RTD vs Thermocouple vs Thermistor: Sensor Selection

Quick answer

For battery-pack temperature monitoring, start with an insulated negative temperature coefficient (NTC) thermistor assembly in thermal contact with the cells. In heating, ventilation and air conditioning (HVAC) equipment, choose an NTC for economical local sensing or a resistance temperature detector (RTD) when sensors must be interchangeable in the field. A 3D-printer hotend should use the sensor assembly and firmware curve specified by the hotend manufacturer. In industrial processes operating at 0–400°C, such as heated pipes or process tanks, consider a suitably rated Pt100 or Pt1000 RTD and check the tolerance class across the full range. A kiln or furnace above 1,000°C may instead require a thermocouple assembly qualified for its operating atmosphere. Type K is one thermocouple category, but the letter alone does not establish an assembly's temperature rating. For precision lab measurements, select a calibrated RTD system for remote probes, or a precision digital temperature IC such as the Texas Instruments TMP117 when the object can be thermally coupled to a printed circuit board (PCB).

Temperature sensors monitor everything from a battery cell's surface to the interior of an industrial furnace. Several technologies measure temperature differently.

• An RTD measures changes in the electrical resistance of a metal element, usually platinum.
• An NTC thermistor also measures resistance, but its resistance falls much more sharply as temperature rises.
• A thermocouple generates a small voltage from a junction of different metals, while a digital temperature IC measures its own chip temperature and reports a digital value.

Each option has different limits on temperature, wiring, accuracy and installation. Selection depends on the application requirements and the complete measurement chain, including mounting, cable, signal-conversion electronics, firmware and calibration.

RTD vs thermocouple vs thermistor comparison

For NTC thermistors, R25 denotes resistance at 25°C. The circuit examples compare a MAX31865 RTD interface, a MAX31856 thermocouple converter, a Vishay NTC device, and the TMP117 digital sensor. Converter ICs provide the readout electronics, while the actual probe or sensing element determines the measurement location.

Table 1. Temperature sensor characteristics and hardware design implications

The NTC range comes from Vishay's component datasheet, and TMP117's operating range comes from TI's TMP117 specifications. Response time and annual drift depend on the specific assembly and test conditions, so a single figure would not describe every sensor in a category.

Choose an RTD if…

An RTD probe combines the sensing element with its housing, cable and other components needed to install it at the measurement point. Consider an RTD when a stable, specified resistance–temperature relationship and replaceable probes are important.

• An industrial controller needs a replaceable probe with a documented resistance–temperature characteristic.
• A precision lab instrument can support probe-specific calibration and a traceable uncertainty budget.
• The cable harness can accommodate separate force and sense conductors.
• An HVAC installation benefits from field replacement without selecting a different firmware curve for each vendor.
• A hotend explicitly supports an RTD cartridge and its matching interface electronics.

Pt100 vs Pt1000, classes and usable range

Pt100 is a platinum RTD nominally 100 Ω at 0°C, and Pt1000 is nominally 1 kΩ at 0°C. Neither name specifies accuracy. Pt1000 reduces a given series lead resistance's fractional contribution. However, using the same excitation current increases dissipated power, so compare the excitation and thermal installation rather than resistance alone.

International Electrotechnical Commission (IEC) standard 60751 defines the platinum resistance–temperature relationship. The Callendar–Van Dusen implementation in Analog Devices' MAX31865 RTD converter extends from −200 to +850°C. The characteristic range is not a packaged-probe rating or a promise of Class A accuracy everywhere.

WIKA, an industrial measurement manufacturer, reproduces IEC 60751 thermometer tolerances in information sheet IN 00.17 (November 2020). At temperature t in Celsius:

• Class A: ±(0.15 + 0.0020|t|)°C.
• Class B: ±(0.30 + 0.0050|t|)°C.

The permissible ranges depend on construction:

• Wire-wound: Class A from −100 to +450°C, and Class B from −196 to +600°C.
• Thin-film: Class A from −30 to +300°C, and Class B from −50 to +500°C.

