The demand for pressure measuring instruments arises with the advent of steam age. Mechanical methods of measuring pressure such as Bourdon tubes or bellows, where mechanical displacements were transferred to an indicating pointer were the first pressure instruments. Initially, these tubes were constructed of glass, and scales were added to them as per requirements. However, these mechanical motion balance pressure measuring arrangements were large, cumbersome, and not well suited for integration into automatic control loops. Consequently, as control systems evolve to become more centralized and computerized, these devices were replaced by analog electronic and, more lately, digital electronic pressure transmitters. Pressure transmitters or transducers are ready to use instruments employed for measurement of pressure. These are OEM transducers with
- pressure port
- integrated compensation resistors
- a cable or connector
The terms pressure gauge, sensor, transducer, and transmitter can be used interchangeably. Majority of modern pressure sensors operates on piezoresistance principle. Due to pressure, a material generates electricity at a certain rate, which leads to a specific level of charge flow related with a specific level of pressure. This charge is supplied to a wire which leads to a control panel and display for human analysis.
It is a standardized pressure measurement package which includes following three fundamental components:
- a pressure transducer
- its power supply,
- a signal conditioner/retransmitter used to transform the transducer signal into a standardized output
- In pressure transmitters, process pressures can be transmitted using
- an analog pneumatic (3-15 psig),
- analog electronic (4-20 mA dc),
- or digital electronic signal
When transducers are directly interfaced with digital data acquisition systems and are positioned at some distance from the data acquisition hardware, high output voltage signals are preferred and these signals must be guarded against both electromagnetic and radio frequency interference (EMI/RFI) when traveling longer distances.
Types of Pressure Sensors
Pressure sensing based upon diaphragm technology measures the difference in pressure of the two sides of the diaphragm. Depending upon the relevant pressure, pressure sensors have been classified into following three categories:
- Absolute pressure sensor: It measures absolute pressure using a vacuum as a reference point.
- Gauge sensor: It measures pressure by reference to the ambient atmospheric pressure. "Gauge" pressure is defined relative to atmospheric conditions.
- Differential pressure sensor: It measures the pressure difference between two contacts or ports. Differential pressure transducers are frequently employed in flow measurement where they are used to measure the pressure difference across a venturi, orifice, or other type of primary element. The detected pressure differential is related to flowing velocity and as a result to volumetric flow. Many features of modern pressure transmitters have appeared from the differential pressure transducer.
It is normally referred to as a self-contained indicator which can transform the detected process pressure into the mechanical motion of a pointer. A pressure transducer might join the sensor element of a gauge with a mechanical-to-electrical or mechanical-to-pneumatic converter and a power supply.
- Accuracy: "It refers to the degree of conformity of the measured value to an accepted standard. It is usually expressed as a percentage of either the full scale or of the actual reading of the instrument. In case of percent-full-scale devices, error increases as the absolute value of the measurement drops".
- Repeatability: It refers to the closeness of agreement among a number of successive measurements of the same variable.
- Linearity: It is a measure of how appropriate the output of transducer increases linearly with increasing pressure.
- Hysteresis error: This characteristic explains the phenomenon according to which the same process pressure results in different output signals depending upon whether the pressure is approached from a lower or higher pressure.
- Sensitivity: It determines the amount of variation that occurs in the output voltage as and when the input voltage varies, keeping in view that the measured pressure and the rated pressure of the transducer remains constant.
In industrial applications, good repeatability is considered more significant as compared to absolute accuracy. For pressure variations over a wide range, transducers with good linearity and low hysteresis are the ideal choice.
- Ambient and process temperature variations cause errors in pressure measurements, mainly in detecting low pressures and small differential pressures. For these applications, temperature compensators can be employed.
- Power supply variations also affect the performance of pressure transducers.
Maintenance and Calibration
Pressure sensors call for scheduled, periodic maintenance and recalibration without any exemption. It is obligatory to periodically remove the transducer from the process for the maintenance purpose. It should be ensured that this procedure does not require shutting down the process and does not cause any injury or damage. Since the process fluid may be toxic, corrosive, or otherwise noxious to personnel or the environment, it is essential to guard against the release of such fluids during maintenance.
Pressure transducers can be recalibrated on-line or in a calibration laboratory. Laboratory recalibration is usually chosen over the other one. In the laboratory, there are generally two types of calibration devices:
- Deadweight testers: They provide primary, base-line standards,
- “Laboratory" or "Field" standard calibration devices: These are periodically recalibrated against the primary. These secondary standards are less accurate than the primary, but they provide a more convenient means of testing other instruments.