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Calibration involves comparing a device under test (DUT) with an unknown value against a reference standard with a known value. This process is typically performed to identify the error in the DUT or to verify its accuracy. For instance, consider calibrating a thermometer by measuring its temperature reading in water at the known boiling point of 212 degrees Fahrenheit. To enhance precision, you might use a calibrated reference thermometer with an exact known value to verify the DUT thermometer, as visually determining the exact boiling point can be imprecise. Following calibration, the next logical step might be to make corrective adjustments to the instrument to minimize measurement errors, although technically, this step is distinct from calibration.   International System of Units (SI) – The Top Level of Known Measurement Standards How do we arrive at measurement standards of known values against which we calibrate our devices under test? For the answer, we turn to the International System of Units, abbreviated “SI”, which is derived from “Le Système International d'Unités” in French. The SI consists of seven base units which are the second, meter, kilogram, ampere, kelvin, mole and candela. The seven base SI units are derived from quantities in nature that do not change, such as the speed of light.       Calibration Interoperability A key benefit of having the BIPM manage the SI on a worldwide basis is calibration interoperability. This means that all around the world, we are using the same measurement system and definitions. This allows someone in the U.S. to purchase a 1-ohm resistor in Australia and be confident that it will be 1 ohm as measured by U.S. Standards, and vice versa. In order to have interoperability, we need to have all of our measurements traceable to the same definition.   Calibration Accreditation When calibrations are conducted, ensuring the reliability of the process is crucial. Calibration accreditation instills this trust by giving instrument owners confidence that the calibration was performed correctly. Accreditation signifies that a calibration process has been reviewed and found to comply with internationally accepted technical and quality metrology standards. The global metrology quality standard to which calibration laboratories are accredited is ISO/IEC 17025.   Calibration Certificates A calibration laboratory often provides a certificate with the calibration of an instrument. The calibration certificate provides important information to give the instrument’s owner confidence that the device was calibrated correctly and to help show proof of the calibration. A calibration certificate might include a statement of traceability or a list of the calibration standards used for the calibration, any data resulting from the calibration, the calibration date, and possibly pass or fail statements for each measurement result. Calibration certificates vary because not all calibration laboratories follow the same industry standards, and they also can vary depending on where the calibration fits within the calibration pyramid or hierarchy. For example, the calibration certificate required for a grocery store scale may be very simple, while the calibration certificate for a precision balance in a calibration laboratory may have a lot more technical content. Calibration certificates coming from an accredited calibration process have some very particular requirements which can be found in the international standard ISO/IEC 17025.  

Introductions RTDs use a high-precision resistor, typically made of platinum, whose resistance increases with temperature. They offer high accuracy and stability thus are commonly used in industrial applications and typically come in 2, 3, or 4-wire configurations. Thermocouples consist of a junction between two different metal alloys, generating a voltage that varies with temperature. They are versatile, cost-effective, and can measure a wide range of temperatures. Common types include J, K, T, and N, each defined by the metals used and their specific temperature range and sensitivity. NTC thermistors are resistors that decrease in resistance as temperature increases. They are highly sensitive and respond quickly to temperature changes, making them suitable for applications requiring rapid detection of temperature variations.   Thermocouple, Platinum Resistance & Thermistor Comparison     Platinum Resistance Thermometer Thermocouple Thermistor Sensor Platinum-wire wound or flat- film resistor Thermoelement. two dissimilar metals/alloys Ceramic [metal oxides] Accuracy (typical values) 0.1 to 1.0℃ 0.5 to 5.0℃ 0.1 to 1.5℃ Long term Stability Excellent Variable,Prone to ageing Good Temperature range -200 to 650℃ -200 to1750℃ -100 to 300℃ Thermal response Wirewound-slow Film-faster 1-50 secs typical Sheathed -slow Exposed tip- fast 0.1 to10 secs typical generally fast0.05 to 2.5 secs typical Excitation Constant current required None None Characteristic PTC resistance Thermovoltage NTC resistance (some are PTC) Linearity Fairly linear Most types non-linear Exponential Lead resistance effect 3 8 4 wire-low. 2 wire -high Short cable runs satisfactory Low Electrical“pick-up” Rarely susceptible Susceptible Not susceptible Interface Bridge 2,3 or 4 wire Potentiometric input.Cold junction compensation required 2 wire resistance Vibration effects/shock Wirewound -not suitable.  Film-good Mineral insulated types suitable Suitable Output/characteristic approx.0.4 W/℃ From 10uV/℃ to to 40μv/℃ depending on type -4%/℃ Extension Leads Copper Compensating cable Copper Cost Wirewound-more expensive Film-cheaper Relatively low cost Inexpensive to moderate     How to choose between RTD Sensors and Thermocouples RTD Sensors: Resistance Thermometers (RTDs) use a high-precision resistor, typically platinum, which increases in resistance as temperature rises. The widely adopted Pt100 standard has a resistance of 100.0 Ohