This page provides a comprehensive collection of interview questions and answers related to temperature measurement in industrial and process automation. It is designed to help freshers and experienced engineers understand the core concepts, components, installation methods, and troubleshooting of different temperature sensing devices. The guide includes 50+ carefully structured questions and answers for each major temperature measurement device, including RTDs, thermocouples, transmitters, and pyrometers.
An RTD (Resistance Temperature Detector) is a temperature sensor that works on the principle that the resistance of a metal increases with temperature. RTDs, commonly made with platinum, are known for their high accuracy and stability and are widely used in precision temperature measurement.
A Thermocouple is a temperature sensor made from two dissimilar metals joined together at one end. It generates a voltage that corresponds to the temperature difference between the measuring junction and the reference junction. Thermocouples are robust, cost-effective, and suitable for a wide temperature range.
A Pyrometer is a non-contact temperature measuring instrument that detects infrared radiation emitted by an object. Pyrometers are ideal for measuring high temperatures or moving objects, and are often used in steel, glass, and ceramic industries.
Throughout this page, users will find detailed questions segmented into definition, installation, calibration, accuracy, application, and troubleshooting of each device. The content is crafted to enhance both technical knowledge and practical understanding, making it a valuable resource for job interviews, fieldwork, and instrumentation training.
An RTD is a temperature sensor that operates based on the principle that the resistance of a metal increases with temperature. Platinum is the most commonly used element.
RTDs work by measuring the electrical resistance of a metal element. This resistance changes with temperature, and the change is used to determine the actual temperature.
RTDs operate on the positive temperature coefficient of metals, meaning resistance increases as temperature increases.
PT100, PT500, and PT1000 RTDs are most common. The number refers to the resistance in ohms at 0°C.
Platinum offers excellent linearity, stability, and repeatability over a wide range of temperatures.
Use a thermowell for mechanical protection and process isolation. Insert the RTD to the appropriate immersion depth for accurate readings.
Improper immersion can lead to incorrect temperature readings due to stem conduction error or poor thermal contact.
Good thermal contact ensures fast and accurate temperature sensing by reducing lag and conduction losses.
Yes, but always follow manufacturer recommendations. Horizontal installation must ensure proper immersion and avoid stress on the probe.
A thermowell is a protective sheath that shields the RTD from pressure, flow, and chemical exposure, allowing easy replacement.
Visual inspection, verifying correct wiring, checking sensor resistance, loop checking with transmitter/PLC, and system verification.
Check the number of wires (2, 3, or 4), follow terminal markings on the transmitter, and measure resistance to confirm proper connections.
Loop checking ensures the signal path from the RTD to the control system is functional and accurately processed.
At 25°C, a PT100 RTD typically has a resistance of about 109.73 ohms.
Verify sensor type configuration (PT100, 3-wire, etc.), range settings, output signal (4–20 mA), and grounding.
Use a multimeter to measure resistance and verify it changes with applied temperature. Compare against standard RTD tables.
Exactly 100 ohms.
Use a multimeter: infinite resistance indicates an open circuit, zero resistance indicates a short circuit.
It's the heat loss or gain through the sensor stem that causes inaccurate readings if not immersed properly.
Long wires and temperature variation can introduce resistance error, especially in 2-wire RTDs.
By placing the RTD in a known temperature source (e.g., dry block calibrator) and comparing with a standard reference thermometer.
Dry block calibrators, temperature baths, reference PRTs, and high-accuracy multimeters or calibrators.
±(0.15 + 0.002 × |t|) °C, where t is the temperature in °C.
To detect drift or deviations and ensure measurement accuracy over time.
Yes, periodic re-calibration extends life and ensures accuracy.
A 2-wire RTD has only two connections. Wire resistance adds error unless corrected. Suitable for short runs or low-accuracy applications.
3-wire RTDs use a third lead to help cancel out lead resistance. Most common in industrial use due to accuracy and cost balance.
4-wire RTDs use two wires for current and two for voltage measurement, offering the highest accuracy by eliminating wire resistance completely.
4-wire RTDs are best suited for long cable runs where high accuracy is needed.
Not directly. You’d need a different sensor. Conversion doesn’t remove resistance error introduced by the cable.
RTDs are more accurate, especially in the 0–200°C range, with less drift and higher repeatability.
