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The Temperature Sensors Calculator converts raw sensor output — resistance readings from thermistors and RTDs, or voltage/EMF readings from thermocouples and linear IC sensors — into temperature in Celsius, Fahrenheit, and Kelvin. This tool supports four of the most widely used sensor technologies in electronics and industrial automation: NTC thermistors, PT100 RTDs, Type K thermocouples, and linear analog IC sensors like the LM35.
Temperature measurement is one of the most fundamental requirements in electronics, mechatronics, HVAC, process control, medical devices, and environmental monitoring. The choice of sensor type depends on the temperature range, required accuracy, response time, environmental conditions, and system complexity. Each sensor type has a fundamentally different physical mechanism and output characteristic, requiring different conversion formulas to extract temperature from the raw measurement.
NTC Thermistors (Negative Temperature Coefficient) are resistive sensors made from metal oxide semiconductors. Their resistance decreases exponentially as temperature increases — a 10 kΩ NTC thermistor might measure 32 kΩ at 0°C and only 4 kΩ at 50°C. They are inexpensive, highly sensitive at moderate temperatures (−40°C to +150°C), and simple to interface (just a voltage divider and ADC). The Beta model used here, T = 1 / (1/T₀ + (1/β) × ln(R/R₀)), approximates the Steinhart–Hart equation for most practical temperature ranges.
PT100 RTDs (Resistance Temperature Detectors) are precision platinum wire sensors with a standardized resistance of 100 Ω at 0°C (PT100) and 1000 Ω at 0°C (PT1000). Platinum's resistance increases nearly linearly with temperature, following the Callendar–Van Dusen equation. PT100 sensors are the industrial standard for precision temperature measurement from −200°C to +850°C and are far more accurate and stable than thermistors, though more expensive and requiring more precise signal conditioning.
Type K Thermocouples consist of a junction of two dissimilar metals (Chromel–Alumel) that generates a small EMF (electromotive force) proportional to the temperature difference between the hot junction (measuring point) and the cold junction (reference). Type K is the most commonly used thermocouple, with a range of −200°C to +1260°C and a Seebeck coefficient of approximately 41.276 μV/°C. This calculator uses a linear approximation suitable for the 0–500°C range; for precision use, reference the NIST ITS-90 polynomial tables.
Linear IC sensors like the LM35 from Texas Instruments output a voltage directly proportional to temperature: 10 mV/°C for the LM35, with 0 V output at 0°C. These sensors integrate on-chip calibration and linearization, making them the simplest to use with any microcontroller ADC. They are accurate to ±0.5°C from 0°C to +100°C and require no external calibration or complex math — simply multiply the millivolt reading by 0.1 to get temperature in Celsius.
This multi-sensor calculator enables quick field verification and educational exploration of different sensor technologies, allowing engineers and students to compare the characteristics of each sensor type across overlapping temperature ranges.
Each sensor type uses a different conversion formula:
NTC Thermistor (Beta model): T(K) = 1 / [1/T₀ + (1/β) × ln(R/R₀)], then T(°C) = T(K) − 273.15. Where T₀ is the reference temperature in Kelvin (typically 298.15 K = 25°C), R₀ is resistance at T₀, and β is the material constant (typically 3000–5000 K).
PT100 RTD (linear approx.): T(°C) = (R − R₀) / (R₀ × α), where α = 0.003850 Ω/Ω/°C (IEC 60751 standard alpha coefficient).
Type K Thermocouple (linear approx.): T(°C) = EMF(mV) / 0.041276 mV/°C, valid for 0–500°C range.
LM35 / Linear IC: T(°C) = V_out(mV) / 10 mV/°C.
The Temperature (°C) output is the primary result suitable for most engineering applications. Temperature (°F) is provided for reference in North American contexts and HVAC applications. Temperature (K) is required for thermodynamic calculations, gas law computations, and scientific work. For NTC thermistors, note that the Beta model is an approximation — for temperatures more than ±50°C from the reference temperature, use the full 3-parameter Steinhart–Hart equation for better accuracy (typically within ±0.1°C vs. ±0.5°C for Beta model).
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Results
A 10 kΩ NTC with β=3950 reads 15 kΩ, indicating approximately 13.8°C. This is consistent with the thermistor being in a cool environment slightly below room temperature. The Beta model gives good accuracy within ±30°C of the reference.
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PT100 at 100°C reads exactly 138.5 Ω: T = (138.5 − 100) / (100 × 0.00385) = 100°C. This is the water boiling point at sea level — a useful calibration verification point for PT100 sensors.
NTC (Negative Temperature Coefficient) thermistors decrease in resistance as temperature rises — most common for temperature measurement. PTC (Positive Temperature Coefficient) thermistors increase sharply in resistance above a threshold temperature, making them useful for overcurrent protection (resettable fuses) and self-regulating heaters, not measurement.
Platinum has a highly stable, repeatable, and well-characterized resistance-temperature relationship that has been standardized internationally (IEC 60751). NTC thermistors vary between batches and have higher non-linearity. PT100 sensors are typically accurate to ±0.1°C or better; NTCs are usually ±0.5–1°C and require individual calibration for best accuracy.
Thermocouples measure the temperature difference between two junctions (hot and cold/reference). To get the absolute hot junction temperature, you must know the cold junction (typically the connection terminals on your measurement circuit) temperature and add it to the measured EMF-derived temperature. Most thermocouple interface ICs (like the MAX31855 or MCP9600) include a built-in temperature sensor for cold junction compensation.
Yes. The LM35 operates from 4–30 V supply but its output voltage range (10 mV/°C from 0°C to 100°C = 0–1 V) is well within a 3.3 V ADC input range. For temperatures below 0°C, a small negative bias supply or offset circuit is needed to keep the output above ground. The LM35D version operates down to −55°C with a suitable bias.
The Steinhart–Hart equation is a more accurate model for NTC thermistors: 1/T = A + B×ln(R) + C×(ln(R))³. It requires three calibration coefficients (A, B, C) from the manufacturer's datasheet and provides accuracy better than ±0.01°C over a wide temperature range. The Beta model (used here) is a simplified 2-parameter version adequate for ±0.5°C accuracy within a moderate temperature range of ±50°C around the reference temperature.
IEC 60751 defines the resistance-temperature relationship for platinum RTDs (PT100 and PT1000), specifying the standard alpha coefficient (α = 0.003850 Ω/Ω/°C), the Callendar–Van Dusen equation coefficients for accurate conversion, and tolerance classes (Class AA ±0.1°C, Class A ±0.15°C, Class B ±0.3°C, Class C ±0.6°C). Most industrial PT100 probes conform to this standard.
For outdoor IoT, the DS18B20 digital 1-Wire sensor is extremely popular: it outputs a calibrated digital value over a single wire, operates from −55°C to +125°C, has ±0.5°C accuracy, and integrates A/D conversion on-chip. For lower cost, NTC thermistors in a voltage divider with a microcontroller ADC are widely used. For harsh industrial environments, PT100 with a 4–20 mA transmitter provides noise-immune measurement over long cable runs.
Current flowing through a thermistor for resistance measurement dissipates power (P = I²R), heating the sensor above ambient temperature and causing a reading error. To minimize self-heating, use the lowest measurement current that still gives adequate ADC resolution — typically 10–100 μA for 10 kΩ NTC sensors. The self-heating index (°C/mW) is listed in datasheets; for a sensor with 1°C/mW self-heating, 1 mA through 10 kΩ = 10 mW = 10°C error.
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The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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