RTD Calibration in Rockford

What Is RTD Calibration

RTD (Resistance Temperature Detector) calibration is the process of verifying and documenting the resistance-temperature relationship of platinum resistance thermometers against NIST-traceable reference standards. RTDs operate on the principle that the electrical resistance of platinum changes predictably with temperature, providing highly accurate and repeatable temperature measurements. Calibration ensures that the RTD's measured resistance values correspond to the correct temperatures within specified tolerances.

Several types of RTDs are calibrated: Industrial RTDs (Pt100 and Pt1000, available in both thin-film and wire-wound element constructions), Platinum Resistance Thermometers (PRTs) used as secondary reference standards in calibration laboratories, and Standard Platinum Resistance Thermometers (SPRTs) used as primary reference standards that achieve the lowest measurement uncertainties. Wire-wound elements provide superior accuracy and long-term stability compared to thin-film elements, making them the preferred construction for reference-grade instruments.

The Callendar-Van Dusen equation describes the resistance-temperature relationship for platinum RTDs. Calibration determines the actual coefficients (R0, alpha, delta, beta) for each individual RTD, quantifying any deviation from the standard IEC 60751 reference curve. This per-sensor characterization is essential for achieving the highest measurement accuracy, as manufacturing variations cause each RTD to deviate slightly from the nominal curve. Accurate coefficient determination through calibration is the foundation of traceable RTD-based temperature measurement.

The RTD Calibration Process

Step 1: Sensor Assessment and Classification

A physical inspection of the RTD probe is performed, including evaluation of lead wire configuration (2-wire, 3-wire, or 4-wire), element type identification (thin-film versus wire-wound), and insulation resistance testing. The IEC 60751 tolerance class requirement is reviewed — Class AA, Class A, Class B, Class C, or 1/3 DIN — and the operating temperature range and required measurement uncertainty are assessed. This initial classification determines the calibration method, reference standard, and temperature source to be used.

Step 2: Reference Standard and Equipment Selection

The appropriate reference standard is selected based on the required measurement uncertainty: an SPRT for highest accuracy calibration, a PRT for general-purpose calibration, or a reference RTD for field comparison work. The temperature source is selected accordingly — fixed-point cells for ITS-90 calibration of SPRTs, stirred liquid baths for comparison calibration (providing superior thermal immersion and uniformity), or dry-block calibrators for field calibration. A precision resistance bridge or high-accuracy digital multimeter is selected for the resistance measurement, with the measurement method matched to the RTD's lead wire configuration.

Step 3: Resistance Measurement and Comparison

The RTD under test and the reference standard are immersed in the same stable temperature source. Four-wire resistance measurements are performed to eliminate lead wire resistance errors. Readings are recorded at multiple temperature points across the RTD's operating range, with sufficient stabilization time allowed at each point. Self-heating effects are evaluated and corrected by measuring at two or more excitation currents. The RTD's actual resistance at each temperature point is compared against the IEC 60751 reference curve or the customer's specified tolerance to determine conformance.

Step 4: Callendar-Van Dusen Coefficient Determination

When required, the Callendar-Van Dusen coefficients are calculated from the calibration data. R0 (resistance at 0 degrees Celsius), alpha (temperature coefficient of resistance), delta, and beta coefficients are determined through least-squares curve fitting to the measured resistance-temperature data. These coefficients allow the RTD's unique resistance-temperature relationship to be characterized with maximum accuracy, enabling precise temperature calculation from any measured resistance value. For SPRTs, ITS-90 deviation function coefficients are calculated from fixed-point measurements rather than Callendar-Van Dusen coefficients.

Step 5: Documentation and Certification

A calibration certificate is issued under the scope of an ISO/IEC 17025 accredited laboratory. The certificate includes resistance readings at each test point, deviations from the IEC 60751 reference curve, calculated Callendar-Van Dusen coefficients (when determined), measurement uncertainty at each calibration point, reference standard identification and traceability information, insulation resistance test results, and the complete NIST traceability chain for all measurement results.

