There are many types of instruments used to measure absolute or gauge pressure. Pressure measurement devices can be separated into two categories: mechanical and electronic.
The height of the liquid in a manometer is directly proportional to the hydrostatic pressure . The picture below shows an inclined manometer.
The three major types of manometers are: differential, sealed-end, and open-end.
A differential manometer is different than a sealed-end or an open-end manometer in that the two ends are exposed to different points along the flow path.
The difference in fluid heights yields the pressure difference between the two points. The small difference in liquid heights is more easily measured along the angled line (“L”) than along the vertical stem (“h”).
In sealed-end manometers the open end is open to the fluid of interest, while the closed end encloses a vacuum.
For example, in a barometer, atmospheric pressure, P atm , pushes some of the mercury from the reservoir into and up the glass tube. The height of the liquid column, h, then represents the atmospheric pressure.
In 2007 the European Union banned the use of mercury in barometers due to the high toxicity of mercury. They were followed in 2011 by the United States.
In open-end manometers one arm is exposed to the fluid being monitored, and the other arm is open to the atmosphere. The picture below shows an open-end manometer from a unit operations laboratory. It is actually over six feet tall, so only some parts are shown here.
In this example, the system pressure is greater than atmospheric pressure. The gas in the left arm then pushes some of the liquid in the manometer into the other arm. The resulting height difference h represents the pressure difference between the system and the atmosphere.
When the ends of the manometer tube are exposed to different pressures, the high pressure pushes down on the fluid and the level drops, forcing the fluid level to rise in the low pressure arm. The pressure difference can be determined from the difference in height. Click on the picture below for a hotspot on manometer fluid
On the left the hydrostatic head or static head of a water manometer is used to measure the pressure between the surface water and groundwater. Manometers are also useful in the medical field. For example, the sphygmomanometer to the right is used to measure blood pressure.
- Easy to read, height difference is proportional to pressure
- No moving parts
- Cannot measure high pressures with great accuracy
- Small pressure range
- Contain fluids which can be dangerous when exposed to high temperatures, limiting manometers to low temperature systems
Besides measuring pressure, pitot tubes also measure velocity and are sometimes called velocity gauges or anemometers.
Placement of holes on the pitot tube greatly affects the type of pressure to be measured. For example, the schematic drawing below shows the pitot tube measuring both static and total pressure.
A pitot tube can be placed along the streamline parallel to the moving fluid. At the head of the pitot tube is a tiny opening to allow minimum flow of the moving fluid. The fluid travels up the pressure indicating device, i.e. manometer. The manometer height then reflects the pressure in the moving fluid.
Other variations of the pitot tube, such as the one below, have holes on the sides instead of one in front to measure static pressure. The fluid outside of the tube forces the fluid within the tube up to the indicating device where the pressure can be read.
Below on the left are examples of different head types. Pitot tubes must be placed in the direction of fluid flow, otherwise measurements will be erroneous. Pitot tubes with large pressure holes and bevels inside, shown in the schematic below on the right, are recommended for best results because they are less sensitive to misalignment.
(Reference: Folsom, R.G. (1955). “Review of the Pitot Tube”, University of Michigan Industry Program of the College of Engineering. pg. 36.)
Pitot tubes, such as the one below, are used extensively to measure pressure (or air velocity ) of aircrafts. Various designs are available, depending on aircraft specifications.
The pitot tube from NASA’s X-31 fighter jet is shown below. The jet travels at almost 1,000 mph with an altitude of 40,000 ft, whereas other aircraft only travel at around 250-500 mph.
- Suitable for moving fluid
- Measures both static and dynamic pressure
- Must be placed in the direction of fluid flow, otherwise measurements will be in error
Simplicity and small size makes the bourdon gauge a popular pressure measuring device.
The bourdon pressure gauge is very common and probably the most recognizable piece of pressure measuring equipment. Below are two views of a Bourdon gauge after it has been taken out of its casing.
When fluid enters the shaft, it travels up into the hollow curved bourdon tube. The pressure of the fluid forces the tube to straighten, turning the gears and moving the needle. Pictured below on the right is a close up of the gear mechanism.
The bourdon gauge is the most common pressure equipment because it allows for a wide range of pressure measurements. Notice the many uses of the bourdon gauge on these processes.
- Can be used for a wide range or pressures, from 0 to 7000 atm
- Can be used instead of manometers at extreme pressures and temperatures
- Measurement of pressure at high temperatures may cause deformation in the gauge, resulting in systematic errors
General Information/Equipment Design
There are two types of strain gauges: unbonded and bonded. The unbonded strain gauge consists of two wires connected to a fixed frame and two wires connected to a movable armature.
When pressure is applied to the diaphragm , the armature moves, and two wires stretch while the other two contract. The resulting change in electrical resistance reflects the pressure applied.
In bonded strain gauges, on the other hand, the wire filament is embedded in cloth, paper, plastic, or resin and is mounted onto a flexible plate, as shown above. When pressure is applied to the flexible plate, the filament wires stretch or contract. The resulting change in resistance reflects the pressure applied.
Micro-machined silicon pressure sensors with strain gauges can be used in precision pressure transmitters such as the one below. The gauges are isolated from the pressure source by an external diaphragm. Pressure is transmitted through silicon oil located between the diaphragm and the sensor. Digitization of Data is nothing new; however, getting data from a pressure transmitting device in a usable format can get tricky. Modern innovations have allowed real time data capture, providing room for greater improvement in efficiency and reliability. Smart features that allow for such innovation include ethernet cables to connect the devices to export data straight to a monitoring system, predictable maintenance based on the output of continuous pressure data, and faster response times to alert faulty equipment.