Pressure measurement undoubtedly forms an important measurement in industrial process measurement and control. One only needs to consider applications involved with compressed air, the monitoring of gas consumption or the control of refrigeration plants to appreciate the importance of accurate pressure measurement and control.
In line with the advances in electronic technology in recent years, the trend in industry has clearly seen a move away from the older pneumatic transmitters to their electronic counterpart. Pneumatic transmitters do, however, still play an important role in the petrochemical industry for reasons related to intrinsic safety requirements. However, in view of the general trend, all references made to transmitters in this article will be made with electronic transmitters in mind.
An introduction to pressure measurement
Pressure measurement is generally seen as rather elementary but on close inspection one finds that some applications are somewhat confusing. Before a decision can be taken as to the type of transmitter to use, the measurement needs to be clearly understood and defined. A measurement range defined as 0 to 80 kPa does not allow a correct selection of pressure transmitter.
The measurement should be defined in terms of either gauge, absolute pressure or vacuum, before the correct type of transmitter can be selected for an application.
Atmospheric pressure is the force generated by the earth's atmosphere on the earth's surface. The atmospheric pressure at sea level is approximately 101,325 kPa, depending on weather conditions. The absolute pressure varies from place to place depending on the elevations above sea level and the local weather conditions. Gauge pressure is the amount of pressure above or below atmospheric pressure. In other words, gauge pressure represents the difference between the measured pressure and the atmospheric pressure at that location. Vacuum is negative gauge pressure and therefore also represents the difference between the measured pressure and atmosphere.
Note: vacuum is measured with a gauge pressure transmitter, not an absolute pressure transmitter. Clearly we have now seen that all pressure measurements are, in fact, differential measurements because all measurements are referenced to either absolute (0 kPa) or atmosphere pressure (Figure 1). A gauge pressure transmitter therefore has its low pressure side vented to atmospheric pressure. An absolute pressure transmitter has its low pressure side evacuated to absolute zero pressure and then sealed at the factory.
With the type of transmitter now correctly defined, the required range of measurement must be specified. The range limit of a pressure transmitter is the maximum pressure the transmitter can accept, while the span limits of a transmitter refers to the difference between the minimum and maximum input pressure a transmitter can accept and calibrate between zero (4 mA) and full scale (2 mA). All transmitters are therefore specified to minimum and maximum spans as well as range.
Now, to add a little more confusion, the measurement specification should also state whether the measurement is either an increasing/increasing or increasing/decreasing measurement. What this refers to, quite simply, is whether the transmitter output should increase or decrease with an increasing pressure measurement.
The majority or pressure measurement applications are defined as increase/increase, but exceptions do exist. For example, a particular process may require a measurement of vacuum which could require and increase/decrease measurement as an increasing vacuum is in fact decreasing gauge pressure (0-50 kPa vacuum).
Because not all gauge pressure transmitters have the facility to reverse their outputs, a different approach may be required for an increase/decrease measurement.
By substituting the gauge pressure transmitter with a true differential pressure transmitter and connecting the process to the low pressure side of the transmitter, an increase/decrease measurement of vacuum can also be achieved. (Note: the high pressure side of the transmitters must be vented to atmosphere.)
In the case of an increase/increase measurement of vacuum, say 60 to 0 kPa vacuum (negative gauge pressure) with 60 kPa applied, the transmitter will have to be able to be calibrated to zero (4 mA) output and with the transmitter at atmospheric pressure, the output will be full scale (20 mA).
Some applications considered
Clean noncorrosive liquids and gases
Providing the process media is noncorrosive and clean (low percentage solids content), the measurement can be made via direct connection to a standard transmitter.
The process temperature should be noted to ensure that the maximum process temperature falls within the transmitter operating specifications.
In some cases where the transmitter limits are exceeded, some form of cooling will need to be investigated. In most cases, however, an extended impulse line with a pigtail type seal will achieve sufficient cooling to enable the measurement to be made.
High process temperatures are generally associated with unacceptably high local ambient temperatures which can result in transmitter electronics failure, especially when transmitters are mounted in field enclosures close to the point of measurement.
In cases of high ambient temperature, the transmitter should be mounted some distance from the point of measurement where temperatures are more acceptable. In applications involving extremely high temperatures, high temperature chemical seals should be used to protect the transmitter.
Dirty corrosive liquids and gases
Dirty corrosive liquids or gases can obviously not be connected directly to a pressure transmitter. The solids content will tend to block up the impulse line, thereby isolating the transmitter from the process, while corrosive processes could damage the transmitter diaphragm.
Most manufacturers of pressure transmitters offer a wide choice of wetted parts materials which need to be carefully selected to suit the process.
However, in extreme cases it may make both economic sense and be good engineering practice to make use of a suitable chemical seal.
Controlling the process pressure
With the pressure correctly measured, it is now possible to control it at a desired level.
A control valve will need to be installed in the process and a controller selected for the application. A two term (proportional plus integral or PI) electronic or pneumatic controller will suffice for most pressure control applications. Dozens of different makes and sizes of controllers are available and the final choice will depend largely on the quality and accuracy required as well as on financial constraints. The pressure transmitter output is connected to the controller measurement input while the controller output is connected to the control valve. The setpoint can be either manual or remote, depending on the process interaction with other areas of the plant.
The control action of the controller should also be observed. If the process requires an increasing output with increasing measurement, the action is said to be direct, while a decreasing output with increasing measurement is said to be reverse.
Both the proportional and integral control modes require tuning to the process to ensure optimum control. Although various manual methods for tuning controllers exist, it should be pointed out that controllers are now available with auto-tune and self-tune features which can, if used correctly, save time and simplify the tuning of process controllers.
The main difference between auto-tune and self-tune is that auto-tune requires manual intervention to initiate a returning of the controller, while self-tune continually monitors the process and retunes itself whenever the process starts deviating from the setpoint, thus ensuring that the process is maintained at setpoint irrespective of load changes.
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