An RTD Element and Its Input Can Describe Different Resistances

Identify the resistance named by the record

An equipment or fleet report may receive a sensor designation, an input resistance or a converted temperature. Pt100 names a nominal element property at a specified temperature. It does not establish the resistance of a remote input path.

Distinguish resistance across the sensing element from resistance represented at the reader terminals. The leads have resistance too. A terminal total can be correct for the path it describes while leaving the element value unresolved.

For thermal information used alongside AI equipment records, establish which quantity the supplier returns and how the supported element, leads and reader produce it. That relationship gives the number its physical meaning.

References: [TI275A]

RTD-102: two element values produce the same 102 Ω

RTD-102 is an invented resistance example. Stipulate steady ideal values, a series path containing only the element and two leads, and no contact resistance, offsets, noise or self-heating. These are supplied analytical premises, not actual measurements or asserted temperatures.

Record A has element resistance 100 Ω, lead 1 resistance 1 Ω and lead 2 resistance 1 Ω. Its two-wire terminal total is 100 Ω + 1 Ω + 1 Ω = 102 Ω.

Record B has element resistance 101 Ω and lead resistances 0.5 Ω and 0.5 Ω. Its terminal total is 101 Ω + 0.5 Ω + 0.5 Ω = 102 Ω.

Both totals are exact under their premises. A return containing only 102 Ω cannot distinguish the 100 Ω element in A from the 101 Ω element in B. It contains one sum without the information needed to separate its terms.

Printing 102.000000 Ω adds no relationship between those terms. A higher nominal ADC bit count cannot supply it either. If the intended deliverable is element resistance, the missing evidence concerns the lead contribution and the reader's treatment of it. Asking for another decimal place leaves that question open.

References: [TI275A][ADI2016]

Read two-, three- and four-wire claims with the reader

In the elementary two-wire arrangement, the reader includes the element and both series leads. Calling the sensor Pt100 leaves those added resistances present in the input path.

Three-wire arrangements permit compensation whose operation depends on the circuit. TI's SBAA275A, revised March 2023, explicitly identifies matching lead resistances as an assumption of the compensation it describes. The buyer needs the actual reader's explanation of the relevant lead assumptions and compensation; the wire count alone supplies no universal residual-error figure.

A compatible four-wire, or Kelvin, arrangement separates excitation from voltage sensing. With high-impedance sense inputs and negligible sense current, the sensed voltage can describe the element without the excitation-lead drops. Analog Devices' June 15, 2016 discussion distinguishes current- and voltage-excitation examples and explains why the implemented relationship still matters. Four conductors alone do not establish that relationship.

References: [TI275A][ADI2016]

A 100 Ω element can coexist with the 102 Ω current path

Now apply an expressly ideal four-wire current-excitation reader to record A. Keep the same 100 Ω element and the same two 1 Ω current-carrying leads. Stipulate separate high-impedance voltage sensing across the element, negligible sense current and an ideal relationship between the sensed voltage and the excitation current.

Under those assumptions, the reader's equivalent element resistance is 100 Ω. The original excitation path still totals 102 Ω. That series total describes the excitation leads and element; the separate ideal sense branches carry no current.

No resistance has been physically removed. Comparing the two-wire 102 Ω total with the four-wire 100 Ω element value does not establish a temperature change: they describe different boundaries of the same stipulated state. There is no basis here for saying the element cooled.

This result excludes the specified excitation-lead contribution under the ideal premises. It is not a numerical accuracy claim for an actual reader.

References: [ADI2016]

Compare what the two offers actually promise to return

Consider two hypothetical descriptions for a thermal-information input. The first reads: “Pt100, Class A element, 24-bit reader and high-resolution display.” It names an element class and a nominal converter representation, but leaves the reported resistance's boundary unexplained. The buyer cannot tell whether a displayed resistance includes leads or whether a temperature output uses an applicable element relation.

A fuller description reads: “For the identified Pt100 and lead assembly, the supplied resistance is the element-equivalent value from the reader's documented four-wire current-excitation mode. The return identifies the sensor and reader revisions, the applicable sense-input conditions and the complete-chain performance evidence for the stated operating conditions. Any converted element temperature identifies its resistance-to-temperature basis.”

The second description makes the intended sensing quantity and supported element/lead/reader relationship intelligible. Its referenced evidence still needs to substantiate those statements for the actual offered assembly. It does not earn acceptance merely by containing more technical words.

A sensor class concerns the applicable element tolerance. TI directs readers to the element manufacturer's data for construction-dependent tolerance ranges. Neither that class nor a 24-bit label provides a universal accuracy guarantee for the assembled input.

References: [TI275A]

Resolve element temperature and its intended use separately

After the resistance boundary is clear, establish the supported conversion and complete-result evidence for the intended temperature. Analog Devices identifies further influences including ADC input behavior, reference and excitation stability, and element self-heating. Separating lead drop does not settle those contributions.

A measurement result generally includes a measured value, uncertainty and relevant information. Its uncertainty concerns the stated quantity and the information supporting it. Retain the supplied uncertainty basis for the assembled chain rather than substituting the element class or the display's last digit.

For an equipment or fleet report, also identify which physical location and condition the supplied element temperature represents. A sound element result alone does not establish the temperature of every component, a rack or an entire cooling circuit. The appointed technical and requirement owners must determine whether the supplied quantity and performance evidence answer the actual question.

The related measurement-resolution article develops uncertainty and conformity decisions. The ADC article addresses SINAD and effective dynamic resolution. The thermocouple article explains reference-junction compensation for a different sensor mechanism. Those remain separate decisions; RTD-102 resolves which resistance is being represented before a temperature result is used.

References: [ADI2016][VIM29][VIM226]

Scope any physical equipment installation separately

If the selected IT equipment independently needs rack installation, Leviathan's Rack-and-Stack Installation service is a starting point for discussing agreed enterprise or GPU equipment mounting to approved rack positions and handling conditions.

Use the service and project-inquiry links below with a nonconfidential equipment list, approved rack positions, site requirements and requested physical handoff records. That inquiry concerns separately agreed physical IT work, requirements and availability. RTD instrumentation, thermal controls and liquid-side work are outside this inquiry; the resistance example supplies no field procedure or work approval.