Mechanical damage, lead wire issues, element drift, corrosion, or thermal cycling over time.
Lead resistance changes with ambient temperature, especially in 2-wire RTDs, causing errors.
Very stable—often better than 0.05°C/year in controlled conditions.
It refers to how closely resistance changes follow a straight line with respect to temperature. Platinum RTDs are nearly linear up to ~600°C.
Process industries, food & pharma, HVAC, laboratory equipment, and automotive testing.
Yes, RTDs can operate at temperatures as low as -200°C when properly designed and calibrated.
Yes, with proper explosion-proof or intrinsically safe transmitters and sensor heads.
Yes, with sealed probes or thermowells designed for immersion in liquids.
Because of their high accuracy, stability, and compliance with quality standards like FDA and GMP.
Slower response time, higher cost, and limited temperature range compared to thermocouples.
Typically up to 600°C for platinum RTDs, depending on insulation and construction.
The wire-wound or thin-film elements can be damaged by vibration or impact, affecting accuracy.
Yes, repeated expansion and contraction can degrade element or lead wire connections.
Yes, especially in 2-wire configuration. Use 3-wire or 4-wire to compensate.
Loose connections, EMI interference, broken leads, or unstable process temperatures.
Measure resistance. An open circuit shows infinite resistance. Some transmitters indicate it as high out-of-range temperature.
Compare with a calibrated reference instrument or calibrate the transmitter input.
In most cases, RTDs are not polarity sensitive, but in 3-wire or 4-wire systems, improper connections may affect compensation.
Yes, due to mismatched sensor type selection, lead compensation errors, or physical sensor damage.
A thermocouple is a temperature sensor made of two dissimilar metals joined at one end, producing a voltage proportional to temperature.
Thermocouples work based on the Seebeck effect, where a voltage is generated at the junction of two dissimilar metals due to a temperature difference.
Common materials include Iron-Constantan (Type J), Chromel-Alumel (Type K), and Copper-Constantan (Type T).
It is the principle that a voltage (EMF) is generated in a circuit made of two different metals when their junctions are at different temperatures.
Using a temperature transmitter or millivolt meter calibrated for the specific thermocouple type.
Correct polarity, appropriate sheath material, proper junction positioning, and shielding from electrical noise are critical.
Mineral-insulated (MI) thermocouples can be bent within limits. Fabricated sheath types are usually rigid.
At least 10 times the diameter of the thermowell or probe to ensure accurate temperature sensing.
Keep away from high-voltage lines and use twisted, shielded cables to minimize EMI.
Reversed polarity leads to incorrect temperature readings or negative values.
Verify type, check continuity, validate cold junction compensation, and test signal output.
Check transmitter or controller configuration and compare wire colors based on standard codes (e.g., IEC or ANSI).
Heat the tip using a heat gun or hot water and observe signal change on the controller.
It converts the millivolt signal from the thermocouple into a standard 4–20 mA or digital output for controllers.
It corrects for the temperature at the reference (cold) junction, ensuring accurate measurement.
Set the meter to millivolt mode, heat the junction, and look for small voltage changes corresponding to temperature.
If the loop resistance is very high or infinite, the thermocouple may be broken or disconnected.
It can cause signal interference, short circuits, or measurement errors in grounded systems.
Using a temperature simulator or millivolt source to mimic expected signal based on thermocouple tables.
Check integrity of wiring, signal verification, grounding, and loop continuity.
Compare readings to a known temperature source or simulator, adjusting transmitter if needed.
Varies by type — for example, Type K (standard grade) is ±2.2°C or ±0.75% of reading.
Cold junction drift, EMI, poor contact, and incorrect configuration are common error sources.
Yes, using a temperature simulator or reference bath, provided access and conditions allow.
It depends on criticality; typically every 6–12 months in process applications.
Type J, K, T, E, N, R, S, and B are widely used depending on temperature range and environment.
Type J (Iron-Constantan) is limited to 750°C, while Type K (Chromel-Alumel) supports up to 1100°C.
Type T (Copper-Constantan) is ideal for low temperatures down to -200°C.
Type K and Type N are preferred due to their resistance to oxidation at high temperatures.
Type B thermocouples can measure up to 1700°C in laboratory conditions.
It is the reference point at the instrument where the thermocouple wires connect to copper terminals.
Using a precision thermistor or RTD sensor inside the transmitter or input card.
You will get incorrect temperature readings, often with large errors.
Yes, in some systems you can enter a fixed ambient temperature, though auto-compensation is preferred.
A block that maintains the cold junction terminals at a uniform temperature for accurate measurement.
Used in furnaces, engines, process heating, HVAC, and chemical reactors.
They are inexpensive, rugged, and capable of measuring very high temperatures.
They have lower accuracy and are more susceptible to noise than RTDs.
Because RTDs and other sensors may fail or degrade beyond 600–800°C, while thermocouples remain functional.
Yes, if made with food-grade stainless steel sheath and used with sanitary fittings.
Causes include electrical noise, poor grounding, broken junction, or cable issues.
Check for wire breakage, open loop, polarity reversal, or input card fault.
Compare it regularly with a reference sensor; look for gradual changes in output.
Likely due to reversed polarity or faulty cold junction compensation.
Yes, especially if the insulation breaks down or if condensation enters the connection head.
To prevent electromagnetic interference (EMI) from affecting the small millivolt signals.
Gently wipe with alcohol or replace the probe; avoid abrasive cleaning.
Use IEC or ANSI standards; in IEC, red is always negative.
To ensure continuity of the thermoelectric properties and avoid measurement error.
Isolate the loop, ensure system is not under load, and wear proper PPE before removal.
A temperature transmitter is a device that converts the signal from a temperature sensor (like an RTD or thermocouple) into a standardized output signal, typically 4-20 mA or a digital protocol like HART.
The main function is to send accurate temperature readings to the control system over long distances while minimizing noise and signal loss.
A temperature sensor detects the actual temperature, while a transmitter conditions and converts the sensor signal for transmission to control systems.
Common types include head-mounted transmitters, DIN rail-mounted transmitters, and field-mounted transmitters.
Smart transmitters support digital communication protocols (e.g., HART, Foundation Fieldbus) and offer features like diagnostics, configuration, and status monitoring.
It takes input from a sensor (RTD/thermocouple), amplifies and conditions the signal, and transmits it as a 4–20 mA or digital output to a PLC or DCS.
It filters, linearizes, and converts the raw sensor signal to a usable form for output.
It uses an internal amplifier and analog-to-current conversion circuit calibrated to represent the temperature range.
Analog transmitters send continuous current signals (4–20 mA), while digital transmitters send encoded data over communication protocols like HART.
It's a standard for transmitting analog signals over long distances using current, where 4 mA represents zero and 20 mA represents full scale.
As close as possible to the sensor to minimize signal loss and improve accuracy.
Ensure proper location, environmental protection (IP rating), electrical isolation, and secure mounting.
Yes, using extension wires to connect the sensor, especially in high-vibration or high-temperature areas.
It helps reduce electromagnetic interference and ensures signal integrity and safety.
Twisted pair, shielded cables are commonly used to reduce noise pickup.
Verify wiring, power up, calibrate, simulate sensor input, and check output signal consistency.
Using a loop calibrator or multimeter in series to check if the output is within 4–20 mA as expected.
Loop calibrator, multimeter, HART communicator, and configuration software (if digital).
Using a simulator device or a resistance decade box for RTDs, or millivolt source for thermocouples.
Wrong sensor type selected, wiring errors, poor grounding, open loop, or incorrect scaling.
Using a HART communicator or PC software to set sensor type, range, damping, and scaling values.
It defines the lower and upper temperature limits corresponding to 4 mA and 20 mA respectively.
Apply known input values (resistance or mV) and adjust the output to match expected current using software or communicator.
Zero is setting the 4 mA point, span is setting the 20 mA point of the measured range.
Depending on process criticality and manufacturer recommendation, typically once per year.
It is a safety feature where the transmitter outputs 3.6 mA or 21 mA when the sensor fails.
Inject known values and observe the resulting current to verify correct operation.
Wrong input reference, incorrect range setup, unstable input signal, or skipping warm-up time.
Factory calibration is done by the manufacturer to standard specs, field calibration adjusts for actual site conditions.
It can result in inaccurate measurements, faulty control actions, or safety risks.
Accuracy ranges from ±0.1% to ±0.5% of span, depending on the model and sensor type.
