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ASME PTC 19.3-1974 (2004) scan — Temperature Measurement
🌡️ ASME PTC 19.3-1974 (2004) is Part 3 of the Instruments and Apparatus supplements in the ASME Performance Test Codes. It presents a summary discussion of temperature measurement as related to Performance Test Code work, with particular emphasis on basic sources of error and means for coping with them. Replacing an older supplement published during 1952–1961, this edition was approved in 1974 and reaffirmed in 2004. Temperature is measured in nearly every performance test — boilers, steam turbines, gas turbines, compressors, heat exchangers — and its accuracy directly determines the results of efficiency, power, and heat-balance computations, making this Part one of the most-cited instrument supplements in the PTC system.
💡 The Part opens by correcting a common misconception: temperature is generally considered one of the simplest and most accurate engineering measurements, which is decidedly wrong — under some conditions accurate measurement is impossible with present knowledge. The desired accuracy in PTC work is obtained only by suitable precautions in the selection, installation, use, and interpretation of temperature-measuring instruments.
1. Temperature Scales and General Concepts 📌
Chapter 1 covers general aspects of temperature measurement and temperature scales:
- Fahrenheit and Celsius (centigrade) scales are the two generally used engineering scales; the Standard also discusses other scales, such as absolute Kelvin and Rankine;
- Conversions between scales and unit conversion factors are collected in the Appendix for conversion between U.S. Customary and SI units;
- Instrument selection must consider range, accuracy, response speed, installation conditions, and medium characteristics.
| Temperature Scale | Unit Symbol | Description |
|---|---|---|
| Celsius | °C | Ice point 0°C, boiling point 100°C of water |
| Fahrenheit | °F | Ice point 32°F, boiling point 212°F of water |
| Kelvin | K | Thermodynamic scale; 0 K is absolute zero |
| Rankine | °R | Absolute scale of the Fahrenheit system |
⚠️ The temperature basis must be consistent throughout performance-test computations: efficiency, heat balance, and property interpolation are all sensitive to the temperature reference. Reports should state the scale and conversion basis used, and U.S. Customary and SI units must not be mixed.
2. Types of Temperature-Measuring Instruments 🎛️
The Part devotes chapters to each instrument type:
| Instrument Type | Operating Principle | Typical Applications | Notes |
|---|---|---|---|
| Thermocouples | Seebeck effect; EMF from junctions of dissimilar metals | Mainstream industrial measurement; wide range (–200°C to 2000°C+) | Reference-junction compensation, calibration type matching, insulation |
| Resistance thermometers | Metal resistance varies with temperature (platinum most common) | High accuracy, low temperature, laboratory | Lead-wire compensation, self-heating, calibration |
| Liquid-in-glass thermometers | Thermal expansion of liquid | Local indication, calibration reference | Emergent-stem correction, scale errors |
| Filled-system thermometers | Pressure of sealed fluid varies with temperature | Remote indication, local control | Capillary head and ambient effects |
| Radiation thermometers | Measures emitted radiant energy | High-temperature surfaces, non-contact | Emissivity correction, field of view, dust |
| Optical pyrometers | Visual comparison of filament and target brightness | High temperature (>700°C) furnaces/metals | Operator skill, gray-body correction |
| Bimetallic thermometers | Differential thermal expansion of two metals | Local indication | Lower accuracy; general indication |
💡 Thermocouples are the preferred choice for performance tests: wide range, multi-point installation, and compatibility with data-acquisition systems. Use matching extension wire for the calibration type, set the reference junction correctly, and calibrate key points against a standard temperature source before and after the test.
3. Installation and Sources of Error 🧪
Chapter 1 and the instrument chapters discuss installation requirements and error sources. Common accessories include thermowells and other protective fittings. The principal error sources include:
- Conduction error: heat conducted along the well and leads shifts the sensing element temperature away from the medium temperature;
- Radiation error: radiation exchange between the sensor and surrounding walls changes its temperature — significant in high-temperature gas measurement;
- Heat transfer at low velocity: low velocity means a small convective heat transfer coefficient and relatively larger error;
- Aerodynamic heating and high velocity: high-velocity gas stagnates and heats at the probe, requiring a distinction between static and total temperature;
- Gradient error: with a temperature gradient near the measuring point, the sensor reads a weighted average rather than the true medium temperature;
- Dynamic error: when medium temperature changes with time, sensor thermal inertia lags the reading.
🚨 Radiation error in high-temperature flue gas can amount to tens of degrees: a bare thermocouple cooled by cold-wall radiation reads low. Use shielded or aspirated thermocouples or apply radiation corrections — with aspirated thermocouples at furnace outlets where needed — so efficiency computations in performance tests are not biased systematically.
4. Calibration and Engineering Practice Essentials 🛠️
A dedicated chapter covers calibration of instruments:
- Traceability: calibrate against reference temperature sources traceable to national standards;
- Calibration frequency: calibrate key points before the test and spot-check after the test to evaluate drift;
- Installation checks: sufficient insertion depth (typically 10–15 times the well diameter), measuring points away from dead zones and wall effects;
- Multi-point synchronization: sample all temperature points synchronously during performance tests so time alignment does not add uncertainty.
⚠️ Per the Part (and PTC 1 General Instructions) only the measurement methods and instruments specified in individual test codes are mandatory; other methods and instruments treated in the Instruments and Apparatus supplements shall not be used unless agreeable to all parties to the test. Confirm the measurement method in writing before the test.
5. Frequently Asked Questions ❓
❓ Q: Why does insufficient insertion depth cause large errors?
A: With shallow insertion the exposed part of the well forms a conduction path; heat flows along the well to the outside, so the sensing element reads away from the true medium temperature — a classic conduction error. Adequate depth placing the element in the main stream reduces it substantially.
A: With shallow insertion the exposed part of the well forms a conduction path; heat flows along the well to the outside, so the sensing element reads away from the true medium temperature — a classic conduction error. Adequate depth placing the element in the main stream reduces it substantially.
❓ Q: Should static or total temperature be measured in high-velocity gas?
A: It depends on the performance calculation: use total-temperature probes (stagnation wells) for energy balances and compressor/turbine performance; use static measurements for state-point calculations. The Part emphasizes aerodynamic heating at high velocity — the difference grows with the square of Mach number and the two must not be mixed.
A: It depends on the performance calculation: use total-temperature probes (stagnation wells) for energy balances and compressor/turbine performance; use static measurements for state-point calculations. The Part emphasizes aerodynamic heating at high velocity — the difference grows with the square of Mach number and the two must not be mixed.
❓ Q: How do I choose between thermocouples and resistance thermometers?
A: Choose thermocouples (e.g., Type K, Type S) for wide ranges, harsh environments, and faster response; choose platinum resistance thermometers (e.g., Pt100) for narrower ranges requiring high accuracy, stability, and repeatability. Both must be calibrated reliably across the range.
A: Choose thermocouples (e.g., Type K, Type S) for wide ranges, harsh environments, and faster response; choose platinum resistance thermometers (e.g., Pt100) for narrower ranges requiring high accuracy, stability, and repeatability. Both must be calibrated reliably across the range.
🔍 Q: What happens when a radiation thermometer measures surfaces with different emissivities?
A: Radiation thermometers convert temperature assuming a preset emissivity; a surface with lower emissivity than the preset reads low (higher reads high). Look up the surface emissivity and correct for it, or apply a surface coating to standardize emissivity before measuring.
A: Radiation thermometers convert temperature assuming a preset emissivity; a surface with lower emissivity than the preset reads low (higher reads high). Look up the surface emissivity and correct for it, or apply a surface coating to standardize emissivity before measuring.
