A closed-end manometer measures absolute pressure against a sealed vacuum (or trapped gas) reference, whereas an open-ended U-tube manometer reads gauge pressure against the local atmosphere, with the relationship P_abs = P_gauge + P_atm [S1][S2].
For a mercury-filled closed-end barometer, atmospheric pressure is balanced by a column of about 760 mm Hg at standard conditions, giving 1 atm = 101,325 Pa = 1.01325 bar [S3][S5]. Choosing between the two is therefore a question of which reference you need: a perfect-vacuum sealed leg, or simply the ambient air.
Reference Side: Sealed Vacuum vs Open Air
In a closed-end manometer the top of one leg is sealed, and if that sealed space is evacuated, the pressure at point C is zero; atmospheric pressure then equals the weight of the liquid column of height h, which is how the device doubles as a barometer [S3]. This is why a sealed mercury column of about 760 mm is the classical absolute-pressure standard at 0 °C with mercury density 13,595 kg/m³ under standard gravity [S3][S5].
In an open-ended U-tube, both legs are exposed to the atmosphere, so fluid levels sit equal at zero differential and any deflection h is read as gauge head ρgh [S1]. Because the open-ended form has no evacuated reference, it cannot distinguish barometric drift from a true process change, which is its central limitation for absolute work [S2].
Measurement Type: Absolute Pressure vs Gauge Pressure
Open-end manometers measure gauge pressure; differential manometers (both legs connected to the same line) measure pressure drop; sealed-end manometers measure absolute pressure [S2]. The naming convention follows reference choice, not the shape of the glass, so a U-tube with a sealed, evacuated leg is still a closed-end absolute manometer.
For comparison, a Bourdon gauge is also a gauge-pressure device calibrated to read zero at atmospheric pressure, with the C-shaped tube straightening as pressure rises [S2]. Absolute pressure can never be negative because fluids push rather than pull, while gauge pressure can swing from about −1 atm (full vacuum) to arbitrarily high positive values [S1]. When you need P_abs at a point in a process, the only two of these classical instruments that deliver it directly are the closed-end manometer and the barometer [S1][S3].
Decision Matrix: Five Selection Criteria
Reference quality: closed-end with hard vacuum gives the cleanest absolute reference; open-end is only as stable as local atmospheric pressure [S3].
Range and resolution: mercury columns typically span 0–760 mm Hg (0–101.325 kPa) in a standard barometer length; water columns give about 13.6× more deflection per unit pressure, useful for low-pressure drafts and furnace pressure checks but impractical above a few kPa [S3][S5].
Process compatibility: mercury is dense and gives short columns, but is toxic and being phased out of many plants; water, oil, and low-density manometer fluids (SG 0.8–1.0) are common in clean service [S3].
Speed and skill: a U-tube is a liquid balance with no electronics, no power, and no calibration drift, which is why the open-ended form is still the first tool a reliability engineer reaches for on a walk-down [S4].
Absolute vs gauge need: if your downstream calculation uses P_abs, for example a vapor mass balance or a gas-density correction, specify a closed-end with a verified vacuum; if you only need to confirm a line is holding 50 kPag, an open-end U-tube is sufficient and far cheaper to maintain [S1][S2].
Who Each Type Is For
Closed-end absolute manometer is the right call for laboratory barometry, vacuum-gauge calibration, weather-station reference standards, and any low-pressure absolute measurement where the process fluid must be referenced to a hard vacuum rather than a swinging barometer [S3][S5].
Open-ended U-tube manometer is for field and shop-floor gauge-pressure work: filter ΔP, pump suction, condenser back-pressure, gas-burner manifold pressure, and any draft or low-ΔP HVAC check where the absolute value is irrelevant [S1][S2][S4]. If your manometer selection can tolerate the local atmosphere as the reference, an open-end U-tube is the lowest total-cost-of-ownership option on the bench.
Failure Modes and Practical Limits
Closed-end instruments lose their reference if the sealed leg develops a leak, admits vapor, or outgasses from the manometer fluid itself, after which readings drift toward gauge-style behaviour and become useless as an absolute standard [S3]. Mercury vapor pressure at room temperature is small but non-zero, which is why precision barometers correct for temperature and local g, and why the closed-end mercury column is treated as a primary standard rather than a working gauge [S5].
Open-ended U-tubes are vulnerable to parallax when read by eye, to meniscus misreading on dirty fluids, to density errors if the working fluid is water-miscible, and to ambient-pressure swings of typically ±2 kPa during storm passages, which directly bias the gauge reading by the same amount [S1][S2]. For higher accuracy, an absolute pressure transmitter with a vacuum reference diaphragm replaces the fluid column entirely, at the cost of electronics and recalibration.
Comparison Against Adjacent Technologies
Versus a Bourdon gauge: both are simple, both read gauge pressure, but the manometer needs no moving metal, no gears, and no pointer, and trades mechanical wear for fluid-cleanliness discipline [S2]. Versus an absolute pressure transmitter: the transmitter gives a 4–20 mA or digital output and a sealed vacuum reference, but requires power, calibration, and intrusion into the process; the closed-end manometer needs none of these and is the better primary reference. For flow work where differential pressure is the goal, the U-tube is functionally identical to a liquid-column primary element upstream of an open channel flowmeter calculation.
Versus digital deadweight testers and precision pressure transmitters: the closed-end mercury column is still listed by national labs as a primary absolute standard, while field manometers of either type sit firmly in the working-gauge tier [S3][S5]. Pick the manometer for verification, redundancy, and zero-power measurement; pick the transmitter for control, logging, and Hazardous Area compliance. Related decision frameworks for other process comparisons are covered in pieces such as the clamp meter vs multimeter selection guide and the PROFINET vs EtherCAT industrial Ethernet piece, which use the same criteria-based approach.
Standards, Units, and Sourcing
The governing relations are P_abs = P_gauge + P_atm and P_abs = P_atm − P_vac, with the standard atmosphere defined as 101,325 Pa, equivalent to 760 mm Hg at 0 °C, ρ_Hg = 13,595 kg/m³, and standard gravity [S3][S5]. The SI unit is the pascal (1 N/m²), with bar (10⁵ Pa) and kg/cm² (9.807 × 10⁴ Pa) common in European and Asian plant practice respectively [S5].
For sourcing, primary references on closed-end absolute measurement cite the LibreTexts University Physics chapter, the UBC CHBE pressure module, the UTM Manometers page, the Accendo Reliability field article, and the University of Mustansiriyah Chapter 2 lecture PDF, all of which appear in the research material and are independent of any single vendor [S1][S2][S3][S4][S5]. For a working specification, document the manometer fluid, fluid density, column range, leg sealing method, and required reference type (vacuum vs ambient) on the datasheet before purchase.
Trackable signals to watch: tightening mercury restrictions in plant safety programs, which pushes field manometer fluids toward water, oil, and low-SG liquids; growth of digital pressure transmitter adoption that erodes the manometer's role in control loops; and continued use of closed-end mercury columns as primary absolute references in calibration labs.