Pressure Measurement

High-Pressure Sensors


For the purposes of this section, we will define high-pressure instruments as devices that are capable of measuring pressures in excess of 10,000 to 20,000 PSIG (700 to 1,400 bars)

One might group these sensors by other characteristics, such as:
1. Mechanical, such as pressure repeaters, helical bourdon tubes, or dead weight testers
2. Electronic, like the strain gauge devices
3. Very high pressure detectors, as the bulk modulus and the Manganin cells.


Dead-Weight Piston Gauges

As illustrated in the Figure, these are piston meters in which the test pressure is balanced against a known weight that is applied to a known area of ​​the piston. The test pressure is applied by the secondary piston. The main purpose of these
Free piston calipers are a primary standard for calibrating other pressure sensors. The National Standards Office (NBS) has been using these devices for many years. Piston gauges, or deadweight testers, are normally provided
with several interchangeable piston assemblies and weights certified by NBS. They can be used to calibrate at pressure levels as low as 5 PSIG (35 kPa) or as high as 100,000 PSIG (690 MPa). The range has been extended to
even higher pressures, but research on the piston and cylinder material and its treatment to withstand loads is a limitation. Assuming that you want to generate a pressure of 100,000 PSIG while keeping the dead weight under 1000 lb (450 kg), it is necessary to reduce the piston area to 0.01 in2 (6.4 mm2).

This means that a 0.1 in. (2.5 mm) diameter piston will have to support a 1000 lb weight, while also being rotated. The accuracy of dead-weight piston testers has improved over the years. For higher pressure services, the main improvement resulted from controlling the piston-cylinder clearance by pressurizing the outside surface of the cylinder. Thus, the piston-cylinder clearance is kept constant, resulting in a slow rate of fall for the piston unaffected by pressure level. The laboratory piston gauges are standardized by NBS, calibrating the associated weights and measuring the piston diameter. NBS has found these dead-weight testers to be inaccurate to 1.5 parts in 10,000 of the measured pressure at values greater than 40,000 PSIG (280 MPa) and to 5 parts in 100,000 at lower pressures. The inaccuracy of industrial dead weight testers is better than ±0.1% of span. The free-piston gauge is limited to its principal purpose, a primary standard for calibrating other pressure sensors, because it is slow in response and is not practical for direct industrial installation.

Helical Bourdon tube

The helical elements used in this instrument are available with spans up to 0 to 80,000 PSIG (0 to 550 MPa) and can detect pressures with an error of about 1% of span.

Do you want to know more about bourdon tubes


These cells, shown in Figure , are comprised of a hollow cylindrical steel probe closed at the inner end, and a stem that
projects beyond the outer end of the probe. When subjected to process pressures, the active part of the probe contracts
isotropically, causing its tip to be displaced to the right. As a result, the stem moves outward, increasing the distance it
projects beyond the outer end. The stem motion can be detected by electromagnetic pickup, capacitance pickup, or the use of mechanical displacement transmitters (pneumatic or electronic). The unit is available with ranges of 0–50,000 to 0–200,000 PSIG (0–350 to 0–1,400 MPa), and its inaccuracy is ±1 to 2% of full scale. Its advantages, when compared with other high pressure sensors, include its relatively fast response, its remote-reading characteristic, and its design that is absolutely

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