A thin-film Class A label alone therefore does not cover an entire 0–400°C industrial process. Check the actual assembly certificate, construction and allowed exposure. Class tolerances describe uncalibrated interchangeability. Calibrated precision also requires measured corrections and a budget for residual uncertainty.

Design considerations and minimal RTD reference circuit

Use Analog Devices/Maxim MAX31865AAP+T to read resistance from an off-board platinum RTD. Its readout supports two-, three- and four-wire connections. The 15-bit analog-to-digital converter (ADC) has a nominal 0.03125°C resolution, compared with a stated maximum converter accuracy of 0.5°C over operating conditions. Probe, reference-resistor and installation errors are additional.

For a minimal four-wire circuit, useful when lead resistance could affect accuracy, follow the datasheet's separate force/sense topology:

The datasheet recommends RREF of 400 Ω for Pt100 or 4 kΩ for Pt1000. Bypass each supply with 0.1 µF. The typical input capacitor is 100 nF for Pt100 or 10 nF for Pt1000. Follow the recommended supply conditions and the selected package's pin table.

The conversion is ratiometric, meaning the sensor resistance is measured against the reference resistance using the same excitation current. Such a ratio reduces sensitivity to changes in excitation current, although reference tolerance and drift still affect accuracy. Choose excitation from the self-heating and noise budgets, allow settling, and consider switching off the bias between measurements.

Lead resistance also depends on the wiring arrangement:

• Two-wire sensing includes lead resistance.
• Three-wire compensation relies on suitably matched leads.
• Four-wire Kelvin sensing separates voltage sensing from excitation.

Route sense traces to the intended nodes, away from current-carrying detours.

Choose a thermocouple if…

• A kiln requires a probe beyond the suitable RTD assembly's temperature limit.
• A furnace specification identifies atmosphere, sheath, junction construction and replacement interval.
• The machine already has a documented K, J or T harness that must be preserved.
• A hotend manufacturer specifies a thermocouple and compatible electronics.
• A test fixture needs a small junction and can validate its mounted response.

K vs J vs T: range is not permission to use

Thermocouple letters identify different conductor pairs, each with a particular voltage–temperature relationship. Omega Engineering's thermocouple guide describes the common types and their suitable environments:

• Type K uses nickel-chromium/nickel-aluminum, primarily in clean oxidizing or inert environments, with limited vacuum or reducing use.
• Type J uses iron/copper-nickel and has limited high-temperature oxidizing use.
• Type T uses copper/copper-nickel and is commonly used for low-temperature applications.

A furnace atmosphere, wire diameter, insulation or sheath can rule out an otherwise suitable type.

Analog Devices' MAX31856 thermocouple-to-digital converter supports linearization over K −200 to +1372°C, J −210 to +1200°C, and T −200 to +400°C. The ranges describe converter functions, not physical-probe qualifications. IEC 60584-1:2013's official summary separately restricts the Type K reference polynomial to 1,300°C.

TC Ltd, a thermocouple manufacturer, reproduces IEC 60584-1 output tolerances. With t as temperature in Celsius:

• Class 1: K and J have ±max(1.5, 0.004|t|)°C, applicable over −40 to +1000°C for K and −40 to +750°C for J. Type T has ±max(0.5, 0.004|t|)°C over −40 to +350°C.
• Class 2: K and J have ±max(2.5, 0.0075|t|)°C, applicable over −40 to +1200°C for K and −40 to +750°C for J. Type T has ±max(1, 0.0075|t|)°C over −40 to +350°C.

For colder work, such as cryogenic testing, Class 3 K is ±max(2.5, 0.015|t|)°C and T is ±max(1, 0.015|t|)°C over −200 to +40°C. The cited tolerance table does not specify a Class 3 category for Type J. Order conductor material to the required low-temperature class. IEC 60584-1 specifies delivered-wire tolerances, excluding calibration drift during use, so class tolerance alone does not determine total system accuracy.

