3.1 Thermal Science: Temperature, Energy Units, and Heat Capacity
Key Takeaways
- The zeroth law of thermodynamics is what makes thermometry possible: two bodies in thermal equilibrium with a third are in equilibrium with each other, which is why a reference thermometer can be trusted at all
- Q = mcΔT is the working equation for stored heat; water's specific heat of about 4186 J/(kg·K) is roughly ten times copper's 385 and four times air's 1005
- Absolute zero is 0 K = −273.15 °C = −459.67 °F, and every radiometric calculation in Chapter 4 uses absolute temperature, never Celsius
- Latent heat moves energy with no temperature change — about 2257 kJ/kg to vaporise water at 100 °C — which is why wet and evaporating surfaces defeat naive thermal interpretation
- High-specific-heat, high-mass assemblies change temperature slowly, so a survey taken before thermal stabilisation reports the wrong answer regardless of camera quality
The ITC Level II topical outline devotes an entire block to Thermal Science — the concept of temperature, common energy units, heat capacity, the thermodynamic laws, the effects of temperature on materials, and states of matter and energy conversion. Infraspection's curriculum opens with Thermometry Fundamentals for the same reason. Level I lets you point a camera; Level II requires you to explain why a surface is at the temperature you measured, and that explanation is thermal science.
What Temperature Actually Is
Temperature is a measure of the average kinetic energy of the particles in a substance. It is an intensive property: it does not depend on how much material you have. Heat is different — it is energy in transit because of a temperature difference, and it is extensive.
A cup of boiling water and a bathtub of warm water can hold very different amounts of heat at very different temperatures. A thermal imager measures surface temperature only (via emitted radiation); it never measures heat content, internal temperature, or energy flow directly. Every diagnostic conclusion you draw about "how much energy is being wasted" is an inference layered on top of a surface-temperature measurement.
The zeroth law is why thermometry works
| Law | Statement | Why Level II cares |
|---|---|---|
| Zeroth | If A is in thermal equilibrium with C, and B is in equilibrium with C, then A and B are in equilibrium with each other | This is the logical basis for all thermometry, including the contact-thermometer cross-verification in Chapter 7 |
| First | Energy is conserved; ΔU = Q − W | Heat entering a connection must go somewhere — stored, conducted away, or radiated |
| Second | Heat flows spontaneously from hotter to colder; entropy of an isolated system does not decrease | Fixes the direction of heat flow, so a hot spot always implies a source |
| Third | Entropy approaches a constant as temperature approaches absolute zero; absolute zero is unattainable | Defines the absolute scales that radiometry requires |
The zeroth law is the one candidates skip and the one that matters most in practice: it is the formal justification for saying "my calibrated contact probe reads 71 °C, therefore the surface is 71 °C, therefore my camera's emissivity setting is wrong."
Temperature Scales and Absolute Zero
| Scale | Freezing point of water | Boiling point of water | Absolute zero |
|---|---|---|---|
| Celsius (°C) | 0 | 100 | −273.15 |
| Fahrenheit (°F) | 32 | 212 | −459.67 |
| Kelvin (K) | 273.15 | 373.15 | 0 |
| Rankine (°R) | 491.67 | 671.67 | 0 |
Conversions you must be able to do without notes:
- K = °C + 273.15
- °F = (9/5 × °C) + 32
- °C = 5/9 × (°F − 32)
- °R = °F + 459.67
A note on notation that appears on exams: a temperature difference is written C° (or F°), while a temperature value is written °C. Infraspection's standards use this convention rigorously — "ΔT of 15 C°" versus "a surface at 15 °C". A ΔT of 1 C° equals 1 K exactly, but a ΔT of 1 C° equals 1.8 F°, which is why the NETA and MIL-STD tables must never be converted casually.
Critical for Chapter 4: the Stefan-Boltzmann law (T⁴) and Planck's law both require absolute temperature. Substituting Celsius produces nonsense. A surface at 27 °C radiates according to 300 K, and doubling 27 to 54 °C does not double radiant exitance — it raises it by a factor of (327/300)⁴ ≈ 1.41.
Energy Units and Heat Capacity
| Quantity | SI unit | Common alternates |
|---|---|---|
| Energy / heat | joule (J) | calorie (4.184 J), BTU (1055 J), kWh (3.6 MJ) |
| Power | watt (W) = J/s | BTU/hr (0.293 W), horsepower (746 W) |
| Specific heat | J/(kg·K) | BTU/(lb·°F) |
| Thermal conductivity | W/(m·K) | BTU·in/(hr·ft²·°F) |
Specific heat capacity (c) is the energy needed to raise one kilogram of a substance by one kelvin:
Q = m × c × ΔT
| Material | Specific heat, J/(kg·K) | Practical consequence |
|---|---|---|
| Water | ~4186 | Enormous thermal storage — the basis of roof-moisture surveys |
| Aluminium | ~900 | Heat sinks and bus warm and cool moderately fast |
| Concrete | ~880 | Masonry walls lag the air by hours |
| Air | ~1005 | Low mass means low stored energy despite similar c |
| Steel | ~490 | Moderate |
| Copper | ~385 | Conducts superbly but stores little per kilogram |
A worked case: a 12 kg copper bus section absorbing 500 W of resistive loss with no cooling would rise at 500 ÷ (12 × 385) ≈ 0.11 K per second, or roughly 6.5 K per minute. Real bus does not do this because conduction and convection carry heat away — but the calculation shows why a connection can go from "fine" to "alarming" within a single shift after a load step, and why stabilisation time matters before you record a temperature.
Latent Heat and States of Matter
Energy conversion between states happens at constant temperature:
| Transition | Water value | Field consequence |
|---|---|---|
| Fusion (melt/freeze) | ~334 kJ/kg | Ice on a roof or conductor pins the surface near 0 °C |
| Vaporisation (boil/condense) | ~2257 kJ/kg at 100 °C | Evaporating moisture holds a surface cool and hides underlying heat |
This is why a rain-wetted electrical enclosure, a sweating pipe, or a damp masonry wall cannot be graded against ΔT criteria: latent heat is dominating the surface temperature, not the defect. Infraspection's standard requires the thermographer to work when "surface and atmospheric moisture" conditions are favourable, and to record sky and weather conditions with every exception, precisely because of this effect.
Effects of Temperature on Materials
| Effect | What happens | Thermography relevance |
|---|---|---|
| Thermal expansion | Materials grow with temperature | Cyclic heating loosens bolted connections — the root cause behind many hot joints |
| Resistivity rise | Metal resistance increases with temperature | A hot joint gets hotter: more heat raises R, which raises I²R — a runaway loop |
| Insulation ageing | Chemical breakdown accelerates with temperature | The Montsinger rule of thumb: insulation life roughly halves per ~10 C° of sustained overtemperature |
| Phase/state change | Melting, softening, lubricant breakdown | Sets the absolute limits catalogued in Chapter 8 |
| Emissivity change | Oxidation raises emissivity over time | The same connector may need a different ε at each survey |
The resistivity feedback loop is the single most important physical fact behind electrical thermography: a marginal connection does not degrade linearly. That is the justification for the escalating urgency in every ΔT criteria table and for the rate-of-change trending in Chapter 12.
Exam-Style Application
Scenario A — Scale confusion. A report states "ΔT = 30 °F above the reference, which exceeds the 15 C° NETA immediate-repair threshold." Check the arithmetic: 30 F° = 30 × 5/9 = 16.7 C°. It does exceed 15 C°, but only just — and the report should state the ΔT in the same units as the criteria it cites.
Scenario B — Stabilisation. A motor was started 4 minutes before the survey. Its 40 kg cast-iron housing has barely begun to warm. Reporting "bearing normal" is unsupported; the assembly has not reached steady state.
Scenario C — Latent heat masking. An outdoor disconnect is scanned during light drizzle and shows no anomaly. Evaporative cooling can suppress a real ΔT entirely. Reschedule for dry conditions.
Quick Answer: Temperature is average particle kinetic energy and is intensive; heat is energy in transit and is extensive. K = °C + 273.15; radiometry always uses absolute temperature. Q = mcΔT, with water at ~4186 J/(kg·K) far above copper's ~385. Latent heat (~2257 kJ/kg for water vaporisation) moves energy at constant temperature and masks defects on wet surfaces. Rising resistivity with temperature makes electrical faults self-accelerating.
A thermographer must convert a criteria threshold. A published limit is a ΔT of 15 C°. What is the equivalent temperature difference in Fahrenheit degrees?
Why must absolute temperature be used in Stefan-Boltzmann and Planck calculations?
Which thermodynamic law provides the logical basis for cross-verifying an imager against a calibrated contact thermometer?
An outdoor bus connection is scanned during light rain and shows no measurable ΔT against its sister phase. What is the best interpretation?