Signal conditioning: More on temperature transmitters

05/17/2007


Last month’s article on temperature sensors was a record breaker for downloads. One part that deserves more discussion is the topic of signal conditioning and how using a transmitter at the sensor can solve problems encountered with weak voltage or resistance signals from temperature sensors and other types of unamplified sensors.

If you’ve worked with thermocouples, you know that they require specialized cables to match the specific thermocouple type, and specialized I/O cards at your PLC. You also know that the signal voltages put out are only a few milliVolts, so it doesn’t take much RFI, EMI, or other disturbance to corrupt the signal and throw off your reading if it has to be sent over any kind of distance. RTD’s aren’t much better. A 4-20 mA signal is very powerful and robust by comparison. A transmitter or signal conditioner mounted at the sensor can convert the signal and make it much more resistant to interference problems of all types.

Such devices are available from a number of manufacturers and can make life much easier. For example, a signal conditioner can:

  • Support a variety of different sensor types (for example, various thermocouple types or RTDs) allowing you to change the sensor without having to change cables or I/O card;

  • Eliminate lead wire imbalances possible with RTD’s;

  • Prevent ground loops;

  • Generate a variety of outputs, such as 4-20 mA, 0-10 Vdc, etc; and,

  • Reduce the need for specialized and expensive extension cabling.

When retrofitting a sensor with a signal conditioner, it is generally possible to use the old cabling to carry the signal, regardless of the type. In other words, you don’t have to replace thermocouple cable because you’re installing a signal conditioner.

Temperature sensor transmitters are available from:

—Edited by Peter Welander, process industries editor, PWelander@cfemedia.com , Control Engineering Weekly News (Register here and scroll down to select your choice of eNewsletters free.)





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