Compliance & Standards

IEC 60751 — Industrial Platinum Resistance Thermometers

IEC 60751 is the international standard defining the resistance-temperature relationship, tolerance classes, and testing requirements for industrial platinum resistance thermometers. Tolerance classes range from Class AA (plus or minus 0.1 degrees Celsius at 0 degrees Celsius) to Class C (plus or minus 0.6 degrees Celsius at 0 degrees Celsius). Calibration determines conformance to the specified tolerance class by comparing measured resistance values against the standard reference curve at each test temperature.

ITS-90 — International Temperature Scale

Standard Platinum Resistance Thermometers (SPRTs) are the defining interpolating instruments of ITS-90 from the triple point of hydrogen (13.8033 K) through the freezing point of silver (961.78 degrees Celsius). ITS-90 calibration of SPRTs is performed at defined fixed points, providing the lowest achievable measurement uncertainties. Fixed-point cells used include the triple point of water (0.01 degrees Celsius), melting point of gallium (29.7646 degrees Celsius), freezing point of tin (231.928 degrees Celsius), and freezing point of zinc (419.527 degrees Celsius).

ASTM E1137 — Standard Specification for Industrial PRTs

ASTM E1137 defines the dimensional, electrical, and performance requirements for industrial platinum resistance thermometers. The standard specifies tolerance grades and testing procedures for qualification and acceptance testing of PRTs used in industrial applications. Calibration performed to ASTM E1137 verifies that the RTD meets the specified tolerance grade and performance criteria.

Additional Standards

• ISO/IEC 17025 — General requirements for the competence of testing and calibration laboratories
• ASTM E644 — Standard Test Methods for Testing Industrial Resistance Thermometers
• IEC 60751 Amendment 2 — Updated tolerance class definitions and testing requirements
• EURAMET cg-13 — Guidelines for the Calibration of Temperature Indicators and Simulators
• GUM — Guide to the Expression of Uncertainty in Measurement

Industry Applications

Pharmaceutical & Biotech

RTDs are used in reactors, lyophilizers, stability chambers, autoclaves, incubators, and clean room monitoring systems throughout pharmaceutical and biotech facilities. FDA 21 CFR Part 211 GMP requirements mandate documented calibration of all temperature measurement instruments. RTDs are preferred over thermocouples in pharmaceutical applications for their superior accuracy, long-term stability, and measurement repeatability, reducing calibration frequency and measurement risk.

Food Processing & Cold Chain

RTDs are deployed in pasteurization systems, retort processing equipment, fermentation vessels, and cold storage monitoring applications. High measurement accuracy is required at critical control points for HACCP and FSMA compliance. The inherent stability of RTDs reduces calibration frequency compared to thermocouples, lowering the total cost of ownership while maintaining measurement confidence at food safety critical limits.

Chemical & Petrochemical

Process RTDs are installed in reactors, heat exchangers, distillation columns, and storage tanks throughout chemical and petrochemical facilities. Industrial Pt100 and Pt1000 RTDs are calibrated to IEC 60751 specifications for process control accuracy. Calibration intervals and tolerance requirements are determined by the process control strategy and the measurement uncertainty budget for each control loop.

Metrology & Calibration Laboratories

PRTs and SPRTs are used as reference standards in calibration laboratories at all levels of the measurement hierarchy. Regular recalibration against fixed-point cells and higher-level reference standards maintains measurement traceability. SPRTs calibrated at ITS-90 fixed points provide the foundation of the temperature traceability chain, transferring measurement accuracy from the defined fixed points to working-level instruments through an unbroken chain of comparisons.

RTD Calibration in Rockford

On-site RTD calibration is performed at pharmaceutical, food processing, chemical, and manufacturing facilities in Rockford and the surrounding Illinois area. Comparison calibration using NIST-traceable reference standards is conducted by ISO/IEC 17025 accredited laboratories with calibration certificates issued on-site upon completion of all measurement work.

Facilities in Rockford, IL requiring RTD calibration for IEC 60751 compliance, FDA regulatory requirements, or quality system standards are served through Temperature Calibration Specialists. Standard and priority scheduling is available.