Pick the pressure range from the water column above the sensor, not the gallon rating: water is about 0.433 psi per foot, so an 8 ft tank reads 3.46 psi at the bottom and a 0-5 psi transmitter covers it. One analog input per tank, scale 4 mA to empty and the top endpoint to the highest water level, and add an offset if the sensor sits above the bottom.
A tank level input is sized from the water column above the sensor, not from the gallon rating on the tank. An 8 ft tank puts about 3.46 psi on a transmitter at the bottom. The right part is the smallest standard gauge range above that: a 0-5 psi transmitter. A 1000 psi transmitter looking at the same tank resolves the level to more than 2 ft per step, which no batching or pump control can use. This article gives the pressure math behind the range choice, the resolution each option delivers on an 11 ft diameter, 8 ft tall, 6000 gallon tank, the ultrasonic alternative, and the scaling that turns the current loop into feet and gallons.
What size pressure sensor does a water tank level input need?
Choose the smallest standard gauge range above the tallest water the tank can ever reach. Fresh water gives 0.433 psi per foot, so a maximum level of 8 ft is 3.46 psi at a bottom sensor, and that calls for a 0-5 psi transmitter.
The gallon figure never enters the calculation. Six thousand gallons can sit at that depth in a wide tank or much deeper in a narrow one. Only the depth loads the sensing element. Transmitter ranges come in standard steps, 0-5, 0-10, 0-15, 0-25 psi and up, so the decision is to compute the head and take the first step above it.
Most bottom-mounted transmitters for water service put the measurement on the 4-20 mA process signal. The selection page for 4-20 mA tank level sensors in water applications narrows the remaining choices of range, material and mounting.
| Option | Span the input divides | This tank at full | Usable resolution | Verdict |
|---|---|---|---|---|
| 0-5 psi bottom transmitter | 5 psi | 69% of span used | 0.135 in per step | The pick |
| 0-10 psi bottom transmitter | 10 psi | 35% of span used | 0.27 in per step | Pick if the level can surge |
| 0-15 psi bottom transmitter | 15 psi | 23% of span used | 0.41 in per step | Works, resolution wasted |
| 1000 psi line-pressure transmitter | 1000 psi | 0.35% of span used | 6.8 in best case, 2.25 ft planned | Wrong part |
| Top-mount ultrasonic | the sensor's own span | reads height directly | the sensor's stated accuracy | Pick when there is no bottom tap |
Resolution figures assume the 1,024 usable steps explained in the worked example below.
How much pressure does the water put on the sensor?
Compute gauge pressure as water density times gravity times height. The hydrostatic relation pressure = density x gravity x height gives 8.66 psi for a 20 ft column of fresh water and 3.46 psi for the 8 ft one.
That works out to the 0.433 psi per foot used above, and it holds for fresh water in a tank open to atmosphere, which is what a production water storage tank is. Measure the height from the sensing diaphragm to the overflow or the highest switch point. Water above that point is freeboard the control system should never be counting on.
One safety note belongs here: keep the tank vent open. A vent blocked by paint, a screen or ice turns every level change into a pressure change, and the level math in the PLC cannot tell the difference.
Mount the port clear of the pump suction stream too. A transmitter sitting in the draw of a running pump reads the dip in the water around its port. The level sags on screen while the tank stands still.
Worked example: what fits an 11 ft diameter, 8 ft tall, 6000 gallon tank?
This tank takes a 0-5 psi bottom transmitter. The full column is 3.46 psi, that uses 69 percent of the span, and the planned resolution is 0.135 in of water per step, tight enough to watch a batch draw down.
The arithmetic, step by step:
- Head: height x 0.433 psi/ft = 3.46 psi at a sensor on the tank floor.
