Choosing a tank level sensor for an HMI display

What level sensor do I need to show tank level on an HMI?

Order a continuous level transmitter with a 4-20 mA output into one analog input channel on the PLC, because a discrete level switch can only say whether the liquid has crossed one point and can never show the tank as a percentage.

Three facts decide it. What the operators need to see: a value they can trend, or only low and high lamps. What sits behind the panel: an analog input channel, or digital inputs only. Where the sensor can mount, and what the air above the liquid is like. The measurement chain never changes: sensor to analog input to scaled PLC value to HMI tag, and each link has to exist before the next one works. The screen is the last link; it has no way to measure the tank.

Can I just use level switches instead?

You can, but each switch gives one more bit: a float that changes state at its set point shows low or high, not the 3.5 ft between them, so a tank shown as six bands still has no real level value.

A float switch changes state at its operating point and holds there. Four switches give four bands, and a screen built from discrete PLC input and output bits draws those bands well. For pump start and stop, and for alarms, bands are often all the operator needs, at a fraction of the cost of a transmitter loop. What bands cannot give is the space between set points: the PLC receives no value there, so no screen work produces a percentage, a trend or remaining volume from switches alone. If the operator's question is whether to start the pump, order switches. If the question is how full the tank is, order a transmitter and keep one switch for the alarm.

Should I pick an ultrasonic or a hydrostatic sensor?

Pick ultrasonic when you can mount it on top with a clear, vertical beam path through clean air; pick hydrostatic when foam, condensation, vapor or internal ladders and agitators stand in that path, since water pressure at the bottom never cares what is above it.

An ultrasonic transmitter times a sound pulse from its face to the surface and back, so anything that disturbs that path disturbs the reading: foam, heavy vapor, condensation on the face, a fill stream splashing under the sensor, or a ladder, an agitator shaft or a thermometer well inside the beam cone. A hydrostatic transmitter measures the pressure of the water column, about 0.433 psi per foot of water, so it ignores everything floating in the tank; it needs a bottom entry or a cable-hung submersible unit, and a vented tank, because a gauge sensor reads head against the atmosphere above the liquid. A sealed, pressurized tank needs a differential-pressure arrangement, since the headspace pressure adds to the head. Where the echo path is doubtful and no bottom entry exists, radar is the non-contact option left; this comparison of hydrostatic, ultrasonic and radar tank level sensors works through the trade-offs. For a vented water tank with a clean top mount, pick ultrasonic. With foam, steam or internal steel in the beam, pick hydrostatic.

What output signal should the sensor have?

Specify a 4-20 mA output, because the live zero lets the PLC tell an empty tank from a broken wire: the 4 mA zero means empty, full scale means full, and a loop at or below 3.6 mA is a fault signal under the NAMUR NE43 convention.

The live zero is the whole point. An empty tank still sends the 4 mA zero, so a comparison on the raw value turns a dead loop into an alarm instead of a false 0 ft reading; transmitters built to the NAMUR NE43 convention drive the loop low on an internal fault, and the datasheet says so. The span from zero to full scale is 16 mA, and most compact transmitters are two-wire: one pair carries power and signal, excited by the analog card or a separate supply at around 24 V DC, so check which your card provides before ordering, because a passive channel needs that supply on the bill of materials. Current also beats voltage on a plant cable run, which is why a current signal wins on a long sensor run. Set the channel to current and to the transmitter's range before wiring, and read the raw value live while you do it.

Where does an ultrasonic sensor mount on the tank?

Mount the transducer face at least the blind zone printed on its datasheet above the highest liquid, on a nozzle if that clears it, aimed straight down at the surface with no fill stream, wall, ladder or agitator inside the beam cone.

The blind zone is the band right in front of the face where the echo returns too quickly for the receiver to separate it from the transmit pulse, so that band reads as no measurement. It is a per-model number on the datasheet, so read yours and put it on the mounting drawing. Aim the sensor straight down at a flat part of the surface and keep the cone clean: no fill stream dropping aerated water under the sensor, no ladder rungs, no agitator shaft, no thermometer well, no weld seams or bolt heads near the cone wall. Where the highest liquid would reach into the blind zone, raise the face on a nozzle; the worked example below buys its clearance with a 1 ft riser. On a tall narrow tank, a sensor a few degrees off vertical starts reading the wall. Condensation on the face is a mounting problem: move the sensor out of the steam plume or choose a model rated for it.

How do I scale the signal into feet on the PLC?

Scale the raw analog counts linearly between the two endpoints you actually wired: raw 0 at the 4 mA zero means empty and raw 4095 on a 12-bit card at full scale means full, so an 8 ft span across the 16 mA signal gives 6 inches of level per milliamp.

Two settings make the number honest. First the transmitter: most models let you program what the zero and full-scale currents mean, so set 4 mA at empty and the full-scale current at the top of the working span; the signal then rises with the level and no inversion is needed. If the transmitter is configured for distance to surface, the endpoints swap, rising level drives the signal down, and the scaling runs inverted. Second the card: read the raw counts and scale them linearly with level_ft = (raw - raw_low) * span_ft / (raw_high - raw_low). The step-by-step arithmetic sits in scaling a 4-20 mA transducer for an HMI display; the worked example below carries the numbers end to end. Tag the result with its units and a decimal place the counts can support.

What does a worked example look like on an 8 ft tank?

In this example an 8 ft water tank gets a non-contact ultrasonic transmitter on a 1 ft nozzle, a 10 in blind zone, a 12-bit analog input and a display in feet to a tenth of an inch, with every number checked by arithmetic.

Scenario: a vertical, vented water tank 8 ft tall, operators want the level in feet and a low alarm. The transmitter datasheet prints a 10 in blind zone; treat that figure as this example's input and read your own datasheet for the real one. The tank takes a 1 ft nozzle on top, so the face sits 9 ft above the bottom, or 108 in. The nearest readable surface is 108 minus the 10 in blind zone, which is 98 in, or 8 ft 2 in above the bottom: the brim sits 12 in below the face, 2 in clear of the blind zone, so the full 0 to 8 ft span reads.

Configure the span: 4 mA at 0 ft and full scale at 8 ft, so the 16 mA span carries 96 in and the scale is 6 in per mA. Assume a 12-bit card, 0 to 4095 counts across the loop current, about 256 counts per mA. Resolution is 96 in over 4095 counts, 0.023 in per count, so a display step of 0.1 in is honest, and the scale reduces to level_ft = raw × 8 / 4095.

Check the middle rows by hand: at 4 ft the current is 4 + 16 × (4 / 8) = 12 mA and the counts are 4095 × 4 / 8 = 2048; at 7.5 ft the current is 4 + 16 × (7.5 / 8) = 19 mA and the counts are 4095 × 7.5 / 8 = 3839. Set the low alarm at 1.5 ft, which is 7 mA and 768 counts, the high alarm at 7.5 ft, and keep alarm setpoints low-below-high before they reach the PLC. Leave one float switch at 1 ft on a digital input as a backup that fails independently of the loop.

Swap the front end to hydrostatic and only these numbers change: 8 ft of water presses 8 × 0.433 = 3.46 psi on the bottom, so a 0 to 5 psi submersible transmitter sits at 3.46 / 5, about 69 percent of range at the brim, which leaves headroom for surge. The counts at full are 4095 × 3.46 / 5 = 2834, and the conversion back to feet is psi × 2.31, so 3.46 × 2.31 lands on 8.0 ft. Same channel, same tag, same display.

Recommendation: For a vented water tank you can reach from the top, order a non-contact ultrasonic transmitter with a 4-20 mA output, mount its face at least the printed blind zone above the highest liquid, with a nozzle if that is what the clearance takes, scale the 4 mA zero to empty and full scale to the top of the working span, and keep one float switch on a digital input as the low-level backup. Choose a submersible hydrostatic transmitter instead when foam, vapor or internal steel spoils the beam; it lands on the same analog channel and the same display.

How do I verify the level reading before sign-off?

Prove the loop with two physical references before you sign off: read the raw counts and the scaled feet at empty and at a known half tank, confirm the display moves up as the level rises, and only then trust the number on the screen.

1. Read raw counts before the scaled value, at empty and at a half tank checked against a dip tape or a sight glass. Raw should sit near 0 at empty and track the level smoothly.
2. Split the fault. A steady raw with a wrong display points to scaling, tag mapping or the screen; raw pinned at 0 or 4095 inside the working span points to the loop, the channel configuration or the sensor.
3. Prove direction: add a little water and watch the display rise. A display that falls as the tank fills means inverted endpoints from a distance-mode transmitter.
4. Check the fault behaviour during a controlled shutdown: pull a loop terminal and confirm the fault comparison alarms instead of the screen showing an empty tank.
5. If the surface ripples, average in the PLC; averaging eight scale readings in PLC logic with high and low dropped shows one method that keeps alarms off the filtered value.

What should I ask for when ordering?

Before you order, confirm the sensor's stated span covers your level span plus its blind zone, the output is a 4-20 mA loop with two-wire power your card can supply, the process temperature and the cable or nozzle length fit, and the HMI tag carries units.

• Tank numbers: level span, face height above the bottom, highest liquid, nozzle length; the readable band must cover the span with the blind zone subtracted.
• Signal and power: the 4-20 mA loop, two-wire, and who excites it, the analog card or an external supply.
• Channel details: current range configured, and the card's real counts range, which the scaling starts from; the example above assumed one figure, yours may differ.
• Process conditions: vented tank, foam, vapor, temperature at the sensor face, and cable length for a submersible unit.
• Screen side: the tag carries units, the decimal place matches the counts, alarm setpoints go in low before high, and if the level must be recorded, configure the trend or logging function and check where values land.

If the same cell also wants a shift-target board or a maintenance notice screen beside the HMI, that is a separate display fed with its own content; our shop-floor signage plays notice boards offline, and it takes none of the analog loop, which belongs to the PLC alone.

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