Tag Archives: Flow Measurement

Analog Output Accuracy: The Devil In the Details

Flowmeters typically contain multiple components that introduce error into the flow measurement system. A simple flow measurement system may be comprised of a primary flow element and a transmitter that processes signals from the primary flow element. Sometimes, the primary flow element and transmitter are physically integrated together as one piece, such as in potable water meters. More complicated flow measurement systems may include multiple components such as a flow computer or other electronic components that compensate for process pressure, process temperature, or other parameters.

It should not be forgotten that flow measurement systems are “systems” that measure flow. As an example, consider a hypothetical primary flow element that exhibits no error while the transmitter exhibits 5 percent accuracy. In this exaggerated example, the accuracy of the flow measurement system will be 5 percent. Assuming that the flow measurement error is that of the primary flow element only is an error of omission. Users should constantly be on guard to identify this type of error.

In most flowmeters, the primary flow element and transmitter are integrated electronically. For example, the wetted primary flow elements of Coriolis mass flowmeters, thermal flowmeters, and magnetic flowmeters are virtually useless without transmitters that contain their respective flow measurement algorithms and drivers. Therefore, flowmeter performance typically includes the combination of a primary flow element and a transmitter. Further, the performance of most flowmeters is predicated on the calibrated output that is usually the pulse/frequency output of the transmitter.

However, most process control applications of flowmeters involve the use of an analog output such as 4-20 mA to represent 0-100 percent of the desired flow rate. The analog signal is typically generated using circuits that convert the pulse/frequency signal (or its source) to an analog signal. This conversion introduces a measurement error that is constant throughout the signal range, so it can usually be expressed as a percent of full scale. The error introduced is typically between 0.03 and 0.10 percent of full scale depending upon the quality of the converter. To obtain the measurement accuracy of the analog output, this error is mathematically added to the accuracy of the flowmeter.

The analog output error may seem small, but at low flow rates, this error can become significant and actually dominate measurement accuracy. For example, consider a vortex shedding flowmeter that can operate from 10 to 100 units per minute with 0.75 percent of rate accuracy but has an analog output accuracy of 0.10 percent of full scale. At 10 units per minute, the pulse/frequency output has an accuracy of 0.75 percent of rate, whereas the analog output contributes an additional (0.1*100/10) or 1.00 percent rate error, so the measurement accuracy of the analog output is 1.75 percent of rate.

Most suppliers calibrate the pulse/frequency output. They typically state its accuracy as the performance of the flowmeter. The accuracy of the analog output conversion is often buried in the specifications in the fine print. Sometimes, it is not published and must be requested from the supplier. Sometimes the information is forthcoming, but often suppliers do not understand the question and try to state the analog output resolution (say 1 part in 4096, or 0.02 percent) as the analog output accuracy. After further investigation, many suppliers will admit that they do not know the analog output accuracy — even though most of their customers may use that output exclusively for their flow measurements. They also provide further enlightenment when they say that “no one ever asked for this before”.

The burden of obtaining the best flow measurement possible in a given application does not lie with the supplier — it lies with the user. Do not forget the fine points that may lurk in the details and the errors of omission that may be available for the asking.

This article originally appeared in Flow Control magazine (September 2004) at www.flowcontrolnetwork.com.

Multi-Phase Flow Measurement: Considering All the Options to Meet All the Requirements

Some flow measurement applications just stick in your craw and may even keep you up at night. Losing sleep may be an exaggeration, but some applications are ‘tough’. The problem is that these applications usually involve the simultaneous solution of many flow problems, each of which may have a feasible solution when taken individually.

For example, consider the problem of multi-phase flow where individual liquid and vapor flow measurements are needed. Measuring the liquid flow rate and gas flow rates individually might be straightforward, but measuring them in the same stream will create havoc in most flowmeters. Not surprisingly, solutions to this problem include separating the vapor from the liquid and measuring each stream individually. Another approach is to install a flowmeter that can handle this service, such as a Coriolis mass flowmeter with software designed for two-phase flow or a correlation flowmeter that can measure fluid velocity and void fraction.

And what if the stream contains two liquid components? Again separating the components and measuring each stream may be an option, but it is often not practical. In some applications, the densities of the liquids may differ enough such that the flow rate of the individual components can be inferred from measurements of the total flow and liquid density, predicated on a known relationship between density and composition. Another option is to measure the total flow and use an analyzer measurement to infer the flow rates of the individual components.

And what if the stream contains liquids and solids? Separation may be an option that the process will not tolerate.

And what if the stream contains a liquid, vapor and solids? Now the problem is more complicated and the phases may have to be separated in order to make the individual measurements.

And what if the temperature is 500 degC? And what if the pressure is 500 bar? And what if 200:1 turndown is needed? And what if there is only 1 diameter of straight run to install the flowmeter? And what if the pipe is located 10 meters above grade with no access platform? And what if the ambient temperature is 80 degC? And what if…?

The list can go on and on. In most cases, meeting one of these requirements is possible. However designing a flow measurement system to measure under a combination of these conditions is difficult and may require a somewhat unlimited budget. Compromising on the requirements and relaxing budgetary constraints may bring solutions into focus, but the long and short of it is that there just may not be a feasible flow measurement system available.

You just may have to measure what you can feasibly measure as a surrogate for the desired measurement. For example, measuring the individual liquid, vapor and solid flow rates in a stream may not be feasible. However, useful information might be obtained by separating the vapor flow from the other phases, and measuring the liquid/solid flow and the vapor flow. This may not be the ideal, but these measurements could provide useful information about the process at reasonable cost.

It should be understood that in some applications, there just may not be any flow measurement system available at any price that meets all of the various requirements.

Originally published in Flow Control magazine (July 2004) at www.flowcontrolnetwork.com