Sensor drift, EMI, power supply instability, wiring resistance, and ambient temperature effects.
It is the time the transmitter takes to respond to a temperature change, influenced by sensor type and electronics.
Yes, many transmitters detect open circuits and send a fault signal (e.g., output 21 mA).
Check for loose connections, fluctuating supply voltage, EMI, or sensor issues.
They are used in industries like oil & gas, chemical, food processing, HVAC, and power plants.
Transmitters improve signal integrity, enable long-distance transmission, and provide signal scaling and diagnostics.
Most transmitters are single input, but multi-channel transmitters are available for some applications.
The transmitter will not operate and may be damaged if protections are not in place.
Check loop power, wiring, sensor health, configuration, and signal input.
It is a transmitter that supports HART protocol for digital communication over the 4–20 mA signal line.
Remote configuration, diagnostics, multi-variable support, and status monitoring.
Using a HART communicator or compatible software with a HART modem.
It is a fully digital device using Fieldbus protocol for complex automation networks.
It provides internal health monitoring, sensor drift alerts, loop integrity, and ambient effects detection.
Yes, many modern transmitters support digital gateways for IIoT and predictive maintenance.
Based on sensor type, signal output, range, protocol, mounting style, and environmental conditions.
Local temperature indication and diagnostics without needing external tools.
Store in dry, dust-free, static-safe packaging and handle with care to avoid damage.
Isolate power, confirm zero signal, discharge loops, use insulated tools, and follow lockout/tagout procedures.
A pyrometer is a non-contact temperature measurement device that detects thermal radiation emitted by an object to determine its surface temperature.
Unlike RTDs or thermocouples that require direct contact with the process, pyrometers measure temperature remotely using infrared or optical radiation.
Pyrometers are ideal for measuring very high temperatures (up to 3000°C), where contact sensors would be damaged or inaccurate.
Pyrometers are used in steel, glass, cement, and semiconductor industries for monitoring furnace, molten metal, and other extreme heat sources.
Key benefits include non-contact measurement, fast response time, high-temperature capability, and safety in hazardous areas.
The main types are optical pyrometers, infrared pyrometers, and radiation pyrometers.
An optical pyrometer uses visible light to compare the brightness of a hot object with a calibrated filament to estimate temperature.
Infrared pyrometers detect infrared radiation to determine temperature and are commonly used for industrial automation and maintenance.
A radiation pyrometer measures total thermal radiation emitted by a body, often used for high-accuracy remote sensing.
A ratio pyrometer compares radiation at two wavelengths to compensate for emissivity variations or dust interference.
Pyrometers should be installed with a clear line of sight to the target, away from steam, dust, or obstructions, and aligned perpendicularly.
This depends on the model and optics. Short-range pyrometers can be installed within 0.5–5 meters, while long-range models support >20 meters.
The FOV must fully cover the target. An incorrect FOV may result in inaccurate readings due to background interference.
Yes, pyrometers require protection from excessive ambient heat using cooling jackets or air purging systems.
Only certain IR-transparent materials like germanium or sapphire glass are suitable. Standard glass can distort IR signals.
Pyrometers should be calibrated annually or more frequently if used in critical processes.
Emissivity is a material's ability to emit thermal radiation. Incorrect emissivity settings can lead to large measurement errors.
Yes, portable blackbody sources or certified calibration devices are used for on-site or lab calibration.
A blackbody is a reference emitter with a known emissivity of 1.0 used for calibrating IR devices.
Most pyrometers have adjustable emissivity settings via software or onboard controls depending on the material being measured.
Common outputs include 4-20 mA, 0-10V, RS-485, Modbus, and digital relay outputs.
Yes, industrial pyrometers often offer analog and digital outputs suitable for PLC/DCS integration.
Shielded cables, signal converters, or fiber optics may be used to minimize interference.
Pyrometers have response times as fast as 1 ms to 200 ms depending on model and accuracy.
Advanced models come with LCDs or software for real-time display, parameter setting, and data logging.
Emissivity error, FOV alignment, environmental interference, and dirty optics all affect accuracy.
Accuracy can range from ±1°C to ±2% of reading depending on the device and conditions.
Drift refers to gradual deviation of output over time due to optics aging or temperature effects.
