Thermocouples function on the principle of the Seebeck effect, which states that two dissimilar metals combined at two junctions produce an electromotive force, or EMF, at the junctions. The metals react to variations in temperature to create an EMF voltage with regard to the difference in temperature at the junction. RTDs function on the precept that electrical resistance increases with rising temperature. The types of metal used to fabricate the sensor impact accuracy, response time, measurement range and resistance to environmental stressors such as vibration.
Grounded Junction, OMEGACLAD® Probes
Ungrounded Junction, OMEGACLAD® Probes
Thermocouple junctions may be ungrounded or grounded. They are frequently covered with protective metal but may be left exposed to enhance response time. Grounding is often required to prevent accumulation of static charge, which may negatively influence accuracy. However, in case the thermocouple is grounded to machinery or other electrically powered equipment, circuit noise may obstruct the measurement. Several different metal combinations are used in the production of thermocouples. Each is categorized based on temperature range and acceptable measuring environments. Thermocouples encased in metal are fairly robust and, on average, much less susceptible to vibration than RTDs.
Types of RTDs
RTDs are available in thin-film or wire-wound types. Wirewound sensors are very accurate. They are made by winding nickel, copper or platinum wire around a ceramic or glass core to which the wire is also fused. Glass-core sensors can be immersed in a majority of liquids without protection while those with a ceramic core provide stability for extremely high temperature measurements. Platinum is the most favored wire, since it offers the best accuracy over the widest temperature range. ASTM E1137 is the international standard that prescribes tolerances for platinum resistance sensors. It is often used as one of the criteria for choosing a temperature sensor, as RTDs manufactured and tested according to this specification provide better reliability and improved performance.
Thin-film RTDs give considerably more vibration protection compared to wire-wound RTDs. They are formed by depositing a thin film of passivated platinum on a ceramic substrate. An electrical circuit is etched into the material to create the preferred resistance. These sensors show a practically linear temperature-resistance curve. Thus, they offer very accurate and stable measurements over a wide temperature range. Their compact size provides them the benefit of faster response times and better resistance to vibration and thermal shock.
Challenges Presented to Temperature Measurements in the Presence of Vibration
Vibration can result in mechanical stress in the wires of RTDs and thermocouples. Thermocouples are subject to vibration fatigue, which can cause short circuits and insulation failure. This may be apparent from intermittently high readings ensuing from the measurement being taken at the short instead of at the junction. Wire-wound RTDs are particularly vulnerable to vibration damage. The fine platinum wire used to wind the sensor has a standard diameter of 15 to 35 µm and is rather fragile. A damaged or broken RTD sensor wire may result in:
- Noisy signals
- An open circuit
- Sporadically high temperature measurements
Decalibration is another fault condition that may happen in thermocouples exposed to vibration. This is the process whereby the structure of the wire is changed to where the voltage-temperature features no longer comply with international standards. The key concern with decalibration is that the temperature measurements seem to be accurate. The readings will drift progressively over time. Testing the thermocouple against a recognized temperature is the most typical technique of detecting decalibration.
Types of Vibrations that Affect Sensors
Machine vibrations are typical in industrial procedures. They occur due to the movement of pumps, motors or compressors. The propensity to cause damage is proportional to the frequency and amplitude of the vibration. The amplitude is the force being applied to an object that is creating the vibration. For instance, the rotational speed in an electric motor will add to the amplitude of vibration. The faster the motor rotates, the greater the amplitude. Frequency is another factor in the severity of vibration. It is the rate at which a mechanical device travels back and forth under force. A machine can vibrate in several directions with fluctuating rates of frequency and amplitude.
Acoustical vibrations are produced by numerous mechanical systems, such as engines and turbines, as well as vehicle traffic and human voices. When acoustical noise makes contact with a structure, it becomes structural vibration. Sound waves can move anywhere there is air flow; thus, they can occur from any direction. Reverberation is the continuance of sound after the original has stopped. This is the consequence of sound waves reflecting from surfaces. Acoustic features can differ based on the shape and size of objects they reflect from, making it hard to predict how they will react.
Flow-induced vibrations occur due to the interaction of forces between fluid flow and the inertia of structures immersed in or conveying it. Fluid flow is a source of energy that can create structural and mechanical vibration. In cylindrical structures, vibrations are categorized as either axial-flow induced or cross-flow induced, according to the angle of inward flow with respect to the cylinder axis.
Vibration Resistant Thermocouples and RTDs
The OMEGA PR-21SL RTD is designed for use in thermowells and features spring loading to sustain contact between the probe and the thermowell in the presence of static and vibration. This ensures optimal heat transfer between the probe and thermowell and insulates the sensor against vibration. The PR-21SL RTD can be used in two, three or four wire applications, and fits standard 0.26” bore thermowells. A modifiable, self-gripping spring allows it to be employed in shorter thermowells.
OMEGA’s PR-31 RTD probe is vibration resistant and bendable. The probe is made of 316 stainless steel and the mineral-insulated cable allows the probe to be bent. The PR-31 RTD is vibration tested to MIL-STD-202G, Method 204D, Condition A and has a measuring range between -50 and 500 °C. It is available in 100 and 1000 Ω and can be employed in 2, 3 or 4 wire applications.
The M12M Series thermocouple probes can be used exposed, mounted into the procedure, or in a thermowell. The probes are available as a Type J thermocouple with 304 stainless steel sheaths or a Type K thermocouple with Inconel 600 sheaths. The Type K has a temperature range between -40 and 1150 °C, and the Type J has a temperature range of -40 to 600 °C. The M12M is provided as a standard with an ungrounded junction; a grounded junction is optional.
PR-31 RTD Probe
M12M Thermocouple Probe
Choosing the right thermocouple or RTD for an application will enhance performance and prevent sensor damage. Thermocouples are a versatile and economical means of temperature measurement and provide superior protection against vibration. Wire-wound RTDs provide excellent accuracy and a broader measurement range but are not as strong. Thin film RTDs offer very accurate and steady data and provide greater resistance to vibration compared to wire-wound RTDs. OMEGA also has tailored solutions for extremely severe vibration environments.
This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.
For more information on this source, please visit OMEGA Engineering Ltd.