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Contact Temperature Sensors by the Numbers
Thermocouples, platinum resistance thermometers and thermistors: how they work and how they are wired, installed and calibrated, worked through one cure oven
Choose, wire, install and calibrate thermocouples, Pt100s and thermistors with numbers: Seebeck voltages, cold junctions, lead errors, immersion, response time and uncertainty.
Start readingAbout this book
Most temperatures in a factory are measured by touching: a thermocouple in an oven, a platinum resistance thermometer in a pipe, a thermistor on a circuit board. These sensors are cheap, small and familiar, and they are trusted far more than they deserve. Each one reports the temperature of its own tip, which is not always the temperature of the air, metal or liquid around it, and each sits at the end of a chain of wires, junctions and electronics that adds its own errors. A reading shown to a tenth of a degree may be thirty degrees from the truth.
This book works through contact temperature measurement with numbers. It starts with what a sensor actually reads, then treats the three common families in turn: thermocouples and the voltage they make, cold junctions and extension cable, platinum resistance thermometers, their wiring and measuring circuits, and thermistors. It then deals with the installation errors that matter most in practice, conduction along the stem, radiation and contact, and with response time. It finishes with calibration, the uncertainty budget and a full specification for one installation. Each chapter follows one running case, includes a plausible mistake and how it was found, and ends with an exercise and its worked answer.
It is for production, process, quality and maintenance engineers, instrument technicians and technical students. It assumes school algebra and physics, and the physics of temperature itself is introduced where it is needed. Non-contact measurement has its own book, Infrared Temperature Measurement by the Numbers, and this one links to it rather than repeating it.
The running case
Every chapter follows one fictional installation. This is an illustration. A small coating business runs a batch powder-coat cure oven, CV-180, with a working space 3.0 m long, 2.0 m wide and 2.2 m high, heated by electric elements and stirred by a recirculating fan. The powder in use must be held at 180 °C metal temperature for 10 minutes to cure, according to its maker’s data sheet as assumed for this illustration. The oven’s air setpoint is 200 °C. Parts range from 1.2 mm sheet panels to frames with 10 mm steel base plates.
The oven has four kinds of contact sensor. A Pt100 in the supply air duct feeds the temperature controller. A type K thermocouple feeds a separate over-temperature limit controller set at 230 °C. A six-channel data logger with trailing type K thermocouples is used to profile parts through a cure. A 10 kΩ thermistor measures the logger’s terminal temperature for cold-junction compensation, and another watches the control cabinet.
The business, its oven, its powder and its problems are invented. None describes KEVOS® work or a real business.
How to use the book
Read Chapter 1 first: it sets out why a sensor reads its own temperature, which every later chapter builds on. Chapters 2 and 3 cover thermocouples, Chapters 4 and 5 platinum resistance thermometers, and Chapter 6 thermistors; they can be read in any order. Chapters 7 and 8 deal with installation and response time, Chapter 9 with calibration and uncertainty, and Chapter 10 brings everything together into a specification and a proving survey. Every calculation was checked by running it, using the NIST reference functions for thermocouples and the standard Callendar–Van Dusen coefficients for platinum.
Source limits and professional responsibility
The physics is standard and the sensor data comes from published reference functions and makers’ summaries of the IEC standards, which are named but not quoted. Heat-transfer coefficients and sensor dimensions in the running case are illustrations, and the right values for a real installation are the ones measured on it. Ovens, furnaces and their controls are hazardous: over-temperature protection, electrical work and the cure schedules that make a coating fit for use are matters for qualified people working under the applicable electrical and work health and safety rules and the coating maker’s instructions. This book teaches how contact sensors work and how to check them. It is not a procedure for any installation.
Who this book is for
Production, process, quality and maintenance engineers, instrument and electrical technicians, and technical students who specify, install, calibrate or rely on thermocouples, resistance thermometers and thermistors.
What you will learn
- Explain what a contact sensor actually measures, and predict when it differs from the temperature of the product.
- Convert thermocouple voltages with the NIST reference functions, and find the errors that cold junctions and extension cable introduce.
- Calculate Pt100 and thermistor resistances, sensitivities, lead-wire and self-heating errors, and choose a wiring arrangement.
- Estimate immersion, radiation and response-time errors, and design an installation that keeps them small.
- Calibrate a sensor against a reference, build an uncertainty budget and set guard bands on a process limit.
Inside this book
Chapters
- 01What a contact sensor actually readsA sensor reports its own temperature. The heat balance that sets it, air against metal temperature in CV-180's cure oven, and a cure timed from the wrong reading.
- 02Thermocouples and where their voltage is madeThe Seebeck effect, why a thermocouple's voltage is made along the wire and not at the tip, the NIST type K function, microvolt signals, and a junction that moved.