Use proper calibration, correct emissivity settings, clean optics, and align sensor properly.
Yes, stray light and surrounding heat can affect sensor performance unless properly shielded.
Yes, non-contact nature makes pyrometers suitable for moving surfaces like rollers, slabs, or engines.
Used for furnace exit temperature, billet heating, forging, welding, and casting monitoring.
Yes, specialized IR pyrometers are used for transparent or molten glass surfaces.
They monitor kiln shell temperature, flame, and clinker cooling to optimize energy usage.
Yes, infrared pyrometers are widely used in predictive maintenance to detect overheating components.
Limitations include dependency on emissivity, obstruction by dust/steam, and expensive models for harsh environments.
No, they measure only surface temperature. Internal readings require thermocouples or RTDs.
Yes, dust or condensation on optics can block IR and degrade accuracy.
Excessive humidity may affect readings and condensation may distort optics, especially for low-cost models.
They are best for medium to high-temperature processes; for very low temps, RTDs are more suitable.
Possible causes include environmental noise, incorrect emissivity, dust, or power supply instability.
Check if sensor is blocked, wiring is faulty, or output is stuck due to internal malfunction.
Use a blackbody source or cross-check with a calibrated contact sensor in the same application.
Ensure consistent target surface, stable alignment, and use averaging/filtering features.
Yes, extreme heat, vibration, or moisture can damage optics or internal electronics.
Consider temperature range, wavelength, spot size, response time, environment, and target material emissivity.
Laser-targeted models must meet safety standards and should not be aimed at humans or reflective surfaces.
Yes, ATEX-certified and flameproof housing models are available for Zone 1/Zone 2 environments.
Yes, models with Modbus, Profibus, or Ethernet output can be integrated into SCADA for monitoring.
Periodic cleaning of optics, checking cable connections, verifying calibration, and software updates are typical maintenance tasks.
RTD (Resistance Temperature Detector): Measures temperature by resistance change, accurate and stable, used in low-to-medium temperature ranges (up to ~600°C).
Thermocouple: Measures voltage due to temperature difference, wider range (up to 1700°C), faster response but less accurate.
RTD: Linearly increasing resistance with temperature, stable, accurate, suitable for industrial use.
Thermistor: Non-linear resistance, high sensitivity, low-cost, used in narrow ranges like HVAC or medical.
Thermocouple: Works via Seebeck effect, suitable for high temperatures and rough conditions.
Thermistor: Better for small temperature changes with higher accuracy in limited ranges.
RTD: Contact sensor, requires direct contact with surface or media.
Infrared Pyrometer: Non-contact sensor that measures surface temperature by detecting emitted IR radiation.
Thermocouple: Contact-based sensor, can measure internal temperatures.
Pyrometer: Non-contact device ideal for moving, hazardous, or inaccessible objects.
Pyrometer: Measures temperature at a single point.
Thermal Camera: Provides a full thermal image and temperature mapping over an area.
Sensor (RTD/Thermocouple): Provides raw signal (resistance or mV).
Transmitter: Converts sensor output into standard signal (4-20mA or digital) for control systems.
Contact Sensors (RTD, Thermocouple): Must touch the object, slower but generally more accurate.
Non-Contact (Pyrometer, Thermal Camera): Safer and faster for moving or hot objects, suitable for harsh environments.
Temperature sensors are essential for accurate and reliable thermal monitoring in industrial automation. Below is a brief summary comparing the most commonly used types:
| Sensor Type | Contact / Non-Contact | Accuracy | Temperature Range | Output Signal |
|---|---|---|---|---|
| RTD | Contact | High | -200 to 600°C | Resistance (Ω) |
| Thermocouple | Contact | Medium | -200 to 1700°C | Millivolt (mV) |
| Thermistor | Contact | Very High (narrow range) | -50 to 150°C | Resistance (Ω) |
| Infrared Pyrometer | Non-Contact | Medium | 0 to 3000°C | Analog / Digital |
| Thermal Camera | Non-Contact | Medium | -40 to 2000°C | Visual + Digital |
| Transmitter | Depends on sensor | NA (Signal Converter) | — | 4–20 mA / HART / Fieldbus |
Understanding the key features and limitations of each sensor type helps in selecting the right technology for different temperature measurement applications in process industries.