By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed August 03, 2026
A water storage tank level sensor reports how full a tank is. On most sites it also starts and stops the pump that fills or empties it. For industrial and municipal water the choice is rarely decided by accuracy, because every mainstream technology is accurate enough for a storage tank. A submersible transmitter quoted at 0.25 percent of full scale is worth 30 mm on a 12 m tank, which no operator will ever notice.
Three other things decide the job: whether the wetted parts are certified for potable water, whether the control band stops the pump short-cycling, and which field failures the installation is exposed to.
This guide is written for potable water storage, pump station wet wells, cooling tower basins, fire water tanks and process water. It is not about rainwater barrels or RV tanks, which need a different and much cheaper answer. For how each sensing principle works in general, see our guide to level transmitter types; this page stays on water service.
Contents
- Potable water compliance
- Choosing by failure mode
- Head and range
- Pump control deadband
- What fails in service
- Fire water tank supervision
- Choosing by application
- FAQ
Potable water compliance
If the tank holds drinking water, the first question is not which technology. It is whether the parts that touch the water are certified, and most level sensor pages never raise it.
Two standards apply and they are not interchangeable. NSF/ANSI/CAN 61 covers what leaches out of a component into the water. Forty-nine US states have legislation, regulation or policy requiring drinking water system components to comply with it. NSF/ANSI/CAN 372 covers what is contained in the wetted material: a maximum weighted average lead content of 0.25 percent calculated across the wetted surfaces.
Products certified to 61 must also be evaluated against 372, with the exceptions written into the Safe Drinking Water Act. One standard governs what comes out of the material, the other governs what is in it.
The practical consequence for a level sensor is that certification is a property of every wetted part, not of the model number. A submersible transmitter can have a 316 stainless body that is perfectly compliant, and still fail the submittal because of the cable jacket, the diaphragm seal or an O-ring. So ask for the certification against the specific part list. Put that on the submittal rather than accepting “NSF certified” on a datasheet line.
A non-contact instrument sidesteps most of this. A radar or ultrasonic sensor mounted in the roof never touches the water, so the compliance question shrinks to whatever passes through the tank penetration. On potable storage that alone is often the reason to mount above the surface rather than hang a probe in the water.
Choosing by failure mode
Since accuracy rarely separates these instruments on a water tank, the useful comparison is what each one needs from the tank and what actually takes it out of service.
| Technology | What it needs from the tank | Wetted | What takes it out of service |
|---|---|---|---|
| Submersible hydrostatic | A way in from the top and a resting place near the bottom | Yes, fully | Blocked vent tube; cable damage; surge on a long run |
| Radar, non-contact | A roof nozzle with a clear view of the surface | No | Beam clipping on ladders, pipes and the fill stream |
| Ultrasonic, non-contact | Roof nozzle plus clearance below it for the blanking distance | No | Condensation on the face; a full tank entering the blanking zone |
| Float switch | Room to swing without fouling internals | Yes | Cable tangling; rags and grease in a wet well |
| Conductivity probes | Water conductive enough to complete the circuit | Yes | Biofilm and scale on the electrodes; very low conductivity water |
| Guided wave radar | A top opening the full depth for the probe | Yes | Buildup on the probe; needs access to clean it |
| Differential pressure | Bottom and top connections on a closed vessel | Yes | Wet leg filling or drying out and shifting the zero |
Water is an easy medium electrically. Its high permittivity gives radar a strong return from the surface, and it conducts well enough for electrode probes in most supplies. That is why the technology choice on water comes down to mounting and maintenance rather than to signal strength.
Head and range
A submersible transmitter measures the pressure the water column makes above it, and the range you order follows directly from the tank height.
P = ρ g h
A full 11.5 m potable tank: 1000 × 9.80665 × 11.5 = 112,776 Pa, or 112.78 kPa (16.36 psi, 452.8 inH2O)
In imperial terms the working number to remember is that 1 psi is 2.31 ft of water. Water is the one liquid where the density term causes no trouble. Unlike fuel or chemical service, you can size the range straight off the tank height plus a little freeboard.
The general derivation is in our hydrostatic pressure guide. On a liquid with a different specific gravity the same arithmetic needs the real density, which is the trap the diesel tank level guide works through.
One number is worth knowing before you trust a submersible reading. A gauge-referenced transmitter compares the water column against atmospheric pressure through a vent tube in the cable. A 1 kPa change in barometric pressure is 102 mm of water.
A normal weather swing of 3 kPa is therefore worth about 306 mm of apparent level, which is why that vent tube has to stay open and dry. See the vent desiccant point below.
Converting level to stored volume is a separate step, and on a vertical cylinder it is linear. Use the tank volume calculator for the geometry.
Application example
Infrastructure, feedwater storage for a special economic zone. The requirement was continuous level over 1,850 to 11,500 mm at atmospheric pressure, with water temperature 0 to 42 °C and output 4-20 mA. The instrument itself had to sit outdoors between −30 and +60 °C, an ambient far harsher than the water. With a span that wide, the selection went to a non-contact radar transmitter rather than a submersible probe, so that nothing is left hanging in the tank; proposed on that basis.
Pump control deadband
On most water tanks the level signal has a second job: starting and stopping a pump. That is where a good measurement gets ruined by a bad setting.
If the start and stop points are close together, the pump cycles constantly, and motor starts are what wear a pump set out. The working volume between the two points sets the cycle rate.
A wet well 1.5 m across with a 0.5 m band holds π/4 × 1.5² × 0.5 = 0.884 m³. At 10 m³/h inflow that is a start every 5.3 minutes, or about 11 starts per hour.
If the motor is rated for six starts an hour, the band has to grow rather than the pump being replaced. Holding 10 m³/h to six starts an hour needs 1.67 m³ of working volume, which on the same 1.5 m well is a band of 0.94 m. Set the band from the motor rating and the inflow, not from what looks tidy on the display.
Two more rules follow. Alarms should come from a device that is independent of the control transmitter, because a transmitter that fails also fails its own alarm. And where two pumps share a well, alternate them so that one does not take every start.
What fails in service
These are the five things that actually bring water tank level instruments down, and almost none of them appear in the product literature.
Condensation and blanking on ultrasonic. A steel roof in the sun drives moisture onto the coolest surface, which is often the transducer face, and a wet face muffles the pulse. The same instrument also has a blanking distance immediately below it where it cannot measure at all, so on a tank that fills close to the roof the high end of the range can disappear into that dead zone. Allow for both when you set the nozzle height.
The vent tube on a submersible. The desiccant capsule that keeps the vent dry is a consumable. When it saturates, moisture reaches the reference side and the reading wanders with the weather by the 102 mm per kPa worked out above. Put the capsule on a maintenance schedule and check it after any long wet spell.
Surge on long cables. An elevated tank or a remote wet well means a long run of cable, often the tallest conductor around. Fit surge protection at the panel and bond the shield at one end only.
Biofilm on conductivity probes. Electrode probes work because water completes a circuit between them. A film of biological growth or scale raises the resistance across that path until the controller stops seeing the water it is standing in. Clean them on a schedule, and check that the supply is conductive enough for probes in the first place; very clean or demineralized water may not be.
Turbulence at the fill point. Any instrument that follows the surface follows a disturbed one during a delivery. A stilling well quiets it without changing what is measured.
Fire water tank supervision
A fire water tank is a storage tank whose level is a life safety function, and it is handled differently from a process tank.
Under NFPA 25 the water level in a fire protection tank is a supervised, inspected item. Level indication has to be verified during periodic inspection, and high and low level alarms are expected to be present and working. That changes what you are buying. The requirement is not the tightest possible measurement, but a reading that can be proven correct on a schedule, plus alarm contacts that are independent of the indication.
In practice that means a continuous transmitter for the reading, separate high and low level switches for the alarms, and a way to verify the indication without draining the tank. Confirm the inspection frequency and the alarm points against the edition of the standard your authority having jurisdiction enforces.
Choosing by application
Water covers several very different tanks, and the right answer changes with the asset rather than with the fluid.
| Asset | Where to start |
|---|---|
| Potable storage tank | Non-contact radar in a roof nozzle, which keeps the certification question to the penetration only |
| Deep well or borehole | A submersible transducer on a vented cable, with the desiccant on a schedule |
| Pump station wet well | A non-contact ultrasonic transmitter above the water, with an independent float switch for high level |
| Cooling tower basin | A shallow span instrument for makeup control, allowing for splash, drift and a shifting surface |
| Fire water tank | Continuous indication plus separate high and low switches, verifiable on an inspection schedule |
| Ground level process tank | A hydrostatic level transmitter where a bottom connection exists and the water is clean |
Application example
Water service, open tank, Vietnam. The point of measurement was an open tank at ambient temperature and pressure, and the requirement was switching rather than continuous level. The specified parts were a polypropylene float switch on a DN100 JIS 10K flange, offered in both cable-suspended and rod versions so the customer could match the tank internals; proposed on that basis. On an open water tank a switch on a known flange is often a faster answer than a transmitter that then needs a controller behind it.
The full range, including the transmitters behind these applications, is on the level instruments page, and the loop side is covered in our 4-20 mA current loop guide.
Evaporative equipment adds its own losses on top of pump cycling; our cooling tower water level control guide covers makeup sizing and the probe setpoint stack for tower basins.
FAQ
How to check if a water level sensor is working?
Compare it against a second, independent measurement: dip the tank or read a sight glass and check that against the indication. Do it at two different levels, so you can tell a zero shift from a span error. On a submersible, also check that the vent tube desiccant is not saturated. That fault shows up as a reading that drifts with the weather rather than as an obvious failure.
How accurate are water level sensors?
More accurate than a storage tank needs. A submersible transmitter quoted at 0.25 percent of full scale is worth 30 mm on a 12 m tank. Non-contact radar is typically specified in single millimeters. Accuracy is almost never what separates two candidates on water service; mounting, certification and maintenance are.
What are the two main types of water level sensors?
Contact and non-contact. Contact instruments such as submersible transmitters, float switches, conductivity probes and guided wave radar touch the water, which makes wetted material certification relevant. Non-contact instruments such as radar and ultrasonic measure from above the surface and avoid most of that.
How do tank sensors work?
Either by measuring the pressure the water column makes, or by timing a signal reflected off the surface, or by detecting the water at a fixed point. Pressure-based sensors convert head to level. Radar and ultrasonic convert a time of flight into the distance down to the surface. Switches simply close a contact when the water reaches them.
Request a quote
Tell us the tank, whether the water is potable, and what the signal has to drive, and we configure one instrument for the application rather than a shelf part. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.