All About Manometers – What They Are and How They Work

In the realm of precision instrumentation, manometers emerge as indispensable tools designed for the precise measurement of pressure, gauging the force exerted by gases or liquids per unit surface area. This article embarks on a comprehensive journey through the intricate world of manometers, unraveling their diverse types, operational mechanisms, applications, and the critical considerations involved in their calibration. From the familiar sphygmomanometer used in medical settings to the nuanced analog and digital variants, each type of manometer is dissected to provide a thorough understanding of its construction and functionality. Delving into pressure definitions, the elucidates fundamental concepts, including absolute, gauge, and differential pressure, laying the groundwork for a nuanced exploration of these vital instruments, are covered.

This article will describe the different types of manometers, explain how they work, present their applications, and discuss correction factor considerations used for manometers.

What is a Manometer?

A manometer is a precision instrument that is used to measure pressure, which is the force exerted by a gas or liquid per unit surface area owing to the effects of the weight of that gas or liquid from gravity. Depending on the type and how they are configured, manometers can be set-up to provide a measurement of different pressure values. A common type of manometer with which most people are familiar is the one that physicians and medical professionals use to measure and monitor a patient’s blood pressure. This type of manometer is called a sphygmomanometer.

What Does a Manometer Measure?

Pressure Definitions

It is useful to review a few basic principles that relate to pressure. Pressure is a measure of the amount of force (F) that is exerted per unit area (A):

The unit of measure for pressure is, therefore, a force value divided by a squared distance value. In metric units, the unit a measure for pressure is Newtons/(meter)², known as a Pascal (Pa). Other common pressure units of measure include pounds per square inch (psi), millibars, atmospheres (atm), millimeters of mercury (mm Hg), and inches of water (in H₂O).

Pressure can be represented in terms of three specific categories:

Absolute pressure
Gauge pressure
Differential pressure

Absolute Pressure

Absolute pressure measures the value of pressure that is exerted relative to the absolute zero pressure of a vacuum. Gauge pressure is presenting the difference between the measured value of pressure and the local atmospheric pressure (think in terms of a tire pressure gauge). Differential pressure is used to describe making a measurement that is the difference between two (unknown) pressure levels, where there is not a reference pressure being specified, but measuring the amount of pressure by which the two differ is still important.

Therefore, total or absolute pressure can be defined in terms of gauge pressure and atmospheric pressure as follows:

Types of Manometers

Manometers can be broadly classified as being of two principal types, analog manometers and digital manometers, each of which are discussed below.

Analog Manometers and How They Work

Analog manometers make use of a fluid that is contained in a U-shaped tube and operates using the principle of Hydrostatic Balance. The fluid in the tube will settle to equal height in each leg of the tube when both ends are open to atmospheric pressure. But if positive pressure is applied to one of the legs of the U-shaped tube, then the level of liquid will fall in that leg and rise in the other leg. This is because the pressure will force the fluid to fall in the one leg and rise in the other until the weight of the column of fluid that results from the applied pressure is enough to oppose that pressure value. Hence, the vertical distance between the level of the fluid in the two legs of the tube represents a measure of the amount of pressure being applied. These common types of analog manometers are referred to as U tube manometers. The pressure value (P) being observed is a function of the height (h) and density (ρ) of the fluid used in the manometer, the value (g) representing the gravitational constant.

Another type of analog manometer is the well type manometer, sometimes referred to as a cistern manometer. The well type manometer is like the U tube style, the difference being that one of the legs of the U has a cross-sectional area that is much larger than that of the second leg. This arrangement results in a smaller movement of the fluid level in the larger leg when exposed to pressure, effectively allowing the use of a single scale to read to obtain the pressure value, as opposed to two scales in the U tube style.

Inclined Manometers as the name implies are designed with a tube that does not sit vertically, but rather at a shallow angle relative to the horizontal plane. This design allows a relatively small amount of pressure change to be observed by the instrument, thus offering improved sensitivity and resolution.

Another type of manometer is called an absolute manometer. Absolute manometers use a sealed leg that permits only one leg of the manometer tube to be exposed to the outside pressure. On the sealed side, a vacuum condition exists which represents absolute zero pressure sealed by a column of mercury. The manometer is, therefore, measuring absolute pressure rather than gauge pressure or differential pressure. This type of manometer can be either the well style of U tube style described above. Mercury barometers that measure the atmospheric pressure are a common example of an absolute manometer.

