Why Use Temperature Sensors Instead of Direct Wiring?

Transmitters vs. Direct Wiring
When making temperature measurements, two ways have traditionally been employed to get process readings back to a monitoring and control system.

One method is to utilize temperature sensors extension wires to carry the low-level signals (ohm or mV) generated by field-mounted RTD or thermocouple temperature sensors (Figure 1). Another is to install temperature transmitters at or near the measurement point. The transmitter amplifies and conditions the signal from temperature sensors, and transmits it over a twisted wire pair back to the control room (Figure 2).

Direct wiring strategies have generally been considered less expensive and sometimes easier. Transmitter use, because of cost considerations, was often reserved for important loops and temperature sensors applications where signal and loop integrity was a must.

Today, our highly functional, yet very affordable, microprocessor-based field-mount temperature transmitters and temperature sensors are comparable in price to direct wiring strategies. When the additional advantages of using intelligent transmitters are factored in you will, in most applications, also save considerable time and maintenance headaches. This is especially true when the temperature sensors measurement point is located a long distance from the readout and control system.

Here are just some of the reasons why you should use Moore Industries’ intelligent temperature sensors in place of direct wiring strategies:

Cut Wiring Costs
Direct wiring temperature sensors to a control system requires the use of sensor extension wires. Not only are temperature sensors extension wires fragile, they also cost three times more than the common shielded copper wire used for a temperature transmitter’s 4-20mA signal. Using the less expensive wires for temperature sensors, transmitters can pay for themselves in wire and conduit costs alone. The longer the wire run, the greater the potential savings.

In retrofit situations, you may wish to switch to transmitters, but are reluctant to do so because some mistakenly believe that new copper wires must be run to accommodate the 4-20mA. This is not the case. Temperature transmitters can be installed at the temperature sensors, and the in-place RTD or thermocouple extension wires can be used to transmit the 4-20mA back to the control system. This means no additional installation time or material costs (including conduit) will be needed. And you still get all of the advantages of using temperature sensors with temperature transmitters.

Protect Signals From Plant Noise
Common in nearly every industrial environment, RFI (radio frequency interference) and EMI (electromagnetic interference) can negatively affect process signals from temperature sensors. Before you eliminate RFI and EMI as a possible culprit of erratic signals, consider just some of the common sources: Mobile and stationary radio, television, and hand-held walkie-talkies; radio-controlled overhead cranes; radar; induction heating systems; static discharge; high speed power switching elements; high ac current conductors; large solenoids and relays; transformers; ac and dc motors; welders; and even fluorescent lighting.