- Geometry check on the 6,000 gallon nameplate: area = 3.1416 x 5.5 ft x 5.5 ft = 95.0 sq ft, brim-full volume = 95.0 sq ft x 8 ft = 760 cu ft, and 760 cu ft x 7.48 gal/cu ft = 5,685 gallons. A straight cylinder this size is full at about 5,685 gallons, so the nameplate includes freeboard or optimism. Set the top endpoint at the highest water the tank actually reaches and count gallons from there.
- Signal at that level: 4 mA + (3.46 / 5) x 16 mA = 4 + 11.07 = 15.07 mA, which leaves the top of the span unused for anything above the normal fill.
- Resolution: a 12-bit analog input divides the span into 4,096 counts. Count on at least one count of input noise in a real panel, which turns 4,096 theoretical counts into 1,024 steps worth trusting. Then 5 psi / 1,024 = 0.0049 psi per step, and 0.0049 psi / 0.433 psi/ft = 0.0113 ft = 0.135 in of water.
- Volume part way up: at 6 ft, 95.0 sq ft x 6 ft x 7.48 gal/cu ft = 4,264 gallons, the same 6/8 x 5,685 answer a straight-walled tank must give.
Why is a 1000 psi transmitter the wrong part for this tank?
A 1000 psi transmitter spends 3.46 psi of its span on this tank, 0.35 percent of full scale. It resolves the level to 6.8 in of water per count at best, about 2.25 ft per step once input noise is counted.
On the loop, the tank drives the current from the 4 mA zero to 4 + (3.46 / 1000) x 16 = 4.06 mA. The input still divides 1,000 psi into 4,096 counts, so one count is 1,000 / 4,096 = 0.244 psi, which is 0.244 / 0.433 = 0.56 ft, or 6.8 in of water, even before noise. Plan on 1,024 steps and the step is 1,000 / 1,024 = 0.98 psi, about 2.25 ft of water. A control meant to hold a 6 in band cannot do it with a part that steps in feet.
The scaling table is where this failure gets misdiagnosed. Scaling relabels the counts the converter already made; it cannot add steps the signal never contained. Ranges in this class exist for line pressure and hydraulics, so read the printed range on the transmitter datasheet before ordering. One product family usually hides several spans.
Recommendation: for a vented water tank with the transmitter at the bottom, order the smallest standard gauge range above the tallest possible water level, wired 4-20 mA with the low endpoint at empty and the high endpoint at that highest level. For the 8 ft head, order a 0-5 psi part. If the fill valve can push the level above the measured maximum before an alarm stops it, take the next range up and accept 0.27 in per step on this tank.
Should I mount the sensor on top of the tank instead?
A top-mounted ultrasonic sensor is the better pick when the tank has no bottom tap, when the port would sit in sediment, or when the transmitter would sit under the pump suction. It feeds the same analog input the same way.
The sensor times a sound pulse off the water surface and reports distance. Its two-point calibration sets the empty level to the low endpoint and the full level to the high endpoint, and the PLC scaling below is identical to the pressure case. Mount it above the highest water by more than the blanking distance printed in its datasheet, aimed straight down at still water, clear of fill streams and agitator cones. Foam, heavy condensation and vapor absorb the echo, so a tank that froths when it fills wants the bottom transmitter. Tank conditions decide the mounting as much as range does, and the page on water level sensor selection by tank conditions walks those cases.
A submersible level transmitter is the third option. It hangs on its cable from the top and rests near the bottom, so it measures head like the bottom-mounted part while the tank wall stays unbroken. Its vent tube stays dry, because the tube carries the atmospheric reference the measurement needs.
How many analog inputs do three tanks need?
Three tanks measured independently need three analog input channels, one transmitter per tank. The channel count belongs in the decision before the PLC hardware is bought, because two inputs cannot serve three transmitters.
Make a channel list with one row per tank: the transmitter's signal, the input type it needs, the wiring, and the register or tag that holds the result. Then check the channel count in the current documentation for the exact base unit and its analog expansion, because small brick PLCs differ by model and generation in how many inputs the base carries and what the expansion adds. Matching the sensor's signal to the input is the same discipline as choosing a temperature sensor for PLC fan control, where the sensor type and the input have to agree before the scaling means anything.
