If you have one or more of these in your plant, you may have a RFI/EMI problem. The result is sometimes just a minor inconvenience. Other times it can be as serious as a costly plant shutdown.

In a direct wiring scheme, the high impedance, low-level signals generated by RTD (ohm) or thermocouple (mV) temperature sensors are particularly susceptible to the signal degrading effects of RFI/EMI. Compounding the problem, sensor extension wires can behave much like an RFI/EMI antenna by actually drawing plant “noise” to the wires, and affecting weak, low-level signals. Conversely, a properly designed temperature transmitter effectively negates the effects of incoming RFI noise by converting a sensor’s low-level signal to a low impedance, high-level analog signal (typically 4-20mA). This amplified signal is resistant to RFI/EMI, and can accurately withstand long distance transmission from the field, through a noisy plant, back to the control room. When specifying your transmitter, always check for RFI/EMI protection. If there’s no specification given, it’s usually because the instrument is not designed to resist noise. It will probably not perform very well in a noisy plant environment.

Stop Ground Loops

Make sure to choose an isolated transmitter (even today, not all are!). Our transmitter’s input/output/power signal isolation protects against signal inaccuracies caused by ground loops. This is important even when using ungrounded thermocouples because their insulation will eventually break down.

With direct wiring, it is necessary to match the sensor type to input-specific DCS and PLC input cards. Input-specific cards for temperature sensors usually cost a lot more per point than a 4-20mA input card. And since numerous sensor types are routinely used in a plant, a large number of different cards must be ordered and kept on hand as spares. This is not only expensive, but can result in a lot of confusion when installing, maintaining, and replacing equipment. Our temperature transmitters incorporate powerful microprocessors that allow them to be easily configured to accommodate nearly all types of temperature sensors. Their 4-20mA output signal is control-system ready. This allows you to standardize on (and stock) less expensive 4-20mA DCS and PLC input cards.

In an intelligent temperature transmitter strategy, you simply change out the temperature sensors and reconfigure the transmitter to accommodate the different types of temperature sensors. The loop’s twisted pair wiring and existing 4-20mA input boards don’t even have to be touched. Because you never know what sensor you’ll end up with, make sure to go with a universal transmitter that configures to accept all common types of temperature sensors and ranges (Figure 3).

Using temperature transmitters can substantially enhance measurement accuracy. DCS and PLC systems measure readings over the entire (very wide) range of a sensor. It is well known that measuring a narrower range produces far more accurate measurements. Transmitters can be calibrated to any range within a sensor’s overall capabilities. Their measurements are more precise than is possible with most direct wiring strategies. Our transmitters deliver accuracy ratings of ±0.13°C (±0.23°F) when paired with a common Pt100 RTD sensor over a 200° span.