Design considerations and minimal thermocouple circuit

Use Analog Devices/Maxim MAX31856MUD+ to convert the thermocouple's small voltage into a temperature reading. Connect the thermocouple to T+/T− and BIAS to T−, with SPI to the controller. Connect AGND and DGND to circuit ground and leave DNC unconnected as specified in the datasheet's pin description. Supply AVDD/DVDD from the same suitable rail within 3.0–3.6 V. Bypass both with 0.1 µF, use 100 nF across the inputs, and add the recommended matched 10 nF input-to-ground capacitors in high-noise environments.

The MAX31856 thermocouple converter linearizes K/J/T and provides cold-junction compensation (CJC), which accounts for the temperature at the thermocouple's wire-to-copper connections. Its 0.0078125°C output step is resolution rather than total accuracy. Because thermocouple electromotive force (EMF) is measured in microvolts, offsets and unintended metal junctions can produce noticeable errors.

Use connector contacts and extension wires compatible with the selected thermocouple type. Keep the copper-transition junctions and CJC sensor in an isothermal zone, away from regulators, heater drivers and airflow gradients, because CJC assumes nearly equal temperatures at the terminals. Grounded probes need special attention. The operating common-mode window is 0.5–1.4 V, and differential full scale depends on gain. Input survival protection does not make a signal outside the permitted range measurable.

Choose an NTC thermistor if…

• A battery monitor needs multiple economical contact-temperature channels.
• An HVAC product has a bounded operating interval and a controlled sensor BOM.
• The controller already has a suitable ADC and ratiometric reference arrangement.
• A hotend specifies that exact thermistor assembly and resistance curve.
• The firmware can use manufacturer resistance tables or fitted coefficients.

Tolerance, Beta and useful range

For a specific Flux component-library example, Vishay BCcomponents NTCS0805E3103FMT is a board-mounted NTC, rather than a hotend or cell-contact probe. Its resistance at 25°C (R25) is 10 kΩ with ±1% tolerance, and B25/85 is 3570 K with ±1% tolerance. Its zero-power range is −55 to +150°C. Useful measurement range can be narrower when ADC sensitivity, leakage, mounting or uncertainty becomes unacceptable.

Vishay's NTC application note defines B25/85 between 25°C and 85°C. R25 tolerance controls the reference resistance, while Beta tolerance controls the temperature curve's slope. Neither provides a general temperature-accuracy specification. Using a Beta value from the wrong temperature interval can distort calculated readings.

Vishay's Selecting NTC Thermistors (July 14, 2015) gives the following approximation using absolute temperature in kelvin:

$$R(T)=R(T_0)\exp \left[B\left(\frac{1}{T}-\frac{1}{T_0}\right)\right]$$

Beta is useful over restricted intervals. For broader precision work, use the manufacturer's resistance table or the Steinhart–Hart relationship with part-specific coefficients:

$$\frac{1}{T}=A+B_{\mathrm{SH}}\ln (R)+C[\ln (R){]}^3$$

The fitted coefficient $B_{\mathrm{SH}}$ in the Steinhart–Hart equation differs from the simpler Beta constant. Include calibration and model residuals when estimating measurement uncertainty.

Design considerations and minimal NTC circuit

A voltage divider converts an NTC's changing resistance into an ADC input voltage. Connect a precision fixed resistor from the ADC reference or excitation rail to the measurement node, the NTC from the node to analog ground, and the node to an ADC input. Select the fixed resistor near the sensor resistance in the temperature region where sensitivity matters, based on its resistance table and permitted self-heating.

Use the same excitation as the ADC reference where supported. Flux's analog vs digital PCB design guide discusses layout considerations for sensitive analog measurements alongside digital logic. Include divider tolerance, ADC offset and gain, reference mismatch, input leakage and sampling-capacitor settling in the budget. ADC bits alone do not define temperature resolution, so translate each code step through the local divider slope. For low-power measurements, switch excitation between readings if necessary and allow settling before sampling. Check dissipation across the entire interval and detect open- or short-circuit faults before accepting a reading as a control temperature. Browse the Flux thermistor library for candidate parts and verify the exact resistance curve.