A display panel can show the three levels once the inputs exist, but it adds no inputs of its own.
How do I scale the 4-20 mA signal into feet and gallons?
Set the input's scaling so the low endpoint produces 0 ft and the high endpoint produces the height of the highest water level. Convert feet to gallons from the tank geometry, and alarm anything below or above the endpoints as a fault.
The convention carries a live zero: the signal rests at 4 mA with the tank empty, so a broken loop wire that pulls the current to 0 mA arrives as a fault, not as an empty tank. That is why the endpoints belong in the alarm logic and not only on the display. The endpoint math is the same as scaling a 4-20 mA pressure transducer for an HMI display, and it reduces to two lines:
- fraction full = (measured mA - 4) / 16
- feet of water = fraction full x span, divided by 0.433 psi/ft
For the 0-5 psi part that second line becomes feet = (measured mA - 4) x 0.722, so the 15.07 mA reading from the worked example lands at the full height.
If the transmitter sits 1 ft above the tank floor to stay out of sediment, add that 1 ft to every computed height, level = feet + offset. Straight walls let gallons ride linearly with height, fraction full x brim-full gallons; a cone bottom needs the geometry worked per band or a calibration against known volumes. Resolution on this tank is 0.135 in, about 0.011 ft, so two decimals on the feet value is honest and a third decimal only displays noise, which is the situation in PLC pressure scaling showing three decimal places.
What should you check before ordering?
Confirm four numbers before the order goes in: the highest possible water level in feet above the sensor, the printed range on the transmitter datasheet, the analog channel count of the exact PLC base and expansion, and whether the tank vents to atmosphere.
The list, in the order to gather it:
- Height from the sensing point to the overflow or high-level switch, measured, not taken from the tank nameplate.
- The transmitter datasheet's own range and signal, since one product family hides several spans.
- Base inputs plus expansion, one channel per tank, from the documentation for the hardware you are actually buying.
- Vented or sealed tank, because a sealed tank needs the measurement referenced to the space above the water.
- Mounting notes for the PLC math: port clear of the suction stream, and the offset if the sensor sits above the floor.
The decision in one line: depth sets the range, the smallest standard range above the tallest water wins, one channel per tank, and two formulas in the PLC turn the loop into feet and gallons. More control and display notes from the shop floor sit in the factory category.
Questions
Does tank shape change the pressure range I need?
No. Gauge pressure at the sensor depends on the vertical water height alone, so a tall narrow tank and a wide shallow tank filled to the same height need the same range. Shape changes the height-to-gallons conversion instead: a straight-walled vertical tank converts linearly, while a cone bottom or a horizontal capsule needs geometry worked per level band or a calibration against known volumes.
What changes when the tank is sealed instead of vented?
The bottom sensor then reads the water head plus the pressure of the gas space above it, so the level reads high whenever that space is pressurized. The clean fix is a differential transmitter that references the bottom pressure to the space, or a second sensor in the headspace with the PLC subtracting the two. Size the range for head plus the highest space pressure the process can produce.
Is distilled water different from tap water for the psi-per-foot figure?
For sizing, no. That figure comes from the density of fresh water, and dissolved solids in tap water shift density by a small fraction of a percent, far below transmitter accuracy. Temperature matters a little more, under one percent between cold and warm tank water, so the same figure serves all year.
How do I verify the transmitter once it is installed?
Check it at two known levels. Isolate and vent the transmitter with the tank empty and confirm the loop rests at its 4 mA zero. Then fill to a level you have measured with a stick or the tank's own markings, compute the expected pressure as feet times 0.433 psi per foot, and compare it to the scaled reading. If both points sit within the transmitter's stated accuracy, trust the line between them.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.