RTD Temperature Sensor Symbol
Definition: The RTD Temperature Sensor symbol represents a Resistance Temperature Detector — a passive sensing element whose electrical resistance increases predictably with temperature — depicted in circuit diagrams as a rectangular block or resistor-style element with pins A, B, and C, conforming to IEC 60751 (industrial platinum RTDs) and commonly implemented as a PT100 (100 Ω at 0 °C) or PT1000 (1000 Ω at 0 °C) platinum-wire element.
Also known as: PT100, PT1000, resistance thermometer, platinum resistance thermometer, PRT, RTD sensor.
What the RTD Temperature Sensor symbol means
The RTD Temperature Sensor symbol in a circuit diagram represents a precision temperature-sensing element whose resistance varies with temperature according to a well-defined, stable, and repeatable characteristic. Unlike thermocouples that generate a voltage, RTDs are passive elements measured by passing a small current through them and reading the resulting voltage (or using a bridge circuit).
In industrial and instrumentation schematics, the RTD symbol indicates where a calibrated temperature measurement originates. PT100 and PT1000 are the most common RTD types; 'PT' denotes platinum, and the number is the resistance in ohms at 0 °C. RTDs provide the highest accuracy and stability of all common temperature sensors, making them the standard choice for process control, calibration laboratories, and precision measurement systems.
How to identify the RTD Temperature Sensor symbol
The RTD Temperature Sensor symbol is drawn as a rectangle or a resistor symbol (IEC rectangle or ANSI zigzag) with a temperature-sensitive annotation, labelled 'RTD', 'PT100', or 'PT1000'. The symbol shows three pins: A, B, and C (for three-wire connection), where A and C are the current-carrying leads and B is the sensing lead, or in two-wire versions just two terminals. Some symbols show a thermometer icon alongside the resistor element to distinguish it from a fixed resistor.
Function in a circuit
An RTD temperature sensor measures temperature by exploiting the predictable increase in resistance of pure metals (typically platinum) with rising temperature. For a PT100, resistance is 100 Ω at 0 °C and increases approximately 0.385 Ω per °C. A transmitter or signal conditioner passes a constant current (typically 1 mA) through the RTD, measures the voltage drop, and converts the resistance value to temperature using the Callendar-Van Dusen equation defined in IEC 60751. Three-wire and four-wire wiring configurations compensate for lead-wire resistance errors.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60751 defines the resistance-temperature characteristic (R-T curve), tolerance classes (AA, A, B, C), and marking for industrial platinum RTDs. IEC 60617 uses a resistor-style symbol annotated with a temperature-sensitivity indicator (a small 't' or thermometer) per IEC 60617-04. |
|---|---|
| ANSI/IEEE 315 | ANSI/ISA-S51.1 covers process instrument symbols. In ISA practice, RTDs are represented as a diamond-shaped temperature element (TE) tagged symbol in P&ID drawings rather than as an electronic symbol. In circuit schematics, ANSI practice uses a resistor box annotated 'RTD' or 'PT100'. |
| Key difference | IEC 60617 uses an annotated resistor symbol with a thermal sensitivity indicator; ISA/ANSI P&ID practice uses a TE (temperature element) bubble. In circuit-level schematics both use a resistor-style box with RTD/PT100 labelling. |
Terminals / pins
| Pin | Name |
|---|---|
| a | A |
| b | B |
| c | C |
Typical values
PT100: 100 Ω at 0 °C, temperature coefficient α = 0.00385 Ω/Ω/°C (IEC 60751). PT1000: 1000 Ω at 0 °C, same α. Measurement range: −200 °C to +850 °C (platinum). Tolerance class A: ±(0.15 + 0.002|T|) °C. Class B: ±(0.30 + 0.005|T|) °C. Excitation current: 0.1–1 mA (to minimise self-heating).
