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A pressure instrument consisting of a one-piece, collapsible, seamless metallic unit with deep folds formed from thin-walled tubing
Bellows pressure element
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A pressure instrument consisting of a flattened, thin-walled tube. When pressure is applied inside the tube, the tube tip moves, and through an attached pointer or pen, indicates the pressure applied
Bourdon tube
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A pressure instrument consisting of a metallic diaphragm built into housing
Diaphragm pressure gauge
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A diaphragm or bellows pressure gauge, connected between two points and is used to measure differential pressure
Differential pressure (D/P) gauge
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A differential pressure cell that uses a capacitance type pressure sensor. It converts differential pressure to flow,
level, or pressure
Variable capacitance detector
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EXPLAIN how a bellows-type pressure detector produces an output signal
As the inlet pressure to the instrument varies, the bellows will expand or contract. The moving end of the bellows is connected to a mechanical linkage assembly. As the bellows and linkage assembly moves, either an electrical signal is generated or a direct pressure indication is provided
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EXPLAIN how a bourdon tube-type pressure detector produces an output signal
The tube is permanently fastened at one end. Pressure applied inside of the tube causes distention of the flat sections and tends to restore its original round cross-section. This change in cross-section causes the tube to straighten slightly. The tip of the tube traces a curve that is the result of the change in position with respect to the center.
Within limits, the movement of the tip of the tube can then be used to position a pointer or to develop an equivalent electrical signal to indicate the value of the applied internal pressure
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STATE the three functions of pressure measuring instrumentation
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DESCRIBE the three alternate methods of determining pressure when the normal pressure sensing devices are inoperable
- A spare detector element may be used (if installed).
- A local mechanical pressure gauge can be used (if available).
- A precision pressure gauge may be installed in the system
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STATE the three environmental concerns that can affect the accuracy and reliability of pressure detection instrumentation
- Atmospheric pressure
- Ambient temperature
- Humidity
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LIST the parameters that can be measured using a differential pressure (D/P) cell
D/P cells are used for many applications, such as flow, pressure, and level measurements
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For a D/P sensing instrument, EXPLAIN the purpose and use of an equalizing valve
D/P cells usually have an equalizing valve for removing the instrument from service. The equalizing valve connects both the low-pressure connection and the high-pressure connection across the bellows of the cell. This prevents overloading the measuring element by exposing it to high pressure on one side only when valving in or out of service.
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DESCRIBE instrument response to equalizing valve operation
When the equalizing valve is open, it equalizes pressure across the D/P cell. This produces a minimum differential pressure and yields a low D/P cell output
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EXPLAIN how a strain gauge pressure transducer produces an output signal
A strain gauge measures the pressure applied to a fine wire. The fine wire is usually arranged in the form of a grid. The pressure change causes a resistance change due to the distortion of the wire
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Given a basic block diagram of a typical pressure detection device, STATE the purpose of the following blocks
a. Sensing element
b. Transducer
c. Pressure detection circuitry
d. Pressure indication
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Bellows type differential pressure valve
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Variable Capacitive D/P Cell
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If the spring on a bellows cell weakens over time the gauge will read _____ than when first installed.
higher
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At atmosphere, assume 15 PSI, a gauge reads 50 PSIG the atmosphere increases to 20 PSI. What is the gauge reading?
- 45 PSIG.
- Differential pressure remains the same at 65 PSIA.
- At 15 the DP is 65 50 + 15.
- At 20 the DP is still 65 so the gauge will not read 45
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_________ converts the measurement to an electrical signal or a pneumatic signal that can be routed to an area by an amplifier to be read.
Transducer
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