Surface temperature is a highly desired but difficult measurement. Unlike applications in which transducers in protective tubes are immersed in fluids under controlled or known circumstances, surface measurements require that sensors be outside of vessels where environmental conditions may affect results. In addition, factors such as conformance with surface contours and thermal as well as electrical paths between the sensors and the objects of measurement influence output signal integrity. Selection of sensors and mounting techniques influence measurement system performance. Surface temperature is of direct interest in many applications and is of value for assessing conditions at inaccessible locations in others. Various sensors and mounting techniques are available. Selection criteria include factors appropriate for any temperature measurements such as range, accuracy, ruggedness, and sensitivity to substances likely to contact the element. Additional considerations peculiar to surface applications include the thickness and flexibility of the units as well as heat exchanged with the object of measurement and the surroundings.

1) The surface sensor must be mounted in a manner which permits maximum thermal contact and minimal mechanical strain. RdF Designs are easy to mount properly.

2) The surface sensor must be insulated or isolated to make its temperature as close to that of the surface as possible. This is controlled by the user.

To illustrate these points Figure 1 shows a typical surface sensor installation. The sensor in this case is mounted on a pipe which is carrying a fluid. The point closest to the fluid where a sensor can be mounted practically is the outer wall of the pipe, If the fluid flow is adequate and the temperature fluctuations are not severe, the outer wall temperature will be very close to that of the fluid.

The surface sensor is only able to measure its own temperature and an attempt must be made to make the surface and sensor temperatures the same. This can be done by placing insulation over the sensor to reduce the effects of the environment and by choosing a mounting method that provides good thermal contact between surface and sensor.

The method chosen for mounting the sensor is equally important, while good thermal contact is desired, a large mismatch in thermal expansion coefficients may cause strain in a wire Resistance Temperature Detector sensor. This strain will produce a change in resistance that could be misinterpreted as a temperature change. Most RdF flexible models use miniature supported thin film sensing elements which eliminate strain effects in surface mounted RTDs.

When mounting both resistance and thermocouple sensors the leads should be in contact with the sensing surface for some length to reduce the effects of thermal conduction from the sens-ing wire or junction.

As can be seen by this introduction, an accurate surface temperature measurement is not a simple matter. A review of the app-lication and a procedure for installation should be considered prior to ordering. It is our intention in these application notes to provide general guide lines for the use of surface sensors.