THE TERM “in-situ calibration” means different things to different people. Calibration is a process by which the operation of an instrument is checked against a standard that is known to measure the desired parameter with a high degree of accuracy. A large amount of effort is expended to ensure that the standard achieves and maintains its accuracy. The extent of this effort may not be obvious to its user because off-the-shelf calibrators are engineered to be easy to use and maintain. In contrast, custom calibrators and calibrators manufactured in low volumes tend to require the user to pay more attention to detail.
Recently, especially in the process industries, some new product introductions have raised doubt about what in-situ calibration for flowmeters actually is, and whether it can be duplicated with simulators and calibrators with expanded diagnostics.
Flowmeters are functionally comprised of the flowmeter primary and the flowmeter secondary. The primary is physically involved with the fluid. The secondary, commonly called the transmitter, processes the signal(s) produced by the primary. Examples of primary/secondary pairs are given in Table 1.
TABLE 1: PRIMARY AND SECONDARY FLOW MEASUREMENTS
|Orifice Plate||Differential Pressure Transmitter|
|Magnetic Coil/Electrode Assembly||Magnetic Flowmeter Transmitter|
|Thermal Heater / Temperature Sensor(s)||Thermal Flowmeter Transmitter|
|Paddle Wheel Sensor||Paddle Wheel Transmitter|
|Oval Gear Assembly||Positive Displacement Transmitter|
|Vortex Shedding Assembly||Vortex Shedder Transmitter|
|Parshall Flume||Level Transmitter|
|Coriolis U-tube Assembly||Coriolis Mass Transmitter|
|Ultrasonic Sensor(s)||Ultrasonic Flowmeter Transmitter|
THESE FUNCTIONAL relationships exist even when the primary and secondary are packaged together. For example, a paddle wheel sensor and transmitter may be incorporated into the same housing and sold as a one-piece assembly. This construction reduces the cost of the flowmeter, reduces the electrical installation complexity/cost, and typically places fewer space requirements on the installation. Yet it still has primary and secondary functionality, just in one physical package.
In general, there are three components necessary to calibrate a flowmeter --- the flowmeter primary, the flowmeter secondary, and the calibration standard. By their nature, flowmeter primaries are exposed to the fluid flow. Note that even flowmeters with non-wetted sensors such as clamp-on ultrasonic flowmeters, are exposed to fluid via the ultrasonic energy that is transmitted/received to/from the fluid flow, and because the pipe itself becomes functionally part of the flowmeter primary.
In a flow laboratory, flowmeters can be calibrated by flowing the same fluid through the flow laboratory calibration standard and the “meter under test” (MUT) that is comprised of a flowmeter primary and secondary. This can be done by circulating the liquid (Figure 1), diverting the liquid to a weigh tank (Figure 2), and then circulating again (Figure 3). By comparing measurements from the standard and the MUT (while diverting), the MUT can be adjusted (calibrated) to provide measurements that mimic the standard.
PERFORMING THIS type of calibration in the field (in-situ) is usually impractical or impossible, because flowmeter laboratories are typically relatively expensive, custom-designed installations that involve multiple valves and weigh tanks. One alternative method is to bring the MUT and its associated piping to a flow laboratory for calibration. Another alternative is to calibrate the MUT in the flow laboratory using piping that is identical to the piping in the field installation. In general, these methods are effectively impossible for the overwhelming majority of applications in process plants.
In contrast, in-situ calibration of a flow measurement system involves bringing the calibration standard to the instrumentation installed in the field. Key to this approach is the adequacy of the standard(s) to calibrate the components of the measurement system, and the ability to transport the standard to the field faithfully.
Custody Transfer Applications
The flow measurement derived from a custody transfer flow measurement system is used to calculate the amount of money that changes hands between the past and present owner of the material. Residential water and natural gas meters that determine the homeowner’s consumption are a commonly encountered type of custody transfer flowmeters.