Part 3 - Why Use Temperature Sensors Instead of Direct Wiring?

If you need even better accuracy, you can trim our universal transmitters to precisely match a particular sensor. Even though temperature sensors are designed to have a high degree of conformance to an established curve, each one (even precision temperature sensors) will vary slightly from their stated specification. In the past, transmitters assumed an accurate sensor measurement and processed it accordingly. Our transmitters can be trimmed to match the measurement actually being taken by each individual sensor.

Called Sensor-to-Transmitter Trimming, the transmitter is connected to the sensor and then immersed in calibration baths maintained at stabilized temperatures (Figure 4). The transmitter then “captures” two readings from the sensor representing the upper and lower range values, and stores them in non-volatile memory. The transmitter uses these values to compensate for deviations between the sensor’s stated linearization curve and its actual measurements. When our transmitters are paired with a 1000 ohm RTD, this technique results in amazing measurement accuracy of up to ±0.014°C (±0.025°F) over a 100° span.

To further enhance measurement accuracy, our transmitters can be trimmed to respond to two data points within the selected zero and span measurement range. This advantage allows a complete range to be monitored, while placing measurement emphasis on a specific segment of the range most critical to the process. For example, in Figure 5, the actual sensor curve is used in place of the ideal RTD curve between 20°C and 27°C. This provides incredible precision over a portion of span, while measuring the remainder of the span with our transmitter’s usual outstanding accuracy.

Simplify Engineering and Mis-Wiring
In place of numerous sensor lead-wire and DCS/PLC input board combinations, engineering designs and drawings will only need to show one wire type (twisted wire pair) and one input board type (4-20mA). This one wire and one input board system means maintenance is greatly simplified, and the chances of loop mis-wiring are virtually eliminated.

Ease Future Upgrades
Throughout the lifetime of a process, enhancements are routinely made to accommodate the manufacture of upgraded or even completely new products. Process changes may require different measurement ranges or greater temperature accuracy than was previously required. Either of these conditions may necessitate a change in the type of temperature sensors that are used.

In a direct wired system, changing temperature sensors generally means removing existing, and pulling new extension wire. This is because extension wire must be matched to the sensor type. Additional costs are incurred when the control system’s costly input boards (if also sensor-type dependent) must be replaced to accommodate the new temperature sensors.