Temperature and Thermometers Lecture

Vocabulary flashcards covering key terms and principles related to temperature measurement, scales, calibration, and the design of various thermometers.

Temperature

A physical property that determines the net direction of heat flow; heat moves from a body at higher temperature to one at lower temperature until equilibrium is reached.

Thermal Equilibrium

Condition in which two bodies in contact have the same temperature and no net flow of thermal energy occurs between them.

Thermometric Property

Any physical property of a substance that varies continuously and reproducibly with temperature and can be used to measure it.

Thermometer

An instrument that determines temperature by monitoring changes in a chosen thermometric property.

Fixed Point

A reproducible reference temperature (e.g., ice point, steam point, triple point) used to calibrate a thermometer.

Ice Point

Temperature of pure melting ice at standard atmospheric pressure; defined as 0 °C or 273.15 K.

Steam Point

Temperature of steam above pure boiling water at standard atmospheric pressure; defined as 100 °C or 373.15 K.

Empirical Celsius Scale

Centigrade temperature scale with 100 equal divisions between ice point (0 °C) and steam point (100 °C).

Kelvin Scale

Absolute (thermodynamic) temperature scale starting at absolute zero; 0 K = −273.15 °C.

Absolute Zero

Lowest possible temperature (0 K) where the pressure and volume of an ideal gas would be zero and molecular kinetic energy is minimal.

Triple Point of Water

273.16 K (0.01 °C) where ice, liquid water, and water vapour coexist in equilibrium; the second fixed point on the Kelvin scale.

Thermodynamic Scale

Temperature scale independent of any particular substance, based on absolute zero and the triple point of water.

Celsius–Kelvin Conversion

T(K) = θ(°C) + 273.15; a change of 1 K equals a change of 1 °C.

Linear Calibration

Assumption that a thermometric property varies linearly with temperature, allowing a straight-line scale between fixed points.

Non-Linear Calibration

Situation where a thermometric property does not vary linearly with temperature; requires a calibration curve to determine temperatures.

NTC Thermistor

A thermistor with a Negative Temperature Coefficient; its electrical resistance decreases as temperature increases.

Mercury-in-Glass Thermometer

Liquid-in-glass thermometer that uses the volume (length) of mercury as the thermometric property.

Sensitivity (Thermometer)

Degree to which a small temperature change produces a large, easily measurable change in the thermometric property.

Range (Thermometer)

Span of temperatures over which a thermometer can reliably operate without damage or loss of accuracy.

Thermal Heat Capacity (of Thermometer)

Amount of heat required to change the thermometer’s temperature; large heat capacity slows response and can cool the sample.

Resistance Thermometer

Device that senses temperature through the change in electrical resistance of a conductor, commonly platinum.

Platinum Resistance Thermometer (PRT)

High-accuracy resistance thermometer using a platinum coil whose resistance changes nearly linearly with temperature.

Temperature Coefficient of Resistivity (α)

Fractional change in resistance per degree Celsius at a reference temperature (usually 20 °C).

Wheatstone Bridge

Circuit used in resistance thermometers to measure small changes in resistance accurately.

Thermistor

Semiconductor temperature sensor with a large, usually non-linear change of resistance with temperature; very fast response.

Thermocouple

Temperature sensor made of two dissimilar metals; generates an emf proportional to the temperature difference between junctions.

Seebeck Effect

Generation of an emf in a circuit of two dissimilar metals with junctions at different temperatures (basis of thermocouples).

Peltier Effect

Heating or cooling that occurs at a junction of two dissimilar conductors when an electric current passes through it.

Thermopile

Series combination of multiple thermocouples to increase output emf and sensitivity.

Inversion Temperature (Thermocouple)

Temperature above which the emf of a given thermocouple decreases instead of increasing.

Constant-Volume Gas Thermometer

Highly accurate thermometer that measures temperature by the pressure change of a fixed mass of gas kept at constant volume.

Ideal Gas Assumption

Approximation that a dilute gas obeys p ∝ T at constant volume, forming the basis for gas thermometers and the Kelvin scale.

Empirical Centigrade Scale

Any centigrade temperature scale based on a chosen thermometric property calibrated at the ice and steam points, not necessarily identical to Celsius.

Reason Thermometers Disagree

Different thermometers use different properties that rarely vary perfectly linearly with temperature, leading to differing readings between fixed points.

Volume of Liquid Thermometric Property

Expansion of a liquid (e.g., mercury, alcohol) in a capillary tube as temperature rises.

Volume of Gas at Constant Pressure

Thermometric property where gas volume increases with temperature when pressure is held constant.

Pressure of Gas at Constant Volume

Thermometric property exploited by constant-volume gas thermometers; pressure rises linearly with absolute temperature.

Length of Metal

Thermometric property where the length of a metal rod expands with increasing temperature.

EMF of Thermocouple

Voltage produced between two dissimilar metal junctions that varies with temperature difference.

Mercury Advantages

Good conductor, visible, does not wet glass, expands uniformly, high boiling point, allowing a moderately wide range.

Mercury Disadvantages

Poisonous, freezes at −39 °C, has relatively large heat capacity and slow response; glass expansion introduces errors.

Alcohol Thermometer

Liquid-in-glass device using dyed alcohol; suitable for low temperatures due to lower freezing point than mercury.

Capillary Bore

Narrow tube inside a liquid-in-glass thermometer that magnifies small volume changes into readable length changes.

Sensitivity Enhancement (Mercury Thermometer)

Achieved by thinner capillary bore, larger bulb, and thinner glass walls to accentuate mercury movement.

Platinum Properties for PRT

Chemically inert, high melting point, nearly linear resistance-temperature relation, sizable α for good sensitivity.

Mica Support (PRT)

Insulating structure that holds the platinum coil and minimises mechanical strain and electrical interference.

Copper Leads (PRT)

Low-resistance connections ensuring measured resistance changes arise from the platinum coil, not the leads.

Negative Temperature Coefficient (NTC)

Characteristic of some semiconductors where resistance decreases as temperature increases.

Self-Heating Error (Thermistor)

Temperature rise of a thermistor caused by its own measurement current, potentially skewing readings.

Semiconductor Avalanche (Thermistor)

Runaway increase in current and temperature that can damage a thermistor exposed to very high heat.

Rapid Response Thermometer

Device with low thermal mass (e.g., thermocouple, thermistor) that quickly follows changing temperatures.

Bulky Thermometer

Large instruments like constant-volume gas thermometers that have slow response and are unsuitable for field use.

Calibration Curve

Graph of thermometric property versus temperature used to interpret readings when the relation is non-linear.

Interpolation

Estimating an unknown temperature lying between two calibrated points on a linear or calibration graph.

Extrapolation

Extending a calibration line beyond measured points to estimate temperatures outside the tested range.

Empirical Temperature

Temperature value assigned using a specific thermometric property and fixed-point calibration, without relying on theory.

Ideal Gas Thermometer Accuracy

Considered highest because gas behaviour most closely follows the kinetic theory, making its readings near-thermodynamic.

Thermometer Range Selection

Choice of thermometer depends on required temperature span, accuracy, response time, and measurement environment.

Seebeck emf Magnitude

Typically a few millivolts per 100 °C, requiring sensitive voltmeters or amplifiers for precise thermocouple readings.

Laser-Guided Infrared Thermometer

Non-contact thermometer that aims with a laser and measures thermal radiation using a solid-state detector (often a thermistor or IR sensor).

Sensitivity vs Linearity Trade-Off

Highly sensitive sensors (e.g., thermistors) often have non-linear responses, whereas linear sensors (e.g., PRTs) may be less sensitive.