| Radar | Ultrasonic | Hydrostatic | Capacitance | |
|---|---|---|---|---|
| Contact with medium | Guided: yes | No | Yes | Yes |
| Affected by foam | Little | Badly | No | Little |
| Affected by vapour | Little | Yes | No | No |
| Affected by dust | Little | Badly | n/a | Little |
| Affected by density change | No | No | Yes | No |
| Coating tolerance | Good | Good | Good | Moderate |
| Solids | Yes | Limited | No | Yes |
| Relative cost | High | Medium | Low | Medium |
Not "water" — is there foam on top, is there vapour above it, does the density change with temperature, does it coat the walls? Each of those knocks out a technology.
A hydrostatic transmitter measures head, and head is density times height. If the liquid is hot in the morning and cold at night, or the concentration changes with the batch, the level reading changes with it even though the level did not.
Ultrasonic needs a clean echo off a flat surface through clear air. Foam absorbs it, vapour bends it, dust scatters it. It is an excellent instrument on the right duty and a nuisance on the wrong one.
Radar and ultrasonic need a nozzle on the top of the vessel with a clear view down. If the vessel has an agitator, internal pipework or a mixer shaft in the way, guided wave radar or a bottom-mounted hydrostatic device is the practical answer.
This kills more transmitters than any measurement problem. SS316L is standard; corrosives need Hastelloy, PTFE lining or a ceramic diaphragm.
4-20 mA for a simple loop, HART if you want diagnostics and remote configuration, Modbus if it goes to a PLC over a bus. Changing this later usually means changing the instrument.
Hydrostatic submersible or a flanged hydrostatic transmitter. Simple, cheap, accurate enough.
Guided wave radar with a coated rod, or a differential pressure transmitter with remote seals.
Non-contact radar, or capacitance with an insulated cable probe.
Guided wave radar. The signal travels along a probe in contact with the liquid, so foam on the surface interferes far less than it does with non-contact ultrasonic.
Yes — it measures pressure head, so the scaling depends on the liquid density. If the density varies, the level reading varies with it.
Non-contact radar can see false echoes from a moving shaft. Guided wave radar with a rod or cable, positioned away from the agitator, is usually the safer choice.
A hydrostatic transmitter on an open tank of a clean liquid of constant density. It is hard to beat on cost per point.
When you only need to know "full" or "empty" — a float, fork or capacitance switch costs a fraction of a transmitter and there is nothing to calibrate.
Tell us what is in the vessel — foam, vapour, dust, coating, changing density. That decides the technology before price does.
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