Where the RTD Temperature Sensor symbol is used
- Industrial process control — measuring fluid, gas, and surface temperatures in pipelines, reactors, and heat exchangers connected to PLC/DCS analog input cards
- HVAC systems — duct air temperature measurement and chilled-water supply/return monitoring using PT100 sensors with 4–20 mA transmitters
- Calibration laboratories — primary temperature reference standards where PT100 class AA accuracy (±0.1 °C) is required
- Food and pharmaceutical processing — sanitary RTD probes measuring CIP (clean-in-place) and sterilisation temperatures for validation records
- Motor winding temperature monitoring — embedded PT100 sensors in motor stator windings for thermal-overload protection
- Battery management systems — measuring cell temperature for charge and discharge thermal de-rating in industrial battery packs
Example
In a PLC-based boiler control system, the RTD Temperature Sensor symbol shows a PT100 element connected in three-wire configuration — pins A and C to the current source terminals and pin B to the sense input of an analog input module — with a 4–20 mA transmitter converting the resistance reading to a loop current proportional to 0–200 °C for the PLC to monitor and control steam pressure.
Key facts
- PT100 (IEC 60751 Class B) is the global standard industrial RTD: 100 Ω at 0 °C, α = 0.00385 Ω/Ω/°C, tolerance ±0.30 °C at 0 °C.
- Three-wire RTD wiring compensates for lead resistance by using two leads for current excitation and one lead plus a reference lead for voltage measurement, eliminating the lead-resistance error present in two-wire configurations.
- Four-wire Kelvin connection eliminates lead resistance entirely by using separate current and voltage leads, achieving the highest accuracy for laboratory and calibration-grade measurements.
- RTD accuracy is superior to thermocouples and thermistors: PT100 class A achieves ±0.15 °C at 0 °C versus ±1–2 °C typical for Type K thermocouple in the same range.
- Self-heating error occurs when the excitation current through the RTD generates resistive heating; the industry standard is to limit excitation to 1 mA or less to keep self-heating below 0.1 °C in typical installations.
- IEC 60751 defines four tolerance classes — AA (tightest, ±0.1 °C at 0 °C), A (±0.15 °C), B (±0.30 °C), and C (±0.6 °C) — which correspond directly to the sensor's measurement accuracy.
- The reference designator for RTDs in instrument loop diagrams is TE (temperature element) per ISA-5.1; in circuit-level schematics the designator RT or TH is commonly used.
Frequently asked questions
What does the RTD temperature sensor symbol mean in a circuit diagram?
The RTD temperature sensor symbol represents a precision resistive element whose resistance changes predictably with temperature, used to measure temperature in industrial and instrumentation circuits. Pins A, B, and C correspond to the three-wire connection for lead-resistance compensation.
What does the RTD symbol look like?
The RTD symbol is drawn as a rectangle (IEC style) or zigzag (ANSI style) resistor shape labelled 'RTD', 'PT100', or 'PT1000', often with a small thermometer or temperature-sensitivity indicator. In P&ID instrument diagrams it appears as a TE (temperature element) bubble per ISA-5.1.
What is the difference between PT100 and PT1000?
Both PT100 and PT1000 are platinum RTDs with the same temperature coefficient (α = 0.00385 Ω/Ω/°C per IEC 60751). PT100 has a base resistance of 100 Ω at 0 °C; PT1000 has 1000 Ω at 0 °C. PT1000 is preferred for long cable runs because its higher resistance makes lead-wire resistance errors proportionally smaller.
What standard defines RTD temperature sensors?
IEC 60751 defines the resistance-temperature characteristic, tolerance classes (AA, A, B, C), and marking conventions for industrial platinum RTDs. Instrument loop diagram symbols are defined by ISA-5.1. IEC 60617 provides the generic circuit symbol.
Why use a three-wire RTD instead of two-wire?
A two-wire RTD connection includes lead-wire resistance in the measurement, causing a positive temperature error (approximately 0.3 °C per metre of cable for 24 AWG wire). A three-wire connection uses a third lead to measure lead resistance and subtract it, reducing lead-wire error to near zero for most industrial applications.
What is the reference designator for an RTD?
In P&ID instrument loop diagrams (ISA-5.1), the RTD is tagged TE (temperature element). In circuit-level schematics, common designators are RT (resistive thermometer) or TH (thermistor/thermal). No single IEC 60617 designator letter is universally standardised for RTDs in circuit schematics.
What is the temperature range of a PT100 RTD?
Platinum PT100 RTDs can measure from −200 °C to +850 °C per IEC 60751. Industrial installations typically use the range −50 °C to +400 °C. Standard tolerance Class B accuracy at 0 °C is ±0.30 °C; Class A is ±0.15 °C.
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