In system design, monitoring ambient temperatures is essential for accurate measurements. Last week, Heraeus Nexensos, a platinum temperature sensor developer, announced the pilot production of a new microRTD temperature sensor. "RTD" refers to resistance temperature detectors included in the device. 

Rendering of the dimensions of Heraeus Nexensos' microRTD. Image (modified) used courtesy of Heraeus Nexensos

While resistance temperature sensors are passive components, these devices are essential in applications ranging from battery monitoring to labs on a chip. 

What Is a Resistance Temperature Sensor?

A resistance temperature detector is a temperature sensor that has its resistance increase when the temperature rises. This increase in resistance is a measure of the change in temperature. An RTD sensor is typically made from elements such as: 

• Platinum (Pt) with a temperature coefficient of 0.00385
• Copper (Cu) with a temperature coefficient of 0.00427
• Nickel (Ni) with a temperature coefficient of 0.00672

As a passive component, an RTD sensor needs current to pass through it to produce a voltage so it can measure changes in the sensor’s base resistance as temperature rises or falls.

RTD sensors feature various lead wire configurations, opening doors for a wide range of use cases. Accuracy improves as wire count goes up: a two-wire sensor is suitable for applications that require less accuracy while three-wire sensors are more accurate, owing to the compensation loop in the sensor’s configuration. Four-wire sensors are the most accurate because they measure sensor resistance without the influence of the lead wires.

RTD sensors come in several lead wire configurations. Image used courtesy of TE Connectivity 

RTD sensors are large compared to other temperature sensors such as thermocouples. However, micro RTU (remote terminal unit) sensors are now produced to meet the demands of miniaturization and component flexibility.

RTD Sensors vs. Thermocouple Sensors

In contrast to the RTD sensor is the thermocouple. A thermocouple features two dissimilar metals wires welded together to make a junction. A thermoelectric potential or voltage is produced between the metal wires when there is a temperature difference in the junction. The voltage is measured and compared to the voltage produced by a reference junction to determine the temperature of the sensing junction.

Thermocouples have a wide measuring range compared to RTD sensors. A typical measuring range of an RTD sensor is -240°C to +650°C while a typical measuring range of a thermocouple temperature sensor is -270°C to +2,320°C.

In terms of accuracy, RTD sensors perform much better than thermocouples. Usually, thermocouples are susceptible to noise. Additionally, their bent wires cause poor accuracy and relatively high drift.

An array of RTD platinum thin-film elements. Image used courtesy of TE Connectivity

Because of the quick change in the temperature in a thermocouple's hot junction, this temperature sensor is more suitable for applications that require a faster response time. Thermocouples also cost less than RTD sensors.

Ultimately, an engineer's preference for an RTD sensor or a thermocouple will largely depend on the application. 

Heraeus Nexensos Takes the Lead in Producing microRTDs

A microRTD is a specific type of ultrathin, miniaturized temperature sensor that measures ambient temperature or monitors the temperature of critical components in system designs.

Pulling from the recent example from Heraeus Nexensos, a microRTD can, for instance, have a footprint as small as 0.6 mm x 0.3 mm. Heraeus Nexensos says that its microRTD is highly accurate, fast, and flexible. Its bendable structure is also said to conform over a variety of surfaces ranging in temperatures from -40 °C to +140 °C. Heraeus Nexensos says the sensor is suitable for use in applications that require temperature control on localized hot spots.

The product is still in its pilot testing phase, but the manufacturer claims a large-scale production facility will be constructed in the next phase.