Line temperature sensor failure

Temperature sensors are divided into two categories: thermocouple (TC) and resistance temperature detector (RTD).
There are many resources to discuss the selection process. Therefore, in this discussion, we will focus on the performance of these resources in operation and the problems that may occur with each resource.
The sensor element is usually enclosed in a stainless steel jacket and then inserted into a thermowell.
Some sensor and thermowell combinations are paired as matching pairs, designed for precise fit to achieve optimal heat transfer and high accuracy.
Other methods use sharded components, which are closely matched by the installer.
Some users prefer to connect each TC or RTD directly to the host system input, but this will complicate the installation and affect the performance.
For example:
● The thermocouple cable from the sensor to the input card must match the sensor. If you need to install different types of sensors, you must change the wiring.
● Similarly, the input card must match the sensor, although some input cards allow multiple options.
● Weak signals from TC temperature sensor or RTD temperature sensor cannot be sent remotely, and will be subject to problems caused by electrical interference.
Adding a temperature transmitter near the sensor eliminates all of these problems:
● 4 – 20 mA with HART or digital protocols such as FOUNDATION fieldbus provides a more powerful signal that can be sent over longer distances.
● No special wiring or input card is required.
● Most transmitters are used with a variety of RTD temperature sensors and thermocouple types for easy sensor replacement when required.
● The multiplexer can capture data from multiple sensors and send it back through a cable.
● Smart transmitters can collect and send diagnostic, calibration and other data.
● Wireless HART transmitter is also an option without wiring and control system input (may be in short supply). These smart transmitters have built-in power modules that can operate for many years without any maintenance.
Although these functions are very useful, the most important progress in many applications is the intelligence of the transmitter: the ability to combine sensors and transmitters into intelligent instruments that can send diagnostic information to the host system.
When the temperature sensors have mechanical aging or wiring and termination problems, they will show obvious signs.
These can be found by the transmitter and used to attract attention before the initial problem escalates to a fault.
Many temperature measurement applications are affected by electrical noise, spikes, and signal loss.
The noise may come from electromagnetic interference, usually caused by radio, motor and lightning.
Other problems may be caused by wiring problems, mechanical shock or vibration. These can be detected, diagnosed and even corrected by complex transmitters.
Even if the sensor and transmitter are tightly coupled, noise or differential pressure can still be a problem, so most users should apply damping to suppress the impact.
Although damping can improve stability, it will increase the response time when the process temperature changes rapidly.
A better approach is to use the built-in signal verification function of the transmitter as part of its signal processing and diagnostic functions.
The thermal inertia of the temperature sensor in the thermowell makes it physically impossible to measure the extremely fast temperature change (i.e., 200 ° C to 400 ° C) within half a second.
Even if the transmitter sees this transient change between consecutive readings, it can reasonably assume that the change is a spike (or a drop if the change is negative) and simply repeat the last good measurement.
This method provides stability, no damping or slow response, and prevents the entire system from being damaged unnecessarily. However, it should not be applied to the place where the rapid full range deviation can be legally seen during measurement.
Although extreme mechanical shock events may damage the sensor, most failures are caused by continuous vibration, loose terminals, corrosion of connections, or chemical attack.
These weaken the sensor and wiring, causing spikes and drops in the number to increase over time. These will also lead to sensor drift, reducing accuracy over time.
The transmitter can detect and trend the increasing number of problems, so as to predict the impending failure and warn the maintenance personnel as early as possible to prevent the loss of the total signal.
The signal verification can dig deeply into the condition of the sensor itself, which can improve the safety and reliability of temperature measurement.
TC is generally more popular than RTD when fast response time or high temperature (>600 ° C) is involved and high accuracy is not required.
TC temperature sensors are generally physically more robust than RTD temperature sensors, but they may fail undetectably.
The node at the tip of the connection of different conductors is the temperature measurement point. However, if the insulation is damaged by physical shock or vibration and the two conductors form contact elsewhere, this new contact point will become the temperature measurement point, no matter where it may be.
Since this new junction is always far away from the thermal process, the damaged TC reading will be very low in most hydrocarbon applications, but the opposite is true in low-temperature applications.
Most processes are dangerous when they run too hot, so low readings create a safety risk.
Modern intelligent temperature transmitters can be configured to accept input from RTD temperature sensors or TC temperature sensors. When configured as TC, the transmitter uses its voltage circuit to determine the temperature.
However, the transmitter can also use its resistance measurement circuit (which can be used with RTD) to monitor the resistance of TC.
Although the resistance of the TC cannot be used to determine temperature, it does help detect and predict faults.
The change of TC circuit resistance can imply several things. If the resistance becomes infinite, the circuit is open. If the resistance drops from its normal level, there may be a short circuit. If the resistance increases, the wires or terminals may corrode.
These changes may be immediate, but more often they are gradual, so measuring and trending resistance changes and analyzing the results can be used to predict faults and improve reliability.
When temperature measurement is particularly important to the process, redundant sensors can be selected.
Temperature sensors are relatively cheap, and some transmitters can accept and process signals from multiple sensors. If the measured values from two sensors differ by one amount programmed into the transmitter, it can remind the operator of the problem.
Similarly, if a sensor fails, automatic standby switching allows the transmitter to immediately switch from the primary sensor to the standby sensor, thereby reducing the chance of losing temperature readings. This thermal function can be used with dual element sensors or two independent sensors.