Various fluids are used in analog manometers. Common fluids are shown in Table 1, below, which are sometimes referred to as manometric fluids. By changing the fluid used, the accuracy, range, and sensitivity of the analog manometer can be varied. Fluids with densities higher than water provide higher ranges but lower resolutions. Similarly, lowering the density of the manometric fluid, also called the indicting fluid, will decrease the pressure range but increase its sensitivity.

Table 1 – Examples of Indicating Fluids for Use in Manometers

Indicating Fluid	Temperature range	Specific Gravity*
High Purity Mercury	-30^oF – 200^oF	13.54 @ 71.6^oF
Red Oil #827	40^oF – 120^oF	0.827 @ 60^oF
Red Unity Oil # 100	30^oF – 100^oF	1.00 @ 73^oF
Green Concentrate #1000	40^oF – 120^oF	1.000 @ 55^oF
Acetylene Tetrabromide	40^oF – 100^oF	2.95 @ 78^oF
Dibutyl Phthalate	20^oF – 150oF	1.04 @ 80^oF

Digital Manometers and How They Work

Digital manometers can be used to check gas leaks and faults in air conditioning systems. Also known as electronic manometers, digital manometers do not rely on Hydrostatic Balance of fluids to determine pressure. Instead, they contain a pressure transducer, a device that can convert an observed pressure level into an electrical signal whose characteristic value is proportional to, or a proxy for, the magnitude of the pressure. The elastic portion of the transducer deflects under pressure and that deflection is then converted to a value of an electrical parameter which can be detected and calibrated to a pressure reading. Pressure transducers typically make use of one of three types of electrical parameters – resistive, capacitive, or inductive.

Resistive transducers result in the deformation changing the electrical resistance of a strain gauge.
Capacitive transducers rely on changes to the value of capacitance observed resulting from the deformation changing the relative position of the two plates of a capacitor.
Inductive transducers use the deformation of the elastic portion to alter the linear motion of an attached ferromagnetic core within a coil or inductor. This movement varies the induced emf and AC current generated in the coil.

To perform measurements on very low pressures, there are additional types of pressure transducer styles used, including a Pirani gauge, thermocouple type transducer, and ionization gauge. Low-pressure manometers are also called micromanometers.

Digital manometers some advantages over analog models. Digital manometers:

Are portable in size, weigh less, and feature easy to read displays.
Can interface with a computer or programmable logic controller (PLC).
Do not rely on the use of manometric fluids, some of which (mercury, for example) can be toxic.
Are not subject to issues relating to fluid properties that can impact the accuracy of measurements.
Can correct for deviations from standard conditions via software programming.

As they are not a primary standard, however, they do require periodic calibration against a primary standard.

Fluid Property Corrections Applicable to Manometers

Analog manometers that rely on the properties of fluids are subject to the need for corrections. The density of fluids is not constant with temperature and the gravitational field strength varies as a function of both the elevation above sea level and the latitude. These facts mandate the use of correction methodologies and the need to establish standard references so that a definition of pressure can be established and agreed upon. Reference 5 below contains a complete explanation of the methodologies that apply to these corrections, which are only briefly presented here.

Correction for Fluid Density – adjusts for the fact that the density of the indicating fluid is not constant with temperature
Correction for Gravitation Field – adjusts for the variation in the strength of the gravitational field at a given altitude and latitude, relative to its value at sea level and 45.54^oN lattitude
Correction for Pressure Head – adjusts for the differential between the fluid column density and that of the pressure medium of the same height
Correction for Scale Changes – adjusts for the fact that the marked scale gradations will change their separation distance due to the change in the temperature at which the pressure reading is performed (this due to the thermal expansion/contraction of the material from which the scale is constructed)
Correction for the Compressibility of Fluids – this correction is mainly applicable at higher pressures wherein the fluid density may change because of the fluid’s compression
Other corrections – these include the absorption of gas by the fluid which can alter its density, as well as the capillary effect which impacts how the reading is interpreted from the scale

How Manometers Are Used

Manometers are used in a variety of industries and can measure pressure and flow rate. Common uses include:

HVAC systems maintenance
Meteorological and weather conditions monitoring
Gas pressure monitoring in piping systems
Fluid flow measurements
Physiological measurements such as blood pressure
Monitoring compressor systems operations

Summary

This article presented a brief review of manometers and how they work. For information on other products, consult our additional guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products.