Sensor Placement Tips and Tricks

01 February 2007

Placing a thermocouple inside a furnace seems like an obvious way to measure the temperature of a product being heat-treated. However, some care must be exercised in determining not only what sensor to use, but exactly where to place it.

Even a highly accurate resistance temperature detector (RTD) tucked in the corner of the furnace will only be able to detect the temperature of its immediate vicinity. If the heat distribution within the furnace is uneven, that local temperature may or may not represent the temperature of the products sitting in the middle of the
furnace.

That's a classic mistake that home heating contractors make when installing household thermostats. A mounting location closest to the heater may be convenient for wiring purposes, but if that spot happens to be in a hallway or other dead air space, the thermostat will not be able to determine the average temperature elsewhere in the house.

To avoid such pitfalls, an instrumentation engineer must select locations for his sensors by considering
where (and if) the required data are actually present. That is arguably the most obvious placement issue, but
there are others that may also affect the sensor’s performance in more subtle ways. These include noise, distance to data, and certain factors peculiar to ultrasonic sensors.

Measurement noise
Having too much data at the proposed sensor location can also be a problem, especially when electrical ground loops, mechanical vibrations, radio frequency interference (RFI), and other environmental factors cause measurement noise. RFI noise is especially common in plants that use walkie-talkies, pagers, and wireless networks and in applications where electromechanical contacts generate sparks.

In flow measurement applications, one common source of measurement noise is turbulence resulting from bends, junctions, and valves in a pipe. Turbulence is especially challenging for magnetic flowmeters. The simple fix is to place all magmeters in sections of straight pipe or use alternative flowmeasuring technology such as vortex flow meters that can be calibrated to handle turbulent flows.

Ultrasonic examples
Distance can be an issue when placing ultrasonic proximity sensors. These work by bouncing a pulse of sound waves off the object to be detected. The time required for the pulse to reach the object and return to the sensor indicates the distance between them, but only if the speed of the pulses is known and the object is not so far away that the returning pulse becomes too faint to detect.

Determining the speed of the ultrasonic pulses can be a problem when the object is so far away from the sensor that appreciable variations occur in the temperature of the intervening air. The sensor can measure the temperature of the nearby air mass to determine how fast sound will pass through it, but the sensor
cannot account for changes in the speed of the ultrasonic pulses as they pass through distant masses of air with different temperatures. It can only assume that all of the intervening air is at the same temperature.

So if the temperature of the air varies along the path of the pulse, the sensor will miscalculate the total distance that the pulse has travelled and mis-identify the object’s position. The same problem afflicts ultrasonic level sensors that are placed too far above the surface of the liquid in a tank.

(The full article appears on Control Engineering Europe’s web site, www.controlengeurope.com)


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