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Willow has provided advanced power and precision resistor products to OEM industries since 1985. These products "push" the limits for performance, precision, stability and RTD Sensor Mainreliability.

Over the past 20 years we have developed a unique level of design and manufacturing expertise in wire wound resistor elements - expertise that also applies to custom RTD elements.

We offer no "standard" RTD probes. Instead, we apply our skills to provide custom RTD solutions for our customers.

We have an enviable industry reputation as a quality supplier that meets commitments and provides exceptional service and we certainly know how to make wire wound resistance elements.

Application Notes

Willow has supplied high-precision wire wound resistors for OEM industries since 1985. These precision resistors have tolerances as small as 0.005%. They are extremely stable over time, and operate over wide operating temperatures.

Resistors this precise and stable require unusual design and manufacturing skills that absolutely manage the desirable properties of the resistor element wire, while minimizing undesirable characteristics that cause unpredictable errors and instabilities. These same design and manufacturing skills are as critical in RTD's as they are in precision resistors!

RTD Basics
A fundamental physical property of a metal is that its resistivity changes with temperature. For many metals, this relationship is quite linear over wide temperature ranges making them ideal for measuring temperatures.

The relationship between the temperature and resistivity of a material is its temperature coefficient (TC) or temperature coefficient of resistance (TCR). TC or TCR is usually stated as a constant; parts per million per degree per Centigrade (ppm/ C). This property is the basis for designing RTD's.

A Resistance Temperature Detector (RTD) is a resistor designed to measure temperature using the known resistance vs. temperature relationship of metals. An RTD element is the actual temperature-sensing unit. It is typically unprotected and offers only the element leads for termination's. An RTD probe is an assembly composed of an element, a sheath or tube to protect the element and provide a thermal path to the measurement, lead wires, and a termination or connection.

RTDs are often made with platinum, nickel or nickel alloy wire as their sensing resistance element. Platinum is a popular choice; it can withstand high temperatures and maintain excellent stability. It has relatively high resistivity, and is highly linear with temperature. Copper has low resistivity, requiring a larger element, but its excellent linearity and low-cost make it an economical alternative in some applications. Balco is nickel-iron alloy with a TC similar to nickel but twice its resistivity.

The resistance of metals is also affected by other factors. This creates some difficult design and manufacturing challenges for RTD products (and precision resistors.) Perhaps the most important is the change in resistance as the resistance element wire is stressed.


TCR (ppm/C)

Nickel 99.9%


Nickel 99.5%








As wire is strained, its cross sectional area diminishes and its length increases resulting in an increase in resistance. Although this principle is used in some sensors (strain gages), the characteristic is very undesirable in precision resistors and RTD's.

The change in resistance with stress must be properly managed in designing and manufacturing precision resistors. A precision resistor must be highly stable in its operating environment, and with time, to maintain its precision tolerance. Designs and manufacturing methods must minimize these internal stress conditions.

These same considerations are equally important For RTDs. The usefulness of an RTD depends upon its resistance changing only as a result of its TCR; if the resistance changes because of other factors such as internal stress, the temperature measurement calibration will be seriously compromised and complicated.

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Willow Technologies Ltd. Unit 3 Borers Yard, Borers Arms Road, Copthorne, West Sussex, RH10 3LH, UK

Tel: +44 (0) 1342 717102, Fax: +44 (0) 1342 717014, Email: