M Series RTD sensors

The M308 should be considered when the sensor must fit within a probe or tube having an internal diameter below 1 mm. It is intended for restricted installation spaces and uses 7 mm leads, compared with the 10 mm leads used by the other core variants. For comparison, the M310 supports a minimum fit diameter of approximately 1.6 mm, the M213 approximately 1.8 mm and the M222 approximately 2.6 mm. The M308 is therefore relevant to wearables, compact medical equipment, battery monitoring and small thermal assemblies. Mechanical clearance, insulation and the effect of encapsulation on response time must still be checked in the completed probe design.

The tolerance class should be selected by considering both the required accuracy and the complete continuous operating range. Class F 0.1 applies from 0 °C to +150 °C, class F 0.15 from -50 °C to +300 °C and class F 0.3 from -70 °C to +500 °C. A tighter class cannot automatically be extended across the wider temperature range assigned to class F 0.3. Class F 0.3 variants may also be used temporarily at temperatures up to +550 °C, but this is not their continuous class range. Engineers should therefore specify the class against every critical measurement point rather than selecting it only from the nominal system temperature.

Pt100 is suitable for established industrial instrumentation and interfaces designed around a 100 Ω sensor at 0 °C. Pt1000 produces a larger resistance change per degree and can reduce the proportional influence of lead resistance, which is useful in compact electronics and lower-current measurement circuits. Pt500 provides an intermediate option and is available within the M222 family. The permissible measuring-current ranges are 0.3 to 1 mA for Pt100, 0.1 to 0.7 mA for Pt500 and 0.1 to 0.3 mA for Pt1000. The final choice should account for the analogue front-end range, wiring configuration, cable resistance, electrical noise and allowable self-heating error.

Self-heating is controlled by limiting electrical power in the RTD and ensuring adequate thermal coupling to the measured medium. The specified measuring currents should be treated as operating boundaries rather than automatic design targets, particularly in still air or poorly conducting encapsulants. At 0 °C, a Pt1000 driven at 0.3 mA dissipates approximately 0.09 mW, corresponding to an estimated temperature rise of about 0.018 to 0.054 K across self-heating coefficients of 0.2 to 0.6 K/mW. Lower current or pulsed excitation can reduce this contribution when the input circuit has sufficient resolution. Validation should be performed after the element is installed because the finished assembly changes the heat-transfer conditions.

The core M Series elements use platinum-clad nickel leads intended for welding, crimping and brazing. The joining method should be selected around the probe material, thermal cycle, electrical resistance and production-volume requirements. Welding can provide a compact metallic joint, while crimping avoids introducing a brazing thermal cycle into the element area. Brazing may suit probe constructions that require a metallurgical joint but demands controlled heat input and suitable process materials. In every case, the sensor body and lead exit should be protected from mechanical loading, and the completed joint should be checked for contact resistance, pull strength and insulation performance.

Response time depends strongly on the selected model, test medium, flow rate and final mounting arrangement. The M308 reaches t0.5 in 0.03 seconds and t0.9 in 0.09 seconds in water flowing at 0.3 m/s, while its corresponding air values at 3 m/s are 1.4 and 3.5 seconds. In water flowing at 0.4 m/s, the M310 is rated at 0.04 and 0.12 seconds, the M222 at 0.05 and 0.15 seconds, and the M416 at 0.06 and 0.18 seconds. Air response is slower because convective heat transfer is lower. Probe walls, potting compounds, sleeves and thermal contact resistance will normally increase the response time of the finished assembly.

The elements are specified for vibration of at least 40 g from 10 to 2,000 Hz and shock of at least 100 g with an 8 ms half-sine pulse, but both ratings depend on installation. These values do not remove the need for strain relief, lead support and controlled encapsulation around the sensor body. Established core variants show a typical R₀ drift of 0.04% after 1,000 hours at +500 °C, which is approximately equivalent to 0.10 °C near 0 °C for a 3850 ppm/K platinum characteristic. Repeated thermal cycling and assembly stress can add further error. Qualification testing should therefore use the actual probe construction, mounting orientation and anticipated vibration spectrum.

Standard catalogue elements should not automatically be treated as carrying a complete automotive qualification or submission package. The platinum characteristic, vibration capability and operating-temperature range may suit automotive sensing tasks, but project requirements can also include IMDS records, PPAP documentation, traceability controls and application-specific validation. These requirements must be confirmed for the selected order number before it is released into the design. The intended joining process, encapsulation material, thermal-cycling profile and failure-mode controls should also be reviewed at assembly level. Early confirmation avoids selecting an electrically suitable RTD that cannot meet the programme documentation or production approval process.