IoTClass Inside the measurement
A closer look · 01

Sensor

Look inside
a sensor.

Platinum RTD

An RTD senses temperature with a metal coil or film, usually platinum. As the metal warms, its electrical resistance rises. The measuring circuit sends a small current through the element and measures its resistance. The IoT device then uses an RTD equation to turn that resistance into a temperature reading. The change is almost a straight line, but precise measurements need an equation that accounts for its slight curve.

Watch the connecting wires: their resistance can affect the reading, especially with a low-resistance RTD. Include lead compensation in the measurement setup.

01

Recognize the element

Recognize the element: Platinum RTD

rtd measurement pathA platinum film changes resistance with temperature; electrical excitation makes resistance measurable. Conceptual element and measurement stages; not to scale.Conceptualexcitation currentplatinumresistancetemperature
The metal element supplies the changing resistance; excitation and an RTD model make it a temperature reading.
02

Peel the teaching model

From case to sensing element

Choose a layer to see the measurement path. The enclosure and board layout are invented teaching choices, not a particular product.

The full text version below describes every layer. All measurement explanations are available without 3D.
Conceptual, not to scale
Case: protective coatingThe coating covers the RTD body, with two leads extending below it.

Drag to rotate; pinch to zoom. With the view focused: arrow keys rotate, + / − zoom, 1–3 choose a layer, Home resets rotation and zoom. Step advances to the next layer; Reset returns to the case and default view. Layer buttons select a view directly. The page does not move on its own.

Layer 1 of 3 / Outside

Case: protective coating

The coating covers the RTD body, with two leads extending below it.

Teaching view: A protective coating covers the platinum sensing film on a pale support. Peel it away to see the metal element whose resistance changes with temperature.

The measurement to rememberA platinum resistance temperature detector (RTD) changes resistance with temperature. An external circuit supplies current so it can measure that resistance. [Cheap Nonlinear Thermistors] [Under the Hood: When Beta Isn’t Enough]

Full text version · every layer

1 · Case: protective coating

The coating covers the RTD body, with two leads extending below it.

Teaching view: A protective coating covers the platinum sensing film on a pale support. Peel it away to see the metal element whose resistance changes with temperature.

2 · Readout: two electrical leads

An external circuit supplies current to measure the RTD resistance. [Under the Hood: When Beta Isn’t Enough]

Teaching view: Two leads connect the platinum film to an external circuit. The circuit passes a small known current through the film and measures the voltage across it. It uses that voltage to calculate resistance, then temperature.

Explore the layers in order to follow the teaching model.

03

Listen

Package narration

Read transcript
Imagine Maya commissioning a connected temperature logger. She has chosen a platinum resistance probe, but she pauses before trusting the temperature it reports. The sensing element and the measurement circuit both matter. Inside the probe, a platinum coil or film changes resistance with temperature. The circuit supplies electrical excitation, measures that resistance, and applies a temperature model. Maya also checks the reference resistor and lead compensation, because cable resistance can affect a low resistance probe's reported temperature. Work through the chapter's P T one hundred example. Its reference resistance is one hundred ohms. At one hundred degrees Celsius, the approximate model multiplies one hundred by one point three eight five. The result is one hundred thirty eight point five ohms. That is a resistance the temperature model can interpret, rather than a temperature supplied directly by the metal. Maya's misconception was that an almost straight response could be treated as perfectly straight. Even this platinum probe curves gently. High accuracy work uses the Callendar Van Dusen equation to account for that curve. The takeaway is to read the platinum element through a suitable temperature model and a carefully checked measurement chain.

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