by John Houldsworth AS APPEARED IN THE OCTOBER, 1996 ISSUE OF INTECH MAGAZINE REPRINTED WITH PERMISSION FROM INTECH/ISA

Leesburg, Va. -- When thermocouples are wired directly to instruments used to control processes, the instruments' thermal behavior can often limit performance. Excessive warm-up times and poor repeatability are not unusual.

Eurotherm Controls Inc has received a patent for a thermocouple interface that helps solve the problem by eliminating warm-up drift and improving immunity to ambient changes.

Thermocouples generate microvolt-level signals dependent on both the "hot junction" and "cold junction" temperatures (CJT). CJT sensing is considered the Achilles heel of temperature measurement. When the effective cold junction is at the input terminals of an instrument, measuring its temperature accurately requires the sensor to be located very close (in a thermal sense) to those terminals. The problem of sensor location is compounded when the instrument plugs into a mounting sleeve.

Where the CJT sensor is mounted on the measurement printed circuit board (pcb) of a controller, the ability of the sensor to acquire the actual CJT at the screw clamp is limited by the high thermal impedance of the connector (-200°C/watt). The relative magnitudes of other thermal paths and the instrument power dissipation (and its variability) will control the size of the CJT error. The most significant of these other paths is not the pcb trace or air convection, but the conduction through and along the glass-epoxy pcb.

It is the pcb size and thermal characteristics that are mainly responsible for the lengthy warm-up time before the heat flow, and its associated temperature error, can reach a steady value. Previous solutions to the problem focused on minimizing thermal impedance, or minimizing the heat flow. The residual error, which is accounted for by a fixed offset adjustment, results in the following symptoms: switch-on errors of 3°C (5°F) or more; warm-up times of 20 minutes or more before operating to specified accuracy; and repeatability spoiled by changes in cabinet airflow and temperature, when opening an air-conditioned cabinet door, for example.

Eurotherm's patented approach, called Instant Accuracy and built into its Series 2000 controller line, accepts there will be a temperature difference due to variable heat flow. Rather than trying to eliminate this physically, it is eliminated mathematically by measuring the heat flow and correcting for it. Thus, the error vanishes under all conditions.

The heat flow in the connector region is measured as a temperature difference between two points on the pcb close to the connector. From this difference, a correction offset is computed to extrapolate to the temperature of the real cold junction. The required "computation" is simplified by using two similar temperature sensors -- one at the edge-pad, one in-board -- and taking a weighted difference of their outputs to produce a single value that behaves as if it were from a "phantom" sensor positioned on the screw clamp. The weights are only a function of pcb layout and, once characterized, can be fixed as resistor values in the CJT circuit. (See figure 1).

Comparative tests with equivalent competitive instruments illustrate the improved performance. It has been estimated that time saved making good product could be as high as 10% in an eight-hour day.

Applications include IR paint curing and porcelain coating, fast furnaces, calibrators, lab equipment, and those applications in which repeatable control is essential, including crystal growing, fibers, and glass.

The technology that is descibed here is applicable to other industries. Eurotherm is now interested in licensing out for use in non competing business areas such as sensors. With this technique, sensors may be enhanced and/or simplified.