry box above the highest level and fit a desiccant or breather element.
You can shorten it if the vent tube is left open at the cut end and terminated in a dry, vented junction box. Avoid inline splices, which usually close the tube; it is better to order the cable to the installation depth.
About 10.2 mm of water per millibar of air-pressure change. A 30 mbar weather swing is about 306 mm, or 3.06% of a 0–10 mWC range (calculated); a sensor sealed at sea level and used at 1,500 m reads about 1.71 m low.
Record barometric pressure at the site at the same time and subtract it: h = (P_abs − P_baro) ÷ (ρ·g). The barometer must be close to the well or tank and logged on the same time base as the level sensor.
Only partly. The vent references the outside air, but in a closed tank the gas above the liquid can be at a different pressure, and that difference adds to the reading. Closed or pressurised tanks need a differential measurement that references the head space.