Part 4 - Why Use Temperature Sensors Instead of Direct Wiring?
Lower Maintenance Time and Expense
Temperature transmitters have come a long way since the days of fixed-range, inflexible instruments. Moore Industries transmitters are not only universal in regards to input type and range; they also incorporate powerful sensor diagnostics that save considerable time and money.
Temperature transmitters with intelligent diagnostic capabilities help you keep track of sensor operation and quickly find and diagnose sensor failures. Capable of continually monitoring the temperature sensors, if a wire breaks or otherwise stops sending a signal during operation, the transmitter sends the output upscale or downscale to warn of sensor burnout and other unwanted conditions. Furthermore, our transmitters can tell you which wire has broken via an error message either on an integral digital display or using their PC configuration software. Specific fault messages eliminate the work of removing temperature sensors or checking all of the lead wires to diagnose a problem (Figure 6). During startups, in the middle of the night, or in the middle of winter, this can be a huge timesaving advantage. .
Avoid Lead Wire Imbalances
Where feasible, use 4-wire RTDs, and specify a temperature transmitter that is able to accept a “true” 4-wire RTD input. The advantage is that the fourth wire in a RTD circuit effectively cancels out errors due to resistance imbalances between the leads. Every ohm of imbalance in a RTD sensor’s lead wires can produce as much as a 2.5°C error in the measurement (Figure 7). Serious imbalances may be present from the very first day of commissioning without you even being aware of them. Typical causes include manufacturing variances; lead length differences; loose connections; terminal block corrosion; and work hardening from bending and other stresses.
Our intelligent temperature transmitters are capable of accepting “true” 4-wire RTD inputs and provide a constant current source to the outer leads of the RTD (Figure 8). The voltage drop is measured across the inner leads, which is a high impedance loop. There is essentially no current flow in the voltage loop, so voltage is directly proportional to resistance. Lead resistance is ignored. You will get a very accurate measurement providing the resistance value of the RTD plus corrosion, plus wire resistance, is less than 2,000 ohms (typically). A 4-wire RTD costs about the same as a 3-wire and can be used with less expensive, smaller gauge wire without concern for added resistance.
Temperature Sensors - Part 1 >>
