The Coriolis effect, a subtle and often-misunderstood principle of physics, has been adapted for highly accurate mass flowmeter instrumentation.
The Coriolis effect is generally viewed, if at all, as a “large-canvas” effect. It is yet another consequence of the rotation of the Earth, with many interesting implications such as the rotational direction of air masses in each hemisphere (Figure 1), and curving the north-south path of projectiles and missiles. (It is also associated with swirling of water in a tub as it drains — but that’s another story, later). Its use in instrumentation is yet another example of how a subtle physics principle can be adapted to provide some very tangible benefits. This article will look at the effect and how it used to measure the mass flow of fluids in pipes and tubes.
Material flow (usually specified in kilograms per second or pounds per second) is second only to temperature as the most commonly measured variable in many industrial processes, lab experiments, and, increasingly, in medical studies. Although mass flow is roughly analogous to electrical current, it is more difficult to measure for two main reasons accurately. First, the material itself presents a challenge: It can be water, liquid, slurry, or gas, and corrosive, homogenous, or mixed. Second, changes in material characteristics due to internal and external factors affect measurement.
There are many different flow-meter designs in wide use in industrial and lab settings, and each has virtues and weaknesses. The reason is that while measuring flow is simple in concept, it is often difficult to do well in practice, and especially so in industrial applications. Among the many intrusive (contact) and non-intrusive (non-contact) methods are:
- Magnetic flow meters, which are suitable only for conductive liquids. These meters use coils outside the pipe to generate a magnetic field in the pipe, which in turn induces a sensed voltage as the liquid flows through the field (Figure 2).
- Ultrasonic flow meters, of which there are two basic types. One measures the transit times of an ultrasonic signal traveling with and against the fluid flow and then calculates the mass flow using the differences; the other measures the Doppler shift, which is a function of flowing fluid.
- Vortex flow meters, which deliberately create an internal swirl or vortex, and the rotational rate of this vortex is proportional to the flow velocity.
- Differential pressure flow meters, which measure the pressure drop as the fluid flows across laminar flow plates, and the pressure differential is proportional to the fluid flow.
- Turbine meters, which use an in-stream propeller-like blade that rotates at speed proportional to the flow rate, while a tachometer or magnetic pickup senses the rotation (Figure 3).
Each of these has varying attributes with respect to accuracy, reliability, fluids it can handle, cost, impact on fluid flow, intrusiveness, and other factors. However, most of these flow meters really sense fluid velocity or volume, which must then be converted to the actual mass flow. Then, using the volume or velocity measurement to calculate the mass is simple, in principle: just the basic equation: density = mass/volume. Unfortunately, temperature, pressure, viscosity, and uniformity affect volume and can result in overall errors of 5% or more, depending on the material, flow rate, and other circumstances.
In the next part, we will look at the Coriolis principle as a basis for use in flow meters.
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