In Search Of Universal Sensors

You are working in a process development research laboratory. Wouldn’t it be easy if you could take a flow device off the shelf, install it, and have immediate, accurate and reliable measurements? This is a dream, a fantasy for some. Advances in sensor design, electronics and software, however, is bringing this day closer than ever through recently introduced developments.

Coriolis—The Dream Sensor

The only truly universal fluid flow sensor is the Coriolis mass flowmeter. Historically, Coriolis mass flow measurement has been used in many process industries in which accurate, reliable measurement of liquid or gas mass flow is required. Improvements in sensor design and electronics processing now allow Coriolis sensors to be reduced in size in order to better measure the low flows found in research applications.

Figure 1 depicts Corios mass flow.

A Coriolis sensor uses a vibrating tube to measure the mass flow of liquid, so that the momentum of the moving fluid changes the shape of the vibrating tube. A U-shaped tube is a typical sensor design for these low-flow sensors (as illustrated in Figure 1).

Every Coriolis mass meter has three key components:

• The sensor tube, mounted rigidly on a base, in which all flow goes through without bypass.
• A position sensor that measures the inlet and outlet legs of the tube.
• A magnetic drive that vibrates the tube.

In a no-flow condition, the inlet and outlet legs of the tube vibrate in unison, and the result is a differential electrical signal from the two position sensors of zero time, whereas in flow conditions, the inlet and outlet legs vibrate out of phase, thereby generating a differential electrical signal from the two sensors with a specific time lag.

When the sensor is factory-calibrated using an inert fluid, the amount of time lag created by the moving fluid is directly proportional and linear to the fluid mass flow. Thus, a Coriolis sensor is a universal mass flow measurement device that can be used to measure liquids or gases. Its measurement accuracy is independent of the physical and thermal properties of the fluid, making Coriolis sensing ideally suited for accurate measurement and control of fluids with unknown properties, such as multi-component fluids. Coriolis sensors are also very fast in responding to changes in flow.

Additionally, the natural resonant frequency of a Coriolis sensor can be used to determine fluid density. A heavy fluid in the tube reduces natural frequency, while a light fluid or gas increases it. Fluid density measurement can be useful in confirming the concentration of binary fluids or the fluid identities. Moreover, Coriolis sensors have been combined with a control valve and packaged with integrated electronics to become a compact integrated flow controller.

Thermal—The Practical Sensor

Figure 2 illustrates thermal mass flow.

In many research applications, a pure or binary gas mixture must be measured and controlled. Thermal sensors have traditionally been used, but are limited in applications involving the measurement of multiple gases. Thermal mass flow sensors use the heat capacity of a pure gas to infer the mass flow rate of a gas (Figure 2). The equation to be solved is a simple one: mass flow = (heat added to the gas)/((T2-T1) x (heat capacity of the gas).

A laminar flow element is used to create a linear relationship between pressure drop and flow. This pressure drop forces a very small portion of the gas to pass through the sensor tube in proportion to the overall flow. An integrated control valve, along with digital electronics, completes the gas mass flow controller.

There is a challenge to applying thermal sensors calibrated using a gas like nitrogen to measure a range of gases. The sensor output is affected not only by the change in heat capacity of the gas, but also by the viscosity of the gas related to the generation of pressure drop through the laminar flow element. The goal of universality can be reached through two paths—advances in software and real gas calibrations.

Recently introduced to the marketplace are mass flow controllers that incorporate multi-gas/multi-range functionality. These result in a sensor that is calibrated on nitrogen, but is then capable of accurately measuring and controlling multiple pure gases or gas mixtures.

The core technology used in multi-gas/multi-range is the physical modeling of the gas flows through the sensor with the goal of understanding when the flow path does not follow ideal principles. Along with physical modeling is measuring the response of the sensor that flows an array of actual gases over a broad range of molecular weights. The data collected is then analyzed and results summarized in a database program. A simple connection to a laptop allows for reprogramming the device to a multitude of gases. The operation flow control range for each gas is predetermined and included in the reprogrammed device.

Accuracy of the multi-gas sensor, furthermore, is superior to the traditional thermal sensor. The economic benefit to the user of multi-gas/multi-flow technology is the reduction of the number of devices required to be maintained in inventory to cover a range of gas types and flow ranges. In an ideal world, the Coriolis sensor would be the standard for all fluid measurements in a research environment. But reality sets in, and for many applications, the practical solution is to use a thermal sensor, although the development of the multi-gas sensor results in thermal sensors getting closer to the universal appeal of the Coriolis sensor. 

For more information, please visit www.brooksinstrument.com.