Why this measurement is difficult to get right
Level measurement is decided by the headspace, not by the vessel. Foam, vapour, condensation, agitation, dust and build-up each eliminate instrument families, and the same tank can be straightforward or impossible depending on what the product does when it is stirred. Two tanks of identical size holding the same liquid can require different instruments because one is vented and still and the other is agitated with a vapour blanket.
The second question is whether the requirement is continuous or point. Stock reconciliation needs a continuous measurement with a known resolution across the full span, while a pump running dry needs a switch that operates at one level and is trusted to do so every time. Specifying a continuous instrument where a switch was needed adds cost and maintenance; specifying a switch where inventory was meant leaves the operator without the number they actually needed.
Both cases share a constraint that is easy to miss: the usable range is smaller than the vessel. The dead zone at the top of the measuring span, the nozzle height and the presence of an agitator all remove part of the height from use. The enquiry should give the usable measuring height rather than the tank height, because the two are rarely the same.
What has to be measured
- Continuous level of liquid, slurry or solid in tanks, silos and sumps.
- Interface level between two immiscible liquids.
- Point level for high and low alarms, overfill protection and pump dry-run protection.
- Vessel pressure and temperature where they affect the measurement or the sensor rating.
- The usable measuring height between the highest and lowest levels that must be reported.
Which instrument measures what
| Measurement | What the reading has to establish | Instruments commonly specified |
| Continuous liquid level with vapour or foam present | A stable reading through conditions that defeat acoustic methods, which is a question of what is in the headspace rather than of height | Radar level meters |
| Continuous level in a clean, still headspace | A straightforward height measurement at lower cost than radar, where nothing in the vapour interferes | Ultrasonic level meters |
| Liquids of known and stable density | Level inferred from the pressure the liquid column exerts, which suits clean open vessels and is unaffected by surface conditions | Hydrostatic level transmitters |
| Solid level in silos and bins | A measurement across an uneven, dusty surface where the angle of repose makes the surface irregular | Radar level meters or point switches |
| Overfill and dry-run protection | That a single level has been reached, every time, without a maintenance-dependent continuous reading in the loop | Level switches, including vibrating fork designs |
| Interface between two immiscible liquids | Where the boundary sits, which a surface-measuring instrument cannot report | Guided radar or interface-specific sensors |
The usable range is the distance between the highest and lowest levels to be reported, not the vessel height.
Narrowing the choice for a vessel installation
- Establish the dielectric constant or the density of the product, since several principles depend on it directly.
- Describe what is in the headspace - vapour, foam, condensation, dust, agitation - because that removes families faster than the liquid does.
- Decide whether the duty is continuous inventory or point protection, since the two lead to different instruments entirely.
- Measure the usable height between the highest and lowest levels, allowing for the dead zone and the nozzle.
- Check whether the vessel is pressurised, which constrains both the sensor rating and the process connection.
- Establish the build-up tendency of the product, because fouling on the antenna or sensor is the most common cause of field failure.
- Decide whether the reading must feed a system, which makes the output signal and protocol part of the specification.
- Confirm the process connection and the material in contact with the product, since those follow the vessel rather than the instrument.
Where this application ends and the next begins
| Adjacent application | What separates it from tank and silo level monitoring |
| Water flow measurement | Flow is a volume passing through a pipe, while level is a quantity sitting in a vessel, and only the former can settle a flow-based duty. |
| Water quality monitoring | Composition is irrelevant to level, which is decided by headspace conditions and by the geometry of the vessel instead. |
| HVAC commissioning | That work measures levels rarely and only as a check on a circuit, whereas here the measurement is the primary duty and runs continuously. |
| Product quality inspection | Inspection judges an article against a drawing, while level is an inventory figure that changes with every transfer. |
| Energy efficiency testing | Consumption over a cycle is a different quantity from a stored volume, and it is measured with a power instrument rather than a level one. |
What to send with the enquiry
- Vessel type, height and diameter, and whether it is pressurised.
- The product, with its density or dielectric constant if known.
- What is in the headspace - vapour, foam, condensation, dust, agitation.
- The usable measuring height between the levels that must be reported.
- Whether the duty is continuous measurement or point switching.
- The process connection available and the material it must be in.
- Temperature, pressure and any hazardous-area classification.
- Whether the reading must feed a control or inventory system.
- Quantity required and the date it has to be in use.
- Whether the customer needs alarm contacts as part of the supply.
