As a result of the advanced method of Cold Junction Compensation (CJC) used in Dataforth Corporation’s MAQ20 thermocouple modules, the accuracy of these modules is significantly better than that offered by competing models.
Dataforth Thermocouples
The Dataforth MAQ20-JTC, -KTC, -TTC and -RSTC thermocouple input modules have eight differential input channels, which are grouped into four blocks of two channels. Each block has a terminal for sensor shield ground as well as a dedicated ambient temperature sensor that is optimally positioned to accurately measure the terminal block temperature in any system configuration, any module orientation, and over the entire operating temperature range of -40°C to +85°C. Precise cold junction compensation can only be accomplished with precise measurement of the terminal block temperature where the dissimilar metal junction is formed.
When input sensor connections are spread out over a distance, temperature gradients are naturally present due to convective heating, transient air currents caused by opening and closing instrumentation cabinets, and local heat sources from adjacent electronics. Only when an ambient temperature sensor is integrally mounted with the terminal block and only when that temperature sensor has the same thermal response time as the terminal block can precise cold junction compensation take place over static and dynamic ambient temperature conditions.
Many competing products only have a single temperature sensor to compensate multiple input channels and often this sensor cannot effectively measure gradients or respond with the same thermal characteristic as the input terminal block.
Dataforth’s CJC is specified as:
±0.25°C typical accuracy at 25°C ambient
±1.0°C typical accuracy over -40°C to +85°C ambient
Dataforth’s initial engineering development thermocouple prototype used the standard 20-pin pluggable terminal block used on other MAQ20 input modules, and it used a single, internal sensor.
According to Engineering Manager John Lehman, “We measured up to an 8 degree Centigrade gradient from the top to the bottom of the input terminal block in typical system configurations, so the best we could hope for was to put the sensor in the middle and compensate with +4°C to -4°C accuracy for the top and bottom channels. A 1°C CJC accuracy specification was just not possible.”
Compared to other products, Dataforth’s MAQ20 thermocouple input modules provide better performance at one-half to one-third the cost and in a much more user friendly package! These modules are able to ensure such greatly improved accuracy because of their leading-edge design, which provides the precise terminal block temperature measurements that are essential for precise cold junction compensation.
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