Pressure Instrumentation Questions and Answers

Correct Answer: B. Force per unit area

Pressure is defined as the normal force acting on a surface divided by the area of that surface. Mathematically, P = F/A. Therefore, if the same force is applied over a smaller area, the resulting pressure is higher.

Correct Answer: A. Pascal

The Pascal (Pa) is the SI unit of pressure. One Pascal is equal to one Newton per square meter (N/m²). Bar, PSI, and Torr are also widely used pressure units, but they are not the SI unit for pressure.

Correct Answer: C. Gauge pressure

Gauge pressure is measured relative to the surrounding atmospheric pressure. If the process pressure is equal to atmospheric pressure, gauge pressure is zero. Positive gauge pressure means the process is above atmospheric pressure, while negative gauge pressure indicates pressure below atmospheric pressure.

Correct Answer: B. Perfect vacuum

Absolute pressure uses a perfect vacuum as its theoretical zero reference. Unlike gauge pressure, it does not use atmospheric pressure as the reference. The relationship is Pabs = Pgauge + Patm.

Correct Answer: D. Atmospheric pressure

Atmospheric pressure is produced by the weight of the air column above a location. At standard sea-level conditions, atmospheric pressure is approximately 101.325 kPa absolute, or about 1.01325 bar absolute.

Correct Answer: C. Gauge pressure

A gauge-pressure transmitter uses atmospheric pressure as its reference. Its sensing element effectively compares process pressure against the atmospheric reference. Changes in atmospheric pressure can therefore affect the absolute pressure represented by a gauge measurement.

Correct Answer: A. Pressure below atmosphere

Negative gauge pressure means that the process pressure is lower than atmospheric pressure. For example, a reading of −0.5 bar(g) means the absolute pressure is approximately 0.513 bar(a), assuming atmospheric pressure is about 1.013 bar(a).

Correct Answer: B. Pressure transmitter

A pressure transmitter senses process pressure and converts it into a standardized electrical output, commonly 4–20 mA. Modern transmitters may also provide digital communication such as HART, FOUNDATION Fieldbus, or other protocols.

Correct Answer: D. 4–20 mA

The 4–20 mA signal is one of the most widely used analog standards in industrial instrumentation. Typically, 4 mA represents the lower range value and 20 mA represents the upper range value. The 4 mA live zero also allows some wiring and device faults to be distinguished from a genuine zero measurement.

Correct Answer: B. Strain gauge

A strain gauge changes its electrical resistance when mechanically stretched or compressed. In pressure transmitters, pressure-induced diaphragm deformation produces strain, and the resulting resistance change is measured electronically to determine pressure.

Correct Answer: C. Wheatstone bridge

A Wheatstone bridge is commonly used with strain gauges because it can accurately detect very small changes in resistance. The strain gauges are arranged in bridge arms, and mechanical deformation produces an imbalance voltage proportional to the applied strain.

Correct Answer: D. It tends to straighten

A Bourdon tube is a curved elastic metal tube. When internal pressure increases, its cross-sectional shape changes and the tube tends to straighten. This mechanical movement is transferred through a linkage and gear mechanism to the pointer of a pressure gauge.

Correct Answer: B. Pressure gauge

A pressure gauge provides a local indication of pressure, commonly using a Bourdon tube, diaphragm, or bellows mechanism. Unlike a transmitter, a conventional pressure gauge does not normally provide a standardized electrical output.

Correct Answer: C. Differential pressure

Differential pressure measurement requires two pressure inputs. The transmitter compares the pressure at the high side with the pressure at the low side and produces an output proportional to their difference: DP = PH − PL.

Correct Answer: A. High and Low

The two process connections of a DP transmitter are normally designated H (High) and L (Low). The transmitter measures the pressure difference between these two connections. Correct connection is important because reversing them can produce an opposite or negative DP indication.

Correct Answer: D. Zero DP

When the pressure applied to the high side and low side is exactly equal, their difference is zero. Therefore, a correctly calibrated DP transmitter should indicate zero differential pressure, normally corresponding to 4 mA when the calibrated range starts at zero DP.

Correct Answer: A. Static pressure

Static pressure is the pressure exerted by a fluid independent of its bulk velocity. In a completely stationary fluid, there is no dynamic component due to velocity, so the pressure is associated with the static or hydrostatic condition.

Correct Answer: C. It increases

Hydrostatic pressure increases with depth because the deeper point supports a taller column of liquid above it. The relationship is P = ρgh, so pressure increases directly with liquid height when density and gravitational acceleration remain constant.

Correct Answer: D. P = ρgh

Hydrostatic pressure is calculated using P = ρgh, where ρ is fluid density, g is gravitational acceleration, and h is the vertical liquid height. This principle is fundamental to DP-based level measurement.

Correct Answer: A. Fluid density

The symbol ρ (rho) represents fluid density, normally expressed in kg/m³. A denser liquid produces greater hydrostatic pressure for the same liquid height because the liquid column has greater mass per unit volume.

