It's not all about the sensor: strategies for identifying and reducing sources of temperature measurement uncertainty.(SECOND OF A TWO-PART SERIES)

A basic awareness of the inherent accuracy of particular sensor types is important, but goes only a short way toward the goal of achieving optimally accurate temperature measurement. It is the broader knowledge of how sensor choice, sensor placement and a wide variety of environmental factors can contribute to sensor error, as well as having a familiarity with calibration techniques that will ultimately lead to optimum sensor selection and measurement accuracy.

Location and transient errors

It is nearly impossible to sense temperature exactly where you need it. At the very least, the sensor itself has a finite size that displaces the sensing element from its attachment--resulting in the sensor being at a different location than the desired measurement location. Thermistors and RTD's are at greater risk for location error than an equivalently placed thermocouple--simply because of their size.

If surrounding heat sources and sinks are known, compensation can be made for location errors. However, this can be difficult in many systems and will result in location errors resisting calibration. The simplest solution, which avoids complex calibration techniques, is to utilize a small sensor and place it as close to the temperature source as possible.

Figure 1 illustrates how errors in sensor location can affect temperature measurement accuracy. Location error 'A' is a direct result of the entire sensor being displaced from its desired location, typically because of interference. Location error 'B' is a direct result of the sensor element being displaced from the intended surface by its encasement.

Transient errors are dynamic thermal errors for which compensation is typically very difficult because every material within the thermal system has its own unique thermal conductivity and capacity. Of the three most popular sensor types, it is the thermocouple that best minimizes transient errors because it corresponds to the smallest time constant.

Heat transfer errors