One of the earliest pressure measuring instruments is still in wide use today because of its inherent accuracy and simplicity of operation. It's the U-tube manometer, which is a U-shaped glass tube partially filled with liquid. This manometer has no moving parts and requires no calibration. Manometry measurements are functions of gravity and the liquid's density, both physical properties that make the U-tube manometer a NIST standard for accuracy.
Manometers are both pressure measurement instruments and calibration standards. They range from simple U-tubes and wells filled with liquid to portable digital instruments with a computer interface.
As shown in Figure 1, with each leg of a U-tube manometer exposed to the atmosphere, the height of liquid in the columns is equal. Using this point as a reference and connecting each leg to an unknown pressure, the difference in column heights indicates the difference in pressures (see Figure 2).
Figure 1. With both legs of a U-tube manometer open to the atmosphere or subjected to the same pressure, the liquid maintains the same level in each leg, establishing a zero reference.
Figure 2. With a greater pressure applied to the left side of a U-tube manometer, the liquid lowers in the left leg and rises in the right leg. The liquid moves until the unit weight of the liquid, as indicated by h, exactly balances the pressure.
The fundamental relationship for pressure expressed by a liquid column is:
|Δp = P2-P1 = ρgh||(1)|
|Δp||= differential pressure|
|P1||= pressure at the low-pressure connection|
|P2||= pressure at the high-pressure connection|
|ρ||= density of the indicating fluid (at a specific temperature)|
|g||= acceleration of gravity (at a specific latitude and elevation)|
|h||= difference in column heights|
The resulting pressure is the difference between forces exerted per unit of surface area of the liquid columns, with pounds per square inch (psi) or newtons per square meter (pascals) as the units. The manometer is so often used to measure pressure that the difference in column heights is also a common unit. This is expressed in inches or centimeters of water or mercury at a specific temperature, which can be changed to standard units of pressure with a conversion table.
All pressure measurements are differential. The reference can be zero absolute pressure (a total vacuum), atmospheric pressure (the barometric pressure), or another pressure. With one leg of a manometer open to the atmosphere (see Figure 3A), the measured pressure is that which exceeds atmospheric pressure, which at sea level is 14.7 psi, 101.3 kPa, or 76 cmHg.
Figure 3. Gauge pressure is a measurement relative to atmospheric pressure and it varies with the barometric reading. A gauge pressure measurement is positive when the unknown pressure exceeds atmospheric pressure (A), and is negative when the unknown pressure is less than atmospheric pressure (B).
This measurement is called gauge pressure, and the relationship for a positive pressure is expressed by:
|absolute pressure = atmospheric pressure + positive gauge pressure||(2)|
For a negative pressure (vacuum) measurement (see Figure 3B), the column heights reverse and the relationship is expressed by:
|absolute pressure = atmospheric pressure + negative gauge pressure||(3)|
These pressure relationships are shown in Figure 4.
Figure 4. A graphical representation of positive and negative gauge pressure shows the differential aspect of all pressure measurements, where gauge pressure is the difference between absolute pressure and atmospheric pressure.
A manometer can be designed to directly measure absolute pressure. The manometer in Figure 5 measures the pressure compared to zero absolute pressure in a sealed leg above a mercury column. The most common form of this manometer is the conventional mercury barometer used to measure atmospheric pressure. With just one connection, this configuration can measure pressures above and below atmospheric pressure.
Variations on the U-Tube Manometer
The differential pressure is always the difference in column heights, regardless of the size or shape of the tubes. As shown in Figure 6A, the legs of both manometers are open to the atmosphere and the indicating fluids are at the same level. Connecting the same pressure to the left leg of each manometer causes its level to lower. Because of the variation in volume in the manometer legs, the fluid in each column moves a different distance. However, the difference between the fluid levels in both manometers is identical (see Figure 6B).
Figure 6. The pressure reading is always the difference between fluid heights, regardless of the tube sizes. With both manometer legs open to the atmosphere, the fluid levels are the same (A). With an equal positive pressure applied to one leg of each manometer, the fluid levels differ, but the distance between the fluid heights is the same.