Correct Answer: B. Fluid height

The variable h represents the vertical height of the liquid column above the measurement point. Increasing this height increases hydrostatic pressure proportionally, assuming density and gravity remain constant.

Correct Answer: D. It increases

From P = ρgh, pressure is directly proportional to density. Therefore, if the liquid density increases while height remains unchanged, the hydrostatic pressure also increases. This is why density variations can introduce errors in DP-based level measurement.

Correct Answer: B. Hydrostatic head

Hydrostatic head is the pressure generated by the weight of a liquid column. By measuring this pressure with a DP transmitter, the liquid level can be calculated when the liquid density and tank geometry are known.

Correct Answer: C. DP transmitter

A DP transmitter is specifically designed to measure pressure differences between two points. It is widely used for applications such as filter monitoring, flow measurement across an orifice, tank level measurement, and equipment pressure-drop monitoring.

Correct Answer: D. Pabs = Pg + Patm

Absolute pressure is the sum of gauge pressure and atmospheric pressure: Pabs = Pgauge + Patm. For example, 5 bar(g) at approximately 1 bar atmospheric pressure corresponds to approximately 6 bar(a).

Correct Answer: B. Atmospheric pressure

Zero gauge pressure means the process pressure is equal to atmospheric pressure. At standard sea-level conditions, this corresponds to approximately 1.013 bar absolute, not zero absolute pressure.

Correct Answer: C. Perfect vacuum

Zero absolute pressure is the theoretical condition of a perfect vacuum. It represents the lowest possible reference point on an absolute pressure scale.

Correct Answer: A. Diaphragm seal

A diaphragm seal provides a physical barrier between the process fluid and the pressure instrument. It is particularly useful for corrosive, viscous, crystallizing, sanitary, high-temperature, or plugging services.

Correct Answer: B. Fill fluid

A specially selected fill fluid transfers pressure from the flexible diaphragm to the pressure sensor. The fill fluid must be compatible with the application and suitable for the operating temperature.

Correct Answer: D. Transmit process pressure

An impulse line physically connects the process tapping point to the pressure instrument. It transmits process pressure to the sensing element. Incorrect routing, blockage, leakage, or trapped gas/liquid can cause significant measurement errors.

Correct Answer: B. Three-valve manifold

A three-valve manifold typically contains high-side isolation, low-side isolation, and an equalizing valve. It allows the transmitter to be isolated and both sides to be equalized during commissioning or maintenance.

Correct Answer: D. Isolate the transmitter

Block or isolation valves allow the pressure transmitter to be separated from the process. This is essential for maintenance, calibration, troubleshooting, and safe removal of the instrument.

Correct Answer: A. Five-valve manifold

A five-valve manifold typically provides high- and low-side isolation, an equalizing valve, and separate vent/drain functions. This arrangement is particularly useful for commissioning, calibration, and maintenance of DP transmitters.

Correct Answer: B. Correct zero-point error

Zero adjustment changes the transmitter’s output relationship at the lower-range condition. It compensates for an offset so that the transmitter correctly represents the specified lower range value.

Correct Answer: A. Measurement range

Span adjustment changes the difference between the lower and upper calibrated values. For example, a transmitter may be configured for 0–10 bar or 0–20 bar depending on the required application range.

Correct Answer: C. 10 bar

Span is calculated as URV − LRV. Therefore, for a 0–10 bar range, span = 10 − 0 = 10 bar.

Correct Answer: D. 8 bar

The lower range value is 2 bar and the upper range value is 10 bar. Therefore, span = 10 − 2 = 8 bar.

Correct Answer: B. 12 mA

Fifty bar represents 50% of a 0–100 bar range. For a 4–20 mA transmitter, 50% output is 4 + (0.5 × 16) = 12 mA.

Correct Answer: A. 12 mA

Five bar is 50% of the 0–10 bar range. The corresponding current is 4 mA + 50% of the 16 mA span, resulting in 12 mA.

Correct Answer: D. 20 mA

Twenty bar is the upper range value of the transmitter. Therefore, the transmitter should produce the upper standard output of 20 mA.

Correct Answer: B. 8 mA

2.5 bar is 25% of the measurement span. The output is therefore 4 + (0.25 × 16) = 8 mA.

Correct Answer: A. 0 bar

In a standard linear 4–20 mA transmitter, 4 mA represents the lower range value. Since the LRV is 0 bar, 4 mA corresponds to 0 bar.

Correct Answer: D. 10 bar

Twenty mA represents 100% of the calibrated range. Therefore, for a 0–10 bar transmitter, 20 mA corresponds to 10 bar.

Correct Answer: B. Pressure calibrator

A pressure calibrator generates or measures a known reference pressure and is used to verify the accuracy of pressure gauges, switches, and transmitters.

Correct Answer: C. 100 bar

The span is the difference between the upper and lower range values: 120 − 20 = 100 bar. The span is not the same as the upper range value when the LRV is above zero.

Correct Answer: C. It increases

As the liquid level rises, the height of the liquid column above the lower pressure connection increases. This increases hydrostatic pressure and therefore increases the measured DP.