- 03Cold junctions, extension cable and thermocouple tolerancesCold-junction compensation, extension and compensating cable, reversed and copper leads, tolerance classes and drift, and an over-temperature limit that tripped 25 K late.
- 04Platinum resistance thermometers and the Callendar–Van Dusen equationWhy platinum, the Pt100's resistance curve, converting resistance to temperature properly, tolerance classes and elements, and a straight-line conversion 3 K wrong at 200 °C.
- 05Wiring, self-heating and the measuring circuitTwo-, three- and four-wire Pt100 connections, lead and insulation errors, self-heating, 4–20 mA transmitters and converter resolution, and a three-wire probe wired as two.
- 06Thermistors and the Boltzmann factorWhy a thermistor's resistance falls exponentially, the Beta and Steinhart–Hart equations, sensitivity, self-heating and resolution, and a replacement part that read 4 K low.
- 07Immersion, conduction, radiation and contactStem conduction and the immersion a probe needs, radiation from heaters and cold doors, attaching sensors to surfaces, and a controller sensor moved into a dead pocket.
- 08Response time, lag and thermowellsA sensor's time constant, step and ramp errors, how lag affects profiling and control, what a thermowell costs in response, and a slower probe that tripped the limit every morning.
- 09Calibration, traceability and the uncertainty budgetThe kelvin and ITS-90, the chain to a national standard, ice points and dry blocks, budgets for CV-180's profile and control sensors, guard bands, and a correction applied backwards.
- 10Specifying the sensors and proving the ovenA sensor specification for CV-180, an air uniformity survey, a loaded profile that found the slowest part, change control for sensors, and a cure proved on an empty oven.
Apply This in Practice
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References and source notes
The manuscript and worked examples are original KEVOS® writing. References inform particular concepts; their authors do not endorse this book. Web references were consulted on 28 October 2026.
- C. Kittel and H. Kroemer. Thermal Physics, second edition (1980). Supplied source, © W. H. Freeman and Company, in copyright. Read as a subject map only: the definition of temperature through thermal equilibrium and entropy, the Boltzmann factor, semiconductor carrier statistics, and the appendix on temperature scales and primary and secondary thermometers. No text, derivations as set out, figures, tables or problems are reproduced; the treatment, case, numbers and exercises in this book are its own.
- National Institute of Standards and Technology. NIST ITS-90 Thermocouple Database (NIST Monograph 175 reference functions) (1993, online database current). Source of the type K reference function and its coefficients, from which every type K voltage in the book was computed and checked against the published table values (for example 4.096 mV at 100 °C, 8.138 mV at 200 °C and 41.276 mV at 1,000 °C).
- TC Ltd. How accurate is a thermocouple? (2026). Maker's summary of the IEC 60584 tolerance classes, used for the type K class 1 (±1.5 °C from −40 to 375 °C) and class 2 (±2.5 °C from −40 to 333 °C) values.
- TC Ltd. What is the standard and tolerances for RTD Pt100 sensors? (2026). Maker's summary of the IEC 60751 tolerance classes for platinum resistance thermometers, used for classes AA, A and B: ±(0.1 + 0.0017|t|), ±(0.15 + 0.002|t|) and ±(0.3 + 0.005|t|) °C.
- Thermosense. RTD information sheet (2026). Source of the Callendar–Van Dusen equation and the standard coefficients A = 3.9083 × 10⁻³, B = −5.775 × 10⁻⁷ and C = −4.183 × 10⁻¹² (C below 0 °C only) used for every Pt100 resistance in the book.
- Bureau International des Poids et Mesures. The International System of Units (SI), ninth edition (2019). Source of the 2019 definition of the kelvin through the fixed value of the Boltzmann constant, 1.380649 × 10⁻²³ J/K.
- H. Preston-Thomas, Metrologia 27, 3–10. The International Temperature Scale of 1990 (ITS-90) (1990). Named as the definition of the temperature scale in use, with its fixed points and interpolating instruments. Only its general structure is described; no tables are reproduced.
- International Electrotechnical Commission. IEC 60584-1 Thermocouples, Part 1: EMF specifications and tolerances; IEC 60751 Industrial platinum resistance thermometers and platinum temperature sensors (editions as called up). Named by number and scope as the standards behind thermocouple and platinum sensor tolerances. Values used in the book come from the makers' published summaries above, not from the standards themselves.
- National Measurement Institute Australia; National Association of Testing Authorities. National Measurement Institute and NATA accreditation (2026). Named as Australia's custodian of the national measurement standards and the body that accredits calibration laboratories, as the basis of traceable calibration.