Carrying this variation in tube sizes further is the well-type (or reservoir) manometer (see Figure 7). As pressure is applied to the well, the level falls slightly as compared to the level rise in the column. By compensating the column's scale graduations to correct for the well drop, it is possible to make a direct reading of differential pressure. There are connection guidelines placed on well-type manometers, compared to the U-tube style:
- Connect pressures higher than atmospheric to the well; connect pressures lower than atmospheric to the tube.
- For differential measurements, connect the higher pressure to the well.
- For raised-well manometers, the well connection can be used for gauge and vacuum measurements.
A variation of the well-type manometer is the inclined-tube (or draft gauge) manometer in Figure 8. With an inclined indicating tube, 1 in. of a vertical rise is stretched over several inches of scale length. The inclined-tube manometer has better sensitivity and resolution for low pressures.
Figure 8. Low pressure and low differentials are better handled with an inclined-tube manometer, where 1 in. of vertical liquid height can be stretched to 12 in. of scale length.
Liquid manometers measure differential pressure by balancing the weight of a liquid between two pressures. Light liquids such as water can measure small pressure differences; mercury or other heavy liquids are used for large pressure differences. For an indicating fluid 3 times heavier than water, the pressure measurement range is 3 times greater, but the resolution is reduced.
Indicating fluids can be colored water, oil, benzenes, bromides, and pure mercury. When selecting an indicating fluid, check the specifications for specific gravity, operating temperature range, vapor pressure, and flash point. Corrosive properties, solubility, and toxicity are also considerations.
A liquid manometer has limitations. Glass tubing, indicating fluids, and level mounting requirements are more suited to a laboratory than the field. Also, it cannot be interfaced with a computer or PLC. Such limitations can be overcome with digital manometers. These microprocessor-based instruments are available in convenient, portable sizes for ease of use in the field, or in panel or stand-alone mounting styles, with outputs for controlling a process or transferring measurement data.
Variations from standard conditions of density and gravity must be compensated for manually when making pressure measurements with liquid manometers. This is easier with digital manometers, because some of the correction factors for liquid manometers can be ignored and others can be compensated for in software.
With dual ports, swapping sensors is all that is needed to change among differential, gauge and absolute pressure measurements.
Other common features of digital manometers include:
- Onboard memory for data logging or storing min./max. readings
- Averaging a number of readings to dampen pressure pulses
Higher accuracy digital manometers are used to calibrate pressure transmitters and other pressure instrumentation in the field. Digital calibrators are faster and simpler as they require no boxes, gas cylinders, regulators, or weights to set up and have no special platforms or critical leveling requirements. Further comparisons of liquid and digital manometer specifications are shown in Table 1.
|Liquid Manometers||Digital Manometers|
|Range||100 in.||100 in.||20 in.||20-2000 in H2O,
|2000 in H2O,
|Accuracy||±½ of minor scale graduation||±½ of minor scale graduation||±½ of minor scale graduation||±0.025-0.1% F.S.||±0.025-0.1% F.S.|
|Cast iron, stainless steel, PVC, glass, Viton||Stainless steel, glass, Viton||Acrylic, stainless steel, aluminum, glass, Viton||Clean, dry non-corrosive gases; liquids compatible with stainless steel||Clean, dry non-corrosive gases; liquids compatible with stainless steel|
|250 psig||250-500 psig||100-350 psig||2 × range||2 × range|
|Mounting||Wall, table||Wall, table, flush front, pipe||Wall, table||Portable||Portable|
For Further Reading
Massey, B.S. 1989. Mechanics of Fluids, 6th Ed., London: Van Nostrand Reinhold.
Meriam Instrument. 1997. Using Manometers to Precisely Measure Pressure, Flow and Level, Cleveland: Meriam Instrument.
Meriam, J.B. 1938. The Manometer and Its Uses. 2nd Ed., Cleveland: Meriam Instrument.
Omega Engineering. 1999. Transactions in Measurement and Control: Force-Related Measurements, 2nd Ed. Stamford, CT: Putnam Publishing and Omega Press.
Yeager, John, and Hrusch-Tupta, M.A., Eds. 1998. Low Level Measurements. 5th Ed. Cleveland: Keithley Instruments.