Temperature sensor has drift alert feature
June 2001
Temperature Measurement
The sensor drift alert feature of the Rosemount Model 3244MV allows its users to maintain uninterrupted process operation while preserving the high measurement accuracy that their process requires.
Sensor drift
All temperature sensors drift for a variety of reasons, at a variety of rates. In general, the higher the process temperature the faster the drift rate. Platinum poisoning occurs at high operating temperatures and is the main cause of resistance temperature detector (RTD) drifting. In addition, thermocouples drift more than RTDs largely because of impurities in their wires and junctions. For this reason, thermocouples are primarily used in those applications where RTDs cannot be installed, at temperatures greater than 660ºC.
A proactive approach with sensor drift alert
The Model 3244MV sensor drift alert feature alerts the user when sensor drifting exceeds user-defined levels. This alert provides the user an opportunity to replace the sensor before the erroneous readings adversely affect their process and before the sensor fails and an important measurement is lost.
How it works
Drift alert works when either a dual-element sensor or two sensors are used, the 3244MV monitors the differential temperature. When both sensors are working properly the temperature difference between them should be close to zero. However, as the sensors drift at different rates, the difference between the two measurements increase. If the difference should exceed the user-defined maximum acceptable drift, the transmitter sends an alert. If desired, the transmitter can also be set to go into alarm when sensor drifting is detected. The user can then schedule a time to replace the sensor.
Detecting drift while the process is running
Sensor drift alert allows the user to detect sensor drifting without having to remove the sensor from the process. Without sensor drift alert, the only way to determine how much a sensor has drifted since installation is to remove it from the process and calibrate it in a laboratory. A common misconception about a drifting temperature measurement is that calibrating it will alleviate the problem. In reality, it is almost always the sensor that is the source of drifting. A drifting sensor that is left measuring the process could potentially produce worse results than an open sensor could; the sensor may drift so far that it is giving highly erroneous readings, adversely affecting the process. Without the sensor drift alert feature, the user may have no way of knowing that the temperature reading is incorrect. Undetected sensor drifting could result in an off-specification manufactured product and ultimately, an unscheduled shutdown.
Alpret Control Specialists
(011) 249 6700
[email protected]
www.alpret.co.za
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