7. When do I need a transmitter?

There are no firm rules on when to specify transmitters. Each temperature monitoring system has unique cost and accuracy requirements, and unique design problems to overcome. 

In general, transmitters offer three advantages: 

  • Eliminate lead effects from temperature readings
  • Output is more immune to electrical noise
  • Condition the RTD (Resistance Temperature Detector) signal 

A. RTD leads effects: Distance between sensors and control points are an obvious reason to specify transmitters. 

Factors to consider:

  • Transmitters are more accurate than 2-wire RTDs, if this distance is more than a few feet
  • Over large distances, even 3-wire RTD's can be inadequate for the required accuracy
  • 4-wire RTD measurement circuits will effectively eliminate lead wire errors, but the space and cost of two extra leads may exceed the cost of a transmitter
  • High resistance sensing elements reduce the effects of lead resistance, but do not eliminate them
  • RTD measurement circuits are typically low signal (1 mA or less), and are prone to electrical noise – especially in long wire runs. 
  • At distances over 500 feet, a transmitter may be the only way to carry the accuracy of the RTD to the control electronics    

The graph below shows how lead wire resistance introduces temperature measurement error.

RTD 2-lead Temperature Error

RTD 3-lead Temperature Error

B. Electrical Noise: Noise from motors, fluorescent lamps, or other sources will degrade resistance or voltage signals, but has little effect on a transmitter’s controlled current. You may therefore want to use transmitters over relatively short distances in especially noisy areas. The transmitter may even cost less than specially shielded extension wires. Simply twisting the transmitter wire pair is very effective at reducing noise; shielded cable is usually not necessary. 

C. Signal Conditioning: You may want transmitters for their signal conditioning circuitry alone. RTD resistance/temperature curves are non-linear. A transmitter changes the RTD resistance to an industry standard 4-20 mA, while simultaneously linearizing the output with temperature. With the transmitter’s linear response, you don’t need complex equations to interpret readings. 

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