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A pressure sensor responds to a difference: its diaphragm deflects because the pressure on one side is not the same as the pressure on the other. What distinguishes a gauge, absolute or differential transmitter is what sits on that second side.
Three references, one principle
- Gauge pressure
- Measured against local atmospheric pressure. The reference side of the sensor is vented, so a gauge transmitter open to air reads zero. Usually written with a g suffix — bar g or psig.
- Absolute pressure
- Measured against a vacuum sealed behind the sensing element. Open to air, an absolute transmitter reads the local barometric pressure — about 1.013 bar a at sea level under standard conditions.
- Differential pressure
- The difference between two process pressures applied to a high-pressure (H) and a low-pressure (L) port. Neither side needs to be atmosphere, and the line pressure on both sides can be far larger than the difference being measured.
- Sealed gauge
- Referenced to a fixed pressure sealed inside the sensor, usually close to one standard atmosphere. Used where a vent is impractical — for example where moisture could enter through it — typically on higher ranges, where barometric variation is negligible.
- Compound range
- A gauge range spanning vacuum and positive pressure, such as −1 to 3 bar g. It is still gauge-referenced, so the vacuum part of the reading moves with the barometer.
Gauge pressure is a special case of differential pressure with the atmosphere as the low side — which is why a DP transmitter with its L port vented can measure gauge pressure, while a gauge transmitter cannot measure between two process points.
Gauge or absolute: when the difference matters
The two are related by one line of arithmetic: absolute pressure = gauge pressure + local atmospheric pressure. The atmospheric term is not a constant. Near sea level it falls by roughly 12 mbar for every 100 m of altitude, and weather moves it by tens of millibar over a few days. Whether that matters depends on how large it is compared with the pressure being measured.
Engineering note
Worked example — vacuum service
A vacuum dryer operates at 100 mbar a. With the barometer at 1013 mbar, a gauge transmitter reads −913 mbar g. A weather front lowers the barometer to 990 mbar while the dryer is unchanged, and the gauge transmitter now reads −890 mbar g. The display has moved by 23 mbar — 23 % of the actual absolute pressure — with no change in the process. An absolute transmitter reads 100 mbar a throughout.
At high pressure the same barometric swing is irrelevant: on a 100 bar g hydraulic system, 23 mbar is about 0.02 % of the reading. Gauge pressure is also the physically meaningful quantity there, because pipe walls, vessels and tools respond to the pressure above their surroundings.
The practical rule: specify gauge when the question is “how far above ambient?” — pump discharge, compressed-air headers, hydraulic and utility systems, open-tank hydrostatic level. Specify absolute when the physics depends on absolute pressure — vacuum processes, boiling point and vapour pressure, gas density for flow compensation, and barometric measurement.
Differential pressure: two process pressures, one measurement
A differential pressure (DP) transmitter measures the difference between its H and L ports with a single sensing element. It is one of the most versatile instruments in a plant, because many process quantities appear as a pressure difference:
- Flow — across an orifice plate, venturi, flow nozzle or averaging pitot tube. The differential rises with the square of flow, so flow is proportional to √ΔP.
- Level — the hydrostatic head of a liquid column, with the L port vented on an open tank or connected to the vapour space on a closed one.
- Filter and strainer condition — the pressure drop grows as the element loads, giving a maintenance trigger based on condition rather than on time.
- Equipment performance — pressure drop across heat exchangers, packed columns and valves, where a change points to fouling, flooding or blockage.
- Room pressure cascades — differentials of a few pascals to a few tens of pascals between cleanroom grades.
Two ratings on a DP transmitter have nothing to do with its differential span. The static pressure rating is the line pressure the body can carry with equal pressure on both sides — a transmitter measuring 250 mbar across an orifice may sit on a 40 bar g line. The one-sided overpressure rating is what the sensor tolerates when full line pressure reaches one port only, as can happen during commissioning or when an impulse line blocks. A three- or five-valve manifold with an equalising valve, operated in the correct sequence, helps prevent that situation. Static pressure can also shift the zero and span slightly; datasheets list this static pressure effect separately.
Why not subtract two gauge readings?
It is tempting to measure a pressure drop with two gauge transmitters and subtract. The arithmetic shows why that rarely works for small differentials.
