By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed October 16, 2026
A pressure transmitter measures liquid level by reading the hydrostatic head of the liquid above its tap. In head units, level in meters equals the reading in meters of water column (mH2O) divided by the liquid’s specific gravity (SG). The transmitter is then ranged so that 4‑20 mA spans the head at 0% and 100% level: the lower and upper range values (LRV and URV).
Three installation details move the zero, sometimes by meters of water: a transmitter mounted below the 0% level, a liquid-filled wet leg and remote-seal capillaries. A closed tank adds one more question: what the low side of the cell sees. One tank, with one set of numbers in SI and US units, is ranged for each case: open tank, dry leg, wet leg and remote seals.
Contents
- Pressure to level basics
- Choosing the transmitter
- Open tank range
- Closed tank dry leg
- Wet leg range
- Remote seal systems
- Density and temperature errors
- Matching our transmitters
- FAQ
Pressure to level basics
In hydrostatic level measurement, the pressure at a point in the liquid depends on the liquid height above it and on its density. The shape of the tank does not matter (see what hydrostatic pressure is).
P = ρ × g × h so h = P / (ρ × g)
With the conventional water density of 1,000 kg/m³ and standard gravity of 9.80665 m/s², 1 mH2O is 9.80665 kPa and one inch of water (inH2O) is 249.09 Pa, so 1 psi is 27.68 inH2O. Working in head units removes g from the arithmetic: the head in mH2O is SG times the liquid height in meters. Our hydrostatic pressure calculator handles the unit conversions.
Check which water column the transmitter itself uses. Rosemount’s 3051 HART configuration data sheet lists inH2O at 68 °F as the default unit. Its options include inH2O at 4 °C and at 60 °F.
An inch of 68 °F water (998.2 kg/m³) exerts 248.64 Pa, 0.18% less than the 1,000 kg/m³ unit. On the 189 inH2O span used below, that is 0.34 in, small but enough to make a control system (DCS) check disagree with the transmitter.
Choosing the transmitter
The table sorts the common setups, including differential pressure (DP) transmitters, by what the sensor uses as its reference.
| Setup | Use it for |
|---|---|
| Submersible probe | Open tanks, wells and sumps; vented cable |
| Gauge transmitter | Open tanks with a side or bottom nozzle |
| DP, dry leg | Closed tanks with a non-condensing gas |
| DP, wet leg | Closed tanks where the vapor condenses |
| DP, remote seals | Hot, corrosive, viscous or sanitary service |
Avoid an absolute sensor on an open tank. It reads barometric pressure as level, and every 1 kPa change in the weather moves the reading by 102 mm of water. A vented gauge sensor, or a DP cell with its low side open to air, cancels the atmosphere. On a closed tank a gauge transmitter at the bottom reads the gas pressure above the liquid as extra level, so pressurized tanks need DP, or a second transmitter on the vapor space.
If nothing can be mounted below the liquid, a bubbler system reads the same head through a dip tube. Radar, ultrasonic, displacer and other methods are compared in our level transmitter types guide. Battery-powered wireless transmitters can use sealed gauge cells, which bring a similar barometric error; the wireless level transmitter guide works through it.
Open tank range
Example: an open tank holds a liquid with SG 1.20. The transmitter sits 0.40 m (1.31 ft) below the 0% level, and the measured span is 4.00 m (13.1 ft). The liquid in the nozzle and impulse line presses on the cell even at 0% level, so 4 mA sits above 0 kPa, which is called zero suppression. With SG1 for the liquid, h for the transmitter’s height below 0% and L for the span:
LRV = SG1 × h URV = SG1 × (h + L)
LRV = 1.20 × 0.40 = 0.48 mH2O = 4.71 kPa (18.9 inH2O)
URV = 1.20 × 4.40 = 5.28 mH2O = 51.78 kPa (207.9 inH2O)
Span = 4.80 mH2O = 47.07 kPa (189.0 inH2O)
Check one midpoint before commissioning. At 2.00 m of level the cell sees 1.20 × 2.40 = 2.88 mH2O. That is (2.88 − 0.48) / 4.80 = 50% of span, so the output should read 12.00 mA (our 4‑20 mA calculator runs the same check).
Enter the 0.48 mH2O as the LRV rather than zero-trimming it away. A zero trim is done with the cell vented; the standing head is real process pressure, and the pressure transmitter installation guide covers the difference.
Closed tank dry leg
In a closed tank, the gas above the liquid adds its pressure to the bottom reading. With the high-pressure (HP) side of a DP transmitter on the bottom tap, connect the low-pressure (LP) side to the vapor space. The gas pressure then acts on both sides of the cell and cancels. With a gas-filled (dry) leg, the LRV and URV stay at 0.48 and 5.28 mH2O whatever the tank pressure.
The risk is condensate: suppose 0.20 m (7.9 in) of water collects in the low-side leg. The low side gains 0.20 mH2O, and the level reads 0.20 / 1.20 = 0.167 m (6.6 in) low. Slope the leg back to the tank, fit a drain or condensate pot at the low point, or change to a wet leg. Dry legs suit clean, non-condensing gas blankets such as nitrogen.
The cell still carries the full tank pressure on both sides. Check the static pressure rating and, separately, the one-sided overpressure rating. While the manifold valves are being opened or closed, one side can see full tank pressure while the other is vented.
Application example
Pressurized liquid oxygen storage. An oil and gas customer asked for five DP level transmitters with 4‑20 mA output for liquid oxygen storage, specified as a 6,000 L tank at 17 to 20 bar. We quoted the five transmitters. When the level DP is a small fraction of the static pressure on both sides of the cell, as here, the static and flange ratings are the first check.
