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Measuring a 4-20ma Input With a Voltage Input Device

One of the most common data acquisition applications is using a thermocouple to collect temperature data. Many users seem to make the same mistakes when trying to use an A/D board to measure thermocouples. It is often believed that, since thermocouples produce a millivolt output, they can be connected to a voltage input measurement board, but it is not that simple. One must remember that a thermocouple is NOT LINEAR and also it requires COLD JUNCTION COMPENSATION, which the average A/D card does not provide.

Basically, Cold Junction Compensation is required to correct for errors produced when connecting a thermocouple to the screw terminals of any measurement device. Without it, readings will be erroneous by tens of degrees, and to make things worse, the readings will not be repeatable due to ambient temperature swings. Also, if the A/D card does not have any amplifiers on board, it will be necessary to boost the low-level signal produced into a measurable amount.

Here are two recommended methods for measuring thermocouples with a PC.

If you are designing a system from the very start, you should strongly consider purchasing a device that is designed to accept thermocouples directly. This way, the necessary cold junction will be included, as well as amplification and linearization. Then you can simply connect a thermocouple wire directly to the input terminals without any other equipment. The readings will usually be directly in degrees, making the entire process very simple.

What if you already own an A/D board that measures voltage? The solution is very simple. Use Thermocouple Signal Conditioners, which will provide cold junction, linearization, amplification, and as an added bonus, isolation which will prevent ground loops and other noise issues. The problem with them is that one conditioner is generally required per input, and at $200 or more each, could make the application very costly. Besides, thermocouple signal conditioners usually have a very slow response time, making them impractical for certain applications.

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