Fieldbus technology improves flow measurement

As a way to increase reliability and maintain good flow measurements, fieldbus technology provides an opportunity to improve flowmeter performance in a variety of applications.

By David W. Spitzer

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Improving flow measurement, in a majority of cases, can be made by enhancing the performance of one technology--the flowmeter itself. For example, a supplier recently introduced a magnetic flowmeter with an accuracy of 0.15% of rate. With an operating accuracy better than any other magnetic flowmeter currently on the market it’s fair to say that the manufacturer enhanced the device’s design and construction to improve its performance.

Nonetheless, additional flow measurement improvement can be achieved by transmitting signals to the control system or data acquisition system using digital signal transmission in place of analog signal transmission techniques.

Analog Signal Transmission
Flowmeter measurements can be transmitted to the control system in a variety of ways. In process control systems, it is common for the flow transmitter to generate an analog signal (usually 4–20 mA) that is wired to the control system. The analog input module in the control system then converts the analog signal to a digital measurement that is used by the control system.

Many flowmeters are inherently digital devices (to improve stability and performance), there can be as many as three signal conversions taking place when using analog signal transmission:
  1. An analog-to-digital conversation that digitizes the raw sensor signal
  2. A digital-to-analog conversion that converts the transmitter measurement to an analog signal that represents the flow measurement
  3. An analog-to-digital conversion that converts the analog signal that represents the flow measurement to a digital measurement in the control system

The digitization of the raw sensor signal is included within the accuracy specification of the flowmeter. The other two conversions can introduce significant measurement error, especially when the analog signal is low in its range and when environmental factors are considered.

For example, a typical analog output circuit of a magnetic flowmeter will add up to 0.06% of full scale error to the flowmeter accuracy statement. The accuracy of an analog input to a distributed control system (DCS) adds about 0.03% of full scale. In contrast, the accuracy specification of an analog input module for programmable logic controllers (PLC) has been noted to be as high as 0.40% of full scale. The table calculates the percent rate error associated with these signal conversions for a linear flowmeter.

Examination of the table indicates that these conversion errors can be significant, especially at low flow rates. In some applications, the errors caused by conversions at some flow rates can exceed the accuracy of the flowmeter.

Performance can be improved somewhat when the frequency (pulse) output of the flowmeter transmitter is used because the digital-to-analog conversion used to generate the analog signal is not needed. However, the frequency signal must be converted to a digital value in the control system.

Environmental factors, such as ambient temperature variations, can further degrade the performance of each of the conversions. However, suppliers may not publish these specifications even though temperature effects can be significant. For example, consider a converter with a temperature coefficient of 0.005 % of full scale per degree Celsius. This may seem small, but when such a device is located outdoors where the ambient temperature can vary by 25° C, the error introduced could be 25*0.005, or 0.125% of full scale. For a linear flowmeter operating at 10% of full scale flow, the error introduced could be as high as 1.25% of rate.

Noise and Other Interference
Electrical noise, radio frequency interference, electromagnetic interference and other installation-related issues related to the analog signal can also degrade measurement accuracy. Cabling problems, including those stemming from wet signal cables, can partially ground the signal wires and adversely affect accuracy. Loose or poor connections can alter the signal in some installations. Connecting too many receiving instruments in series and/or paralleling additional instruments to voltage signals can introduce error as well. 

Many flowmeters are available with Profibus-PA or Foundation fieldbus transmitters. For those instruments that don't, the segments can be installed to accommodate both.  

Fieldbus Signal Transmission

Fieldbus communication allows the control system to communicate digitally with the flowmeter. As such, the need for the transmitter to generate an analog signal is eliminated. Further, the need for the control system to measure and interpret the analog signal is also eliminated. Eliminating these conversions can improve the quality of flow measurements. In addition, other information, such as secondary measurements and flowmeter status information will also be available using digital communications. Flow measurements that are inherently digital, such as vortex shedders, may not require any analog-to-digital conversion.

There are a number of fieldbuses however. The most commonly applied fieldbuses to process control are HART, Profibus-PA, and Foundation fieldbus (FF). Fieldbus segments (wires) can be used to communicate with multiple transmitters and supply their power (See Figure 1)

A flowmeter with a HART output can generate an analog signal that allows the use of existing wiring and analog signal transmission. Digital communication is implemented by superimposing digital signals on the analog signal, allowing simultaneous analog and digital communication.
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