Wet leg range
When the vapor condenses, the low-side leg fills anyway, so it is filled on purpose with a known liquid. The leg then adds a fixed column to the low side, and the DP at 0% level becomes negative.
Use the same tank with the low-side tap 0.30 m above the 100% level. From the transmitter up to that tap the leg is H = 0.40 + 4.00 + 0.30 = 4.70 m (15.4 ft) tall, and it is filled with water, SG2 = 1.00:
LRV = SG1 × h − SG2 × H URV = SG1 × (h + L) − SG2 × H
LRV = 0.48 − 4.70 = −4.22 mH2O = −41.38 kPa (−166.1 inH2O)
URV = 5.28 − 4.70 = 0.58 mH2O = 5.69 kPa (22.8 inH2O)
The span is still 4.80 mH2O; only the zero has moved. This is zero elevation: 4 mA corresponds to a negative DP. Confirm that the sensor accepts the negative LRV within its limits. Also check the connections: with HP and LP swapped, the output falls as the level rises.
The leg’s density is now part of the zero. If the water in the 4.70 m leg warms from 20 to 40 °C, its density falls from 998.2 to 992.2 kg/m³ (0.6%). The leg loses 0.028 mH2O and the level reads 24 mm (0.9 in) high. A partly drained leg is worse: every 0.10 m of missing fill shifts the reading by 0.10 / 1.20 = 83 mm, so refill legs after maintenance.
Remote seal systems
Diaphragm seals with filled capillaries replace impulse legs when the process is hot, corrosive, viscous or must stay clean. Treat the capillary fill like a wet leg. The column that matters is the vertical distance between the two seals. The transmitter’s own height drops out, because both capillaries carry the same column down to it.
Example: the HP seal sits at the 0% level and the LP seal 4.30 m (14.1 ft) above it. The fill is Silicone 200, SG 0.934 at 25 °C according to Rosemount’s 1199 fill fluid specifications:
LRV = −0.934 × 4.30 = −4.016 mH2O = −39.39 kPa (−158.1 inH2O)
URV = 1.20 × 4.00 − 4.016 = 0.784 mH2O = 7.69 kPa (30.9 inH2O)
The same Rosemount document gives the thermal expansion of Silicone 200 as 0.00108 per °C. Suppose the whole seal system warms 30 °C above its calibration temperature. The fill density drops about 3.1%, the 4.30 m column loses 0.126 mH2O, and the level reads 105 mm (4.1 in) high. One capillary in the sun and the other in shade gives an error that is harder to predict.
Keep capillaries short, run them together and keep them out of direct sun. Range the system with the fill SG at its expected service temperature.
Density and temperature errors
A pressure transmitter measures head, so the level reading is right only if the SG entered is right. Ranged for SG 1.20 in the open-tank example, a full tank (4.00 m) of SG 1.15 liquid produces (0.40 + 4.00) × 1.15 = 5.06 mH2O. The transmitter indicates 3.82 m, which is 183 mm (7.2 in), or 4.6%, low.
Thermal expansion acts in a less obvious way: heat a vertical tank of water from 20 to 80 °C and the level rises. The mass stays the same, and so does the bottom pressure. A 4.00 m column of 80 °C water (971.8 kg/m³) gives the head of 3.89 m of 20 °C water. For inventory by mass that is the right answer; for a high-level alarm it is 106 mm short.
Then convert the transmitter’s accuracy into level. A transmitter specified at 0.1% of a 200 kPa upper range limit has a 0.2 kPa error band. On the 47.07 kPa open-tank span that is 0.42% of span, or 17 mm (0.67 in) of SG 1.20 liquid. Choose the smallest sensor range that holds both the LRV and the URV; full scale vs reading accuracy explains the conversion.
Matching our transmitters
Our hydrostatic and DP transmitters cover the installations above; each page carries the full datasheet.
| Installation | What we offer |
|---|---|
| Open tank or well | Hydrostatic level transmitter: 0 to 200 mH2O, vented-gauge cell, 0.2% of full scale (FS) typical, 4‑20 mA, IP68 probe |
| Closed tank | SMT3151LT DP level transmitter: spans 4 kPa to 2.1 MPa in four codes, 0.1% or 0.5% FS, 4‑20 mA with HART, flanges to ANSI Class 900 or PN100 |
| Hot or corrosive | SMT3151LT with a Hastelloy C, tantalum or titanium diaphragm, fill to 304 °C and an optional low-side remote seal; or a diaphragm seal transmitter |
For the example tank, the 47.07 kPa span fits the 20 to 200 kPa span code (the range option in its order code) of the SMT3151LT. The wet-leg and seal ranges keep the same span but start at about −40 kPa. We confirm that negative LRV against the sensor limits before quoting. The hydrostatic pressure transmitter overview compares submersible and flush-mounted builds.
FAQ
How to calculate LRV and URV for a level transmitter?
Write down the differential pressure at 0% and at 100% level. With SG1 for the process, h for the transmitter’s height below 0%, L for the span, and H with SG2 for a wet leg: LRV = SG1 × h − SG2 × H and URV = SG1 × (h + L) − SG2 × H, in meters of water. Drop the SG2 term for open tanks and dry legs.
How accurate is hydrostatic level measurement for deep tank applications?
In a deep tank the transmitter is rarely the largest error. A 0.1% of span error on a 20 m water column is 20 mm. A 1% error in the assumed SG on the same column is 200 mm. In deep tanks, density changes from temperature layering and composition usually matter more than the transmitter specification.
Request a quote
To get the LRV, URV and span code from us, send the 0% and 100% points and the tap and transmitter elevations. Add the liquid SG range and temperature, the tank pressure, and whether the vapor condenses. Tell us the application and we configure one unit, not a shelf part. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.