Choose a digital temperature IC if…

• The product needs to measure board temperature rather than an inaccessible process probe.
• A lab fixture can thermally couple the measured object to a sensor PCB.
• HVAC electronics need a local reference channel with little analog circuitry.
• A battery product needs board monitoring in addition to insulated cell sensors.
• The bill of materials can support power and an electrically suitable digital bus.

Design considerations and minimal digital circuit

Texas Instruments TMP117AIDRVR, introduced above, measures its own die temperature and reports it digitally over an Inter-Integrated Circuit (I²C) bus. It specifies ±0.3°C maximum over −55 to +150°C. The electrical table uses eight averages, a 1 Hz conversion cycle and an unsoldered DRV thermal pad, with full-range supply conditions of 1.8–5.5 V. Its 7.8125 m°C register step is resolution, not the measurement accuracy guarantee.

To measure PCB temperature, connect V+/GND, a local 0.1 µF bypass, SDA/SCL to the host, and appropriate bus pull-ups. The typical circuit shows 5 kΩ pull-ups and ADD0 connected to a permitted address-selection node. Connect ALERT with a pull-up if used. For high precision on a rigid PCB, TI advises leaving the exposed pad unsoldered, so include the requirement in assembly instructions. Separate nearby heat sources and validate object-to-die gradients. A digital interface removes the need for external analog conversion, although thermal installation error still affects the result.

Cable, PCB and verification checklist

To reduce interference from switching power and heater traces, route balanced signal paths for remote resistive sensors and thermocouples away from noisy circuitry. Flux's PCB EMI guide covers grounding, shielding, filtering and trace routing for interference control. Use twisted or shielded cable as appropriate, with a deliberate shield termination, connector-side protection and symmetric filtering. Check leakage and protection-induced offsets, differential range, common-mode range and isolation requirements independently.

When assessing performance, accuracy means agreement with the true temperature and resolution is the smallest reported increment. System uncertainty also includes calibration, mounting gradients, self-heating, lead resistance, reference error, cold-junction compensation and noise. Build a documented budget rather than treating tolerance limits as automatically independent statistical terms. Validate fault detection, settling and mounted response before choosing recalibration intervals. Flux's industrial sensing application examples illustrate sensor interface architectures. Industrial sensing reference designs can help organize the interface, but the finished probe still needs qualification.

FAQ

RTD vs thermocouple: which is more accurate?

Thermocouple vs RTD: which should a furnace use?

Thermistor vs thermocouple: what should a hotend use?

RTD vs thermistor, or NTC vs RTD: what suits HVAC?

Pt100 vs thermocouple: which suits precision lab work?

Pt100 vs Pt1000: is Pt1000 automatically better?

Sources and verified Flux examples

Off-board RTDs and K/J/T assemblies need separate selection. Reference circuits are starting schematics that require electrical review.

• Analog Devices/Maxim MAX31865AAP+T. Datasheet 19-6478 Rev 3, July 2015.
• Analog Devices/Maxim MAX31856MUD+. Datasheet 19-7534 Rev 0, February 2015.
• Texas Instruments TMP117AIDRVR. Datasheet SNOSD82D, September 2022.
• Vishay BCcomponents NTCS0805E3103FMT. Datasheet document 29044, June 22, 2026.
• IEC 60751:2022, edition 3, official scope. Tolerance numbers use manufacturer reproduction WIKA IN 00.17, November 2020.
• IEC 60584-1:2013, edition 3, official scope and preview. Tolerances use TC Ltd's manufacturer table. Omega's guide provides environment guidance.
• Vishay Selecting NTC Thermistors, document 33001, July 14, 2015. NTC application note, document 29053.

Manufacturer guidance supports the quoted tolerance and component values. Confirm the applicable standards edition and finished probe specifications before procurement.