Temperature Sensors

Back to: Sensors for Vibration Testing

Vibration test engineers may monitor temperature during testing to ensure the ambient/surface temperature does not exceed system limits, especially if the device under test (DUT) or equipment is sensitive to temperature. The temperature change may be due to equipment exertion/friction, or the engineer may intentionally vary the temperature of their test environment to reflect the end-use environment (EUE).

We differentiate between ambient and surface temperature because the test environment’s temperature may fluctuate, but the changes may not transfer to the system’s materials.

Effect of Temperature on Vibration & Vibration Testing

Temperature change can alter a material’s mechanical properties, such as stiffness and damping. Some materials become less stiff as temperature increases, potentially shifting their natural frequencies. This change in material property can alter a system’s dynamic response to vibration excitation.

For example, the paper “FDS for Plastics” discusses that the vibration fatigue of plastics is more complicated than for metals because the effects of strain rate and temperature are more pronounced in plastics.

Temperature fluctuation can also cause materials to expand or contract, potentially introducing mechanical stress/strain into the system. These thermal stresses can influence a component’s resonant frequencies and mode shapes, particularly in DUTs with temperature-sensitive materials.

For systems with rotating machinery or bearings, temperature change can affect lubricant viscosity. Variation in lubricant viscosity alters the system’s frictional forces and damping characteristics, impacting vibration levels, stability, and wear rates.

When Should I Monitor Temperature?

Ambient/surface temperature fluctuation can be negligible during vibration testing, and many basic test setups do not include temperature monitoring. However, some DUTs have temperature-sensitive materials that require the engineer to include temperature sensing.

For example, test standards for electric vehicle (EV) batteries such as IEC 62660-2, SAE J2380, and UN 38.3 include temperature stressors/monitoring because EV batteries will combust under certain conditions.

Some vibration test methods include thermal cycling. Highly accelerated stress testing (HALT) applies accelerated stresses to prototypes in a non-uniform manner, incrementally increasing/decreasing the stress levels and sustaining them for a uniform time between each step. HALT may combine stresses such as temperature, humidity, and vibration simultaneously or back-to-back.

Temperature sensors can validate thermal models during charge/discharge cycles and engineers can link them to safety/shutdown systems, such as in the case of EV battery testing.

When deciding to monitor ambient/surface temperature during vibration testing, an engineer must consider their equipment’s susceptibility to temperature changes. They can determine material behavior by recording the EUE or accessing historical data. Engineers may also consider the standard test setups in their industry and the test methods most fitting for their DUT.

Sensors for Temperature Monitoring

Like vibration sensors, the various types of temperature sensors offer different advantages. When selecting a temperature sensor, engineers must consider the requirements of the test environment, including the temperature range, accuracy, sensitivity, response time, and test setup.

Thermocouples

Thermocouples are a popular temperature sensor option because they offer many advantages relative to their affordable price. They are versatile and relatively easy to use.

A thermocouple is an electrical device composed of two dissimilar metals joined together. It generates a voltage in response to temperature differences without requiring an external power source, which simplifies the measurement setup.

Thermocouple types differ in material composition, affecting their temperature range, sensitivity, and suitability for various environments. Several thermocouple types include:

• Type K: Suitable for general-purpose use; selected for its wide temperature range and durability. Performs well in oxidizing environments.
• Type J: Selected for its accuracy and high sensitivity at low temperatures. Ideal for vacuum and reducing environments.
• Type T: Ideal for low-temperature measurements (-200°C to 350°C). Performs well in both oxidizing and reducing atmospheres.

Regarding vibration test environments, thermocouples are generally rugged and can withstand harsh environmental conditions, including high vibration, mechanical stress, and thermal cycling. Their quick response allows them to accurately track rapid temperature changes, which is particularly useful in dynamic testing environments where temperatures can fluctuate rapidly.

Thermocouples can be susceptible to electrical noise and interference, particularly in high-vibration environments. Shielded or twisted-pair cables can help mitigate these effects. Additionally, signal conditioning, such as cold junction compensation, is necessary for correcting temperature variations at the connection points, and proper calibration is needed for accurate measurements.

Thermocouples are available in small sizes and various forms, making them adaptable to different test setups. While durable, their lifespan can be affected by factors like oxidation or corrosion, so periodic replacement might be necessary to maintain accuracy.

Resistance Temperature Detectors (RTD)

Resistance temperature detectors (RTDs), or resistance thermometers, offer high accuracy, stability, and repeatability over a wide temperature range. Although more expensive than thermocouples, their precision and reliability make them suitable for many test applications.

An RTD is an electrical device made from a material (often platinum) whose resistance changes predictably with temperature. RTDs require an external power source because their temperature measurement derives from a resistance change rather than a generated voltage. They come in various configurations, including thin-film and wire-wound types, and can be adapted to different test setups, including harsh industrial environments.

For vibration testing, RTDs are robust and can withstand moderate mechanical stress and thermal cycling. They provide precise temperature readings in dynamic testing environments where temperature control and monitoring are critical. Although their response time is typically slower than thermocouples, an RTD’s stability and accuracy make it a common choice for applications needing precise temperature measurements.

RTDs, such as Pt100 and Pt1000, offer high linearity over a wide temperature range (-200°C to 600°C for Pt100). The accuracy of RTDs can be very high, often within ±0.1°C, making them suitable for precise applications. Regular calibration is necessary to maintain this accuracy over time.

One potential issue with RTDs is self-heating, where the current passing through the sensor causes it to heat up slightly, affecting the measurement. Engineers can mitigate self-heating by using low current levels in the measurement circuit. Environmental factors such as electromagnetic interference (EMI) can also affect RTD performance. Proper shielding and grounding help minimize these effects for accurate temperature readings.

Thermistors

Thermistors offer high accuracy and sensitivity at an affordable price. Engineers often select thermistors for precise temperature measurements within a narrow temperature range.

A thermistor is a type of resistor whose resistance changes significantly with temperature. Thermistors typically contain metal oxides, which provide predictable resistance-temperature characteristics. They need an external power source and usually operate within specific temperature ranges, making them suitable for applications that require precise temperature control and measurement.

Thermistors come in two main types: negative temperature coefficient (NTC) and positive temperature coefficient (PTC).

• NTC thermistors: Resistance decreases as temperature increases; used for temperature measurement and control in a wide range of applications due to their high sensitivity and precision at lower temperatures.
• PTC thermistors: Resistance increases as temperature increases; used for over-current protection and as resettable fuses in electronic circuits.

Vibration test engineers select NTC thermistors for monitoring and controlling temperatures due to their precision and sensitivity. They can integrate NTC thermistors into various test setups for accurate temperature readings.

Thermistors are not as rugged as thermocouples in extreme conditions but are highly effective in controlled environments where precision is most important. It’s worth noting that thermistors can be sensitive to environmental conditions like humidity, which can affect their resistance and performance. Proper environmental control and calibration can mitigate these effects, ensuring reliable measurements.

Temperature Sensors for Vibration Control

Temperature’s effect on vibration involves interactions between material properties, thermal expansion, lubrication characteristics, and structural dynamics. Engineers must consider these factors when operating mechanical systems subjected to varying temperature conditions to ensure optimal performance, durability, and safety.

Vibration sensors can act as monitoring devices for system safety. As will be discussed in the next lesson, other sensors must be able to operate under a test’s range of environmental conditions, including temperature. Engineers must also consider the effect of temperature on other sensors and equipment.