Process Temperature Sensitivity

Temperature sensitivity of processes should be considered early in the design before specifying equipment. Frequently, designers specify a single thermal measurement device for ease of use, replacement, and inventory. Not a bad idea if all temperatures are near the same range or the process is not sensitive. K-Type thermocouples are very popular due to their wide range of temperatures (-200˚C to 1250˚C), but when measuring high temps of 1000˚C and water temperatures around 50˚C what is the error associated with them?

Many designers do not realize that an error even exists and even process interfaces and data files will be set up with one or two decimal places. The standard error of a K-Type thermocouple is a minimum of 2.2˚C and at temperatures nearing 1000˚C the error nears 7˚C. So with even a 4.4 degree spread on error, reading temperatures in tenths of a degree is useless, requires more data-storage  space and adds irrelevant

information to a display or user interface.

RTDs (Resistance Temperature Detectors) are a cost-effective solution for lower temperature process measurements and do not require high-cost  wire to operate. Generally, RTDs offer greater precision than thermocouples within their operating range, but the type of RTD selected needs to be considered with the length of wire run. RTDs measure by resistance and wires have resistance, that means that longer wire lengths increase resistance.

There are four types of RTDs. Two-wire units are good for short wire lengths, and three-wire RTDs contains a compensation loop that allows increased wire lengths. Four-wire units come in two configurations; one configuration for high precision and another that offers ambient temperature readings and the compensation loop of a three wire.

Cost and inventory are not the only considerations when specifying thermal measurement devices. Consider what you are attempting to achieve with your measurements, consider the configuration of the system and determine the best combination.

How sensitive is your system to temperature error?