Correct Answer: C. Piezoresistive sensor

A piezoresistive pressure sensor uses semiconductor or resistive elements whose resistance changes when mechanical stress is applied. The pressure-induced diaphragm strain is converted into an electrical signal.

Correct Answer: B. 100 kPa

One bar is exactly 100 kPa. It is a commonly used industrial pressure unit, especially in process plants and instrumentation specifications.

Correct Answer: D. Atmospheric pressure

A gauge-pressure transmitter measures pressure relative to the local atmospheric pressure. Its zero point therefore corresponds approximately to atmospheric pressure rather than perfect vacuum.

Correct Answer: A. Perfect vacuum

An absolute-pressure transmitter uses a vacuum reference. This allows it to measure pressure independently of variations in atmospheric pressure.

Correct Answer: B. Filter blockage monitor

A clean filter normally has a relatively low pressure drop. As the filter becomes blocked, the pressure difference across it increases. A DP transmitter can therefore provide an early indication of filter fouling or blockage.

Correct Answer: C. Isolate the sensor from process fluid

A diaphragm seal separates the pressure sensor from the process. It is useful where the process is corrosive, viscous, hot, sanitary, crystallizing, or otherwise unsuitable for direct connection to the sensing element.

Correct Answer: D. 1 N/m²

The Pascal is defined as one Newton per square meter (N/m²). This definition follows directly from the pressure equation P = F/A.

Correct Answer: C. Diaphragm

The diaphragm is one of the most widely used pressure-sensing elements. Pressure causes the diaphragm to deflect, and this mechanical deformation is converted into an electrical signal by technologies such as capacitive or piezoresistive sensing.

Correct Answer: A. Difference between two pressures

A DP instrument measures the difference between two applied pressures. The fundamental relationship is DP = PH − PL, where PH is the high-side pressure and PL is the low-side pressure.

Correct Answer: A. It deflects

Pressure causes the diaphragm to undergo a small mechanical deflection. Depending on the transmitter technology, this movement may be detected as a change in capacitance, resistance, or another electrical parameter.

Correct Answer: A. Incorrect pressure reading

A blocked impulse line prevents the actual process pressure from being transmitted correctly to the sensor. The transmitter may therefore continue displaying an old, delayed, or otherwise incorrect pressure value.

Correct Answer: B. It changes pressure transmission

Gas trapped in a liquid impulse line is compressible and can interfere with the transmission of pressure. In DP level systems, incorrect liquid/gas arrangement can also create additional hydrostatic effects and measurement errors.

Correct Answer: C. To distort pressure transmission

Condensed liquid in a gas impulse line creates an additional liquid head. This extra hydrostatic pressure can be interpreted by the transmitter as process pressure and produce an incorrect reading.

Correct Answer: D. To prevent freezing

Heat tracing keeps the process fluid within a suitable temperature range. It is particularly important when the process fluid can freeze, crystallize, or become highly viscous at lower temperatures.

Correct Answer: A. To maintain process temperature

Insulation reduces heat transfer between the impulse line and the surroundings. Maintaining the process-fluid temperature can prevent condensation, freezing, crystallization, or viscosity changes that could affect pressure transmission.

Correct Answer: C. Ratio of maximum to minimum span

Turndown ratio describes how much the calibrated span can be reduced from the transmitter’s maximum specified span while maintaining specified performance. For example, a 100:1 turndown means a transmitter designed for a 100-unit maximum span may be configured for a much smaller span within its specifications.

Correct Answer: A. Density affects hydrostatic pressure

DP level measurement is based on P = ρgh. If liquid density changes while actual level remains constant, the measured hydrostatic pressure changes. Without density compensation, the transmitter can interpret the density change as a level change.

Correct Answer: B. When direct process connection is unsuitable

A remote seal is useful when the transmitter cannot be mounted directly at the process connection. Typical reasons include very high temperature, corrosive fluids, viscous or plugging fluids, difficult accessibility, or requirements for special process isolation.

Correct Answer: A. Absolute pressure transmitter

An absolute pressure transmitter uses a sealed vacuum reference. This means the measured value represents the actual pressure above a theoretical perfect vacuum and is not directly referenced to changing atmospheric pressure.

Correct Answer: C. Equalize both pressure sides

The equalizing valve connects the high-pressure and low-pressure sides of a DP transmitter. Opening it equalizes both sides and protects the sensing element from excessive differential pressure during certain maintenance procedures.

Correct Answer: A. Damp pressure fluctuations

A pressure snubber restricts rapid pressure fluctuations reaching the instrument. It helps protect pressure gauges and transmitters from pulsating service and can improve indication stability.

Correct Answer: D. Protect the gauge from heat

A siphon creates a water or condensate seal between the hot steam and the pressure gauge. This prevents high-temperature steam from directly entering the gauge and protects the sensing mechanism.

Correct Answer: B. Liquid height and density

Hydrostatic pressure depends on P = ρgh. Therefore, liquid density and liquid height are the primary process variables affecting the pressure at the measurement point, while gravity is normally considered constant.