A differential pressure (DP) transmitter measures level by detecting the hydrostatic head pressure created by the liquid column above a reference point. It does not measure level directly. It measures a pressure difference, and the control system converts that reading into a level value using the known specific gravity of the process fluid.

This method works in both open and closed tanks, with or without remote seals, across almost any process liquid. The transmitter itself does not change between applications — what changes is how it is connected and how its range is set.

The basic principle

Hydrostatic pressure follows a simple relationship: pressure equals the height of the liquid column multiplied by the specific gravity of that liquid. At the bottom of a tank holding 1,000 mm of water (SG = 1.0), the hydrostatic head is 1,000 mmH₂O. At the bottom of the same tank filled with crude oil (SG = 0.85), the head at the same level is 850 mmH₂O.

A DP transmitter has a high-pressure port and a low-pressure port. The difference between what acts on each port is what the transmitter reports. Everything about level measurement with a DP transmitter follows from that single fact.

Open tank measurement

In an open tank, the low-pressure port is vented to atmosphere. Only the liquid head acts on the high port. At minimum level (0%), the differential pressure is 0. At maximum level, the differential pressure equals the head of liquid at full span.

If the tank is 2,000 mm tall and the process fluid has a specific gravity of 0.9, the transmitter range is set from 0 mmH₂O (0% level) to 1,800 mmH₂O (100% level). The transmitter outputs 4 mA at empty and 20 mA at full.

Transmitter position can affect this calculation if the high-pressure impulse line runs from the bottom of the tank down to the transmitter and is filled with process fluid. In that case, the static head of fluid in the impulse line must be accounted for in the lower range value (LRV).

Closed tank: dry leg

In a closed tank, the vapor space above the liquid is not at atmospheric pressure. The low-pressure port connects to the top of the tank through an impulse line filled with gas or vapor — this is the dry leg. Because gas has negligible density, it contributes no additional head to the low-pressure side.

The calculation is the same as an open tank: the transmitter sees process fluid head on the high side and near-zero pressure on the low side. Closed tanks with dry legs give a positive calibration range.

Closed tank: wet leg

When the process fluid condenses or when the vapor cannot be used in the low-side impulse line, the line is filled with a reference liquid — typically glycerin, silicone oil, or another compatible fluid. This is the wet leg.

The wet leg puts a constant, known head on the low-pressure port at all times. At 0% level, this head is higher than the process fluid head on the high side, giving a negative differential pressure. At 100% level, the difference narrows but typically remains negative. The transmitter is ranged to this negative span, with the LRV being the larger negative number and the URV being the smaller one.

When you see a transmitter ranged with LRV greater than URV in absolute terms on a closed tank, a wet leg on the low side is almost always the reason.

Remote seal (diaphragm seal) systems

When process conditions make impulse lines impractical — high-viscosity fluids, slurries, highly corrosive media, or applications where lines would plug — remote diaphragm seals replace the impulse lines entirely. The seal diaphragm mounts directly at the process connection; capillary tubing filled with fill fluid connects it to the transmitter.

With dual remote seals (one at the bottom tap, one at the top tap), transmitter elevation relative to the process connection does not affect the calibration. The same LRV and URV apply regardless of where the transmitter is mounted. This is one of the main practical advantages of dual-seal arrangements.

The fill fluid specific gravity and the elevation difference between the two seals do affect the calculation. A seal system filled with glycerin (SG ≈ 1.26) between taps 1,000 mm apart puts roughly 1,260 mmH₂O of constant head on whichever side it occupies, and this must be factored into the range.

What the transmitter accuracy means for level applications

Transmitter accuracy becomes a level accuracy only after accounting for specific gravity uncertainty. If the process fluid specific gravity varies by ±0.01 from the calibrated value, that variation directly adds to the level error, independent of the transmitter’s own accuracy specification.

On a 2,000 mm water tank calibrated at SG = 1.0, a ±0.075% FS transmitter accuracy translates to ±1.5 mm of level uncertainty from the transmitter alone. Any SG variation adds on top of that.

This is why long-term stability matters as much as reference accuracy for level applications. A transmitter rated at ±0.075% FS accuracy but with ±0.25% URL drift over 12 months will accumulate level error that exceeds its reference accuracy within a single calibration interval.

Choosing the right transmitter for DP level measurement

The measurement range, process fluid, tank type, and required accuracy determine which transmitter fits. For general liquid level applications in water treatment, HVAC, light chemical processing, and general manufacturing, a diffused silicon piezoresistive transmitter covers most requirements at reasonable cost.

Microcyber’s NCS-PT105IIS series uses diffused silicon piezoresistive sensor technology and is available with HART, Foundation Fieldbus, and PROFIBUS PA communication. Key specifications relevant to level applications:

For applications requiring tighter long-term stability or higher static pressure capability — such as differential pressure level measurement on high-pressure reactors or long-interval calibration cycles — the NCS-PT105IIM monocrystalline silicon series offers ±0.2% URL stability over 5 years.

For differential pressure level measurement enquiries, contact info@microcyber.cn or visit the product pages linked below.