Despite ongoing advancements in technology, instrumenting a process still can pose technical challenges. While such projects may seem straightforward, much can go wrong. A successful installation requires:
• matching each variable to be measured with the most appropriate sensor;
• installing, calibrating and interfacing the sensor properly to the controller or recorder/indicator; and
• in some cases, conditioning, converting, compensating or mathematically manipulating the data generated by the sensor to provide meaningful information.
With this in mind, here are common-sense ideas to consider when instrumenting a process to ensure appropriate signal integrity.
SELECTING THE RIGHT SENSOR
Although it's generally obvious what quantity to measure in flow, temperature, pressure and level sensor applications, it's not always obvious which sensor technology best suits the job. A mismatch between the sensing technology and the desired measurement can lead to inaccuracies and degraded control.
Flowmeters. The great variety of flowmeter technologies available complicates the selection task. Some may work equally well in an application while others may pose problems. The technologies fall into five classifications: velocity, inferential, variable area, positive displacement and mass (Table 1).
Many kinds of flowmeters — including electromagnetic (conductive liquids only), vortex and swirl, turbine, and ultrasonic — sense a fluid's average velocity. Multiplying that velocity by the cross-sectional area of the meter or pipe gives volumetric flow rates.
| Flowmeter Types -- Table 1. Devices based on velocity, differential pressure (DP)
and other technologies can measure flow.
When specifying velocity meters, consider the fluid's velocity profile in the pipe, which depends on piping geometry and Reynolds number. If the flow is turbulent (Reynolds number greater than 10,000), the velocity is virtually the same at the pipe's center and inside walls. Otherwise flow velocities across the pipe cross-section differ, making the average more difficult to calculate.
Inferential flowmeters — including differential pressure (DP) flowmeters (the most widely used) such as orifice plates, wedges, venturis, nozzles, flow tubes and pitot tubes — use another measurement (e.g., pressure) that has widely accepted correlations to calculate flow rate. For DP meters, the flow calculation depends on the square root of the measured DP, the fluid density, pipe cross-sectional area, the area through the restriction, and a coefficient that's specific to the device.
A variable area meter or rotameter is simple and inexpensive. It consists of a float within a tapered tube. The float's position is a balance between the upward flow rate and gravity forces acting on it. But its accuracy (±2% of full scale) is relatively low and depends on precise knowledge of the fluid and process. It's also susceptible to vibration and plugging by solids.
Positive displacement meters capture a specific volume of fluid and pass it to the outlet, providing true volumetric flow rates without calculations. They require no power, handle high pressures and provide excellent accuracies. However, they're often expensive and can't deal with multiphase fluids.