Step 1: The duty
A filter on a 10 bar g line has a clean pressure drop of 200 mbar. Maintenance is wanted at 500 mbar.
Step 2: Two gauge transmitters
Each is ranged 0–16 bar g to cover the line pressure. Assume an installed error of ±0.25 % of span for each: ±40 mbar. The two errors are independent, so the calculated difference carries about ±57 mbar (root-sum-square), and up to ±80 mbar in the worst case.
Step 3: One DP transmitter
Ranged 0–1 bar across the filter, with the same ±0.25 % of span: ±2.5 mbar, plus a small static pressure effect given on its datasheet.
Step 4: The result
Against a 200 mbar clean drop, the subtracted value is uncertain by roughly ±28 % and the DP measurement by about ±1.3 %. Only the second can reliably trend filter loading.
Two separate sensors remain the right answer where the taps are too far apart for impulse lines or capillaries — along a pipeline, or on very tall vessels. In those cases, range each sensor as tightly as the duty allows and treat the calculated difference with the uncertainty it actually has.
Choosing the reference
| Measurement need | Specify | Why |
|---|---|---|
| Pump discharge, compressed-air header, hydraulic system | Gauge | What matters is the pressure above the surroundings — the load on pipe, vessel or tool. |
| Vacuum distillation, vacuum drying, condenser vacuum | Absolute | Boiling point and process behaviour depend on absolute pressure; gauge readings drift with the barometer. |
| Density compensation for gas or steam flow | Absolute | Density is a function of absolute pressure — for a gas, proportional to it at a given temperature and compressibility. |
| Level in an open or vented tank | Gauge, or DP with L port vented | Atmospheric pressure acts on the liquid surface and the sensor reference alike, and cancels. |
| Level in a closed or pressurised tank | Differential | The L port connected to the vapour space cancels the vessel pressure. |
| Flow through an orifice, venturi or averaging pitot | Differential | Flow is derived from the pressure drop across the primary element. |
| Filter, strainer or heat-exchanger fouling | Differential | The quantity of interest is a pressure drop that is small relative to line pressure. |
| Cleanroom or containment pressure cascade | Differential (low range) | Differences between rooms are a few pascals to a few tens of pascals. |
Installation details that decide the result
A correctly specified transmitter can still read wrongly if its connection to the process is poor. For impulse lines, the aim is to keep each line full of a single phase:
- Liquids — take taps from the side of a horizontal pipe, not the top (gas collects there) or the bottom (sediment collects there), and mount the transmitter below the taps so bubbles rise back into the process.
- Gases — take taps from the top of the pipe and mount the transmitter above them so condensate drains back.
- Steam — mount the transmitter below the taps with condensate-filled legs, which keep live steam temperature away from the sensor. In DP service both legs must be filled to the same height, or the difference appears as a zero error.
- Freezing, plugging and hot media — heat-trace or insulate lines that can freeze, and use diaphragm seals where the process is viscous, crystallising, corrosive, hygienic or too hot for the transmitter.
Diaphragm seals trade one set of problems for another: their fill fluid expands with temperature and adds a fixed head wherever seal and transmitter are at different heights. On DP systems, keep capillaries equal in length, routed together and as short as practical.
Engineering questions
- Is differential pressure the same as gauge pressure?
- Gauge pressure is a special case of differential pressure in which the low side is the atmosphere. A DP transmitter with its low-pressure port vented measures gauge pressure, but a gauge transmitter cannot measure the difference between two process points.
- When should I specify an absolute pressure transmitter?
- Whenever the process depends on absolute pressure or the barometric variation is significant compared with the measurement: vacuum service, boiling point and vapour pressure, gas density compensation for flow, and barometric measurement. For most positive-pressure utility and process duties, gauge is the right reference.
- Can a differential pressure transmitter measure tank level?
- Yes. Level is calculated from the hydrostatic head as h = ΔP / (ρ·g), so the liquid density must be known. On an open tank the low side is vented; on a closed tank it is connected to the vapour space through a dry leg, a wet leg or a diaphragm seal to cancel the vessel pressure.
- What does the static pressure rating of a DP transmitter mean?
- It is the maximum line pressure the transmitter body can carry when the same pressure acts on both ports. It is separate from the one-sided overpressure rating, which applies when only one port sees line pressure, and from the static pressure effect, which is the small zero and span shift the line pressure causes.