Mechanical Thermometers, RTDs and Thermocouples

Temperature Measurement

Temperature quantifies the average thermal kinetic energy of molecules within a system. Power plants monitor steam temperatures to maximize turbine thermal efficiency, protect superheater metallurgy from high-temperature creep, and ensure boiler flue gases do not drop below the acid dew point.

1. Mechanical Temperature Indicators

• Bimetallic Dial Thermometers: Construct of two dissimilar metal strips with widely different coefficients of thermal expansion (typically brass or bronze bonded to Invar, a nickel-iron alloy with near-zero thermal expansion). When heated, the higher-expansion metal lengthens faster than the lower-expansion metal, forcing the composite strip to bend toward the low-expansion side. In industrial thermometers, the bimetallic strip is wound into a tight, multi-helix coil inside a protective stainless steel stem. The helical winding converts axial bending into pure rotational movement, turning a central pointer shaft across a dial without complex linkages.
• Filled Thermal Systems: Consist of a sensing bulb immersed in the process fluid, connected via a fine capillary tube to an elastic Bourdon tube or bellows indicator. The entire sealed system is filled under pressure with a fluid classified under SAMA (Scientific Apparatus Makers Association) standards:
  • Class I: Liquid-filled (non-mercury hydrocarbon or silicone fluid; linear scale).
  • Class II: Vapor-filled (contains volatile liquid and vapor in equilibrium; non-linear vapor pressure curve; bulb must be located at the liquid-vapor interface).
  • Class III: Gas-filled (typically pressurized nitrogen; wide temperature range from $-130^\circ\text{C}$ to $+540^\circ\text{C}$).
  • Class V: Mercury-filled (phased out due to environmental and toxic safety regulations).

2. Resistance Temperature Detectors (RTDs)

RTDs operate on the physical principle that the electrical resistance of pure metals increases predictably and linearly with rising temperature (a Positive Temperature Coefficient - PTC).

• Pt100 Platinum RTDs: Platinum is the international industrial standard due to its chemical inertness, resistance to oxidation, mechanical stability, and exceptional linearity across a broad temperature span ($-200^\circ\text{C}$ to $+650^\circ\text{C}$). The standard Pt100 sensor possesses an electrical resistance of exactly $100.00\text{ }\Omega$ at $0^\circ\text{C}$ and a temperature coefficient of resistance ($\alpha$) of $0.00385\text{ }\Omega/\Omega/^\circ\text{C}$ (the European DIN/IEC 60751 standard), providing a sensitivity of approximately $0.385\text{ }\Omega/^\circ\text{C}$.
• Bridge Wiring Configurations: Because RTD lead wires possess electrical resistance ($R_L$) that fluctuates with ambient temperature, measuring circuits must cancel lead wire effects:
  • 2-Wire Circuit: The simplest circuit, but lead wire resistance ($2 R_L$) is in direct series with the RTD element. A total extra loop resistance of 1 Ω gives about +2.6°C error near 0°C; two leads of 1 Ω each give about +5.2°C under that linear approximation. Strictly limited to short distances ($< 3\text{ m}$).
  • 3-Wire Bridge Circuit (Industry Standard): Adds a third compensating wire connected to the opposite leg of a Wheatstone bridge. One lead wire is placed in series with the RTD, while the second lead wire is placed in series with the bridge reference resistor. Because both lead wires experience identical ambient temperature changes, their resistances cancel out in the bridge balance equation. The third wire connects the detector voltmeter and carries virtually zero current.
  • 4-Wire Kelvin Circuit: Supplies a constant excitation current through two outer leads and measures the voltage drop across the RTD via two high-impedance inner sense leads. greatly reduces lead-resistance effects; utilized in high-precision laboratory and turbine efficiency testing.

3. Thermocouples

Thermocouples operate on the Seebeck Effect discovered by Thomas Johann Seebeck in 1821: when two strips of dissimilar electrical conductors are joined at both ends to form a closed loop, an electromotive force (EMF, measured in millivolts - $mV$) is generated whenever a temperature gradient exists between the two junctions.

• Measuring (Hot) Junction vs. Reference (Cold) Junction: The hot junction is exposed to the process temperature being measured ($T_{\text{process}}$). The cold junction terminates at the measurement instrument or transmitter ($T_{\text{ref}}$). The generated Seebeck voltage is proportional to the temperature difference between the junctions: $V_{\text{Seebeck}} \propto (T_{\text{hot}} - T_{\text{cold}})$.
• Cold Junction Compensation (CJC): Because a thermocouple only measures the temperature differential between junctions, the actual process temperature cannot be known unless the reference junction temperature is accounted for. In modern transmitters, an internal solid-state temperature sensor (such as an RTD or thermistor) measures the temperature of the terminal block, and an electronic compensation circuit adds an equivalent voltage offset (electronic ice-point compensation) to calculate the true absolute temperature.
• Thermocouple Types:
  • Type J (Iron / Constantan): Usable from $-40^\circ\text{C}$ to $+750^\circ\text{C}$. White positive wire (Iron, magnetic), Red negative wire (Constantan, non-magnetic). Well suited for reducing (oxygen-deficient) atmospheres and vacuum furnaces. Rapidly oxidizes in steam service above $538^\circ\text{C}$.
  • Type K (Chromel / Alumel): The universal power plant thermocouple, usable from $-200^\circ\text{C}$ to $+1,250^\circ\text{C}$. Yellow positive wire (Chromel), Red negative wire (Alumel, magnetic). Highly resistant to oxidation; standard for boiler furnace gas, superheater tubes, and turbine exhaust duct monitoring.
• Extension Wires: Signals from thermocouples must be carried to transmitters using specialized extension or compensating wire manufactured from alloys matching the thermoelectric properties of the thermocouple. Using standard copper wire creates unwanted secondary thermocouple junctions at the terminal block, causing severe measurement errors.

4. Thermowells

Industrial temperature sensors are almost never inserted directly into high-pressure pipes or vessels. Instead, they are housed inside a thermowell—a solid, pressure-tight metal cylinder drilled with a blind bore, manufactured from bar stock (such as 316 stainless steel, Inconel, or chrome-moly alloy) and welded, flanged, or threaded into the pipe wall.

• Functions: Thermowells protect delicate RTDs and thermocouples from mechanical impact, fluid erosion, corrosive chemicals, and line pressure. An intact, correctly designed thermowell can permit sensor removal while retaining the process boundary, but only under the authorized procedure. The thermowell itself is a pressure-retaining part and cannot be removed as though it were the sensor.
• Vortex Shedding & Wake Frequency: High-velocity steam or water flowing across a thermowell creates alternating low-pressure vortices downstream (von Kármán vortex street). If the vortex shedding frequency matches the natural mechanical resonant frequency of the thermowell, fatigue damage and failure can result. Thermowells in high-pressure steam service must be engineered in accordance with ASME PTC 19.3 TW wake frequency calculations.

Source notes

Official scope and technical references checked 2026-10-17: Reference 1; Reference 2. Archived DOE handbooks are used for physical principles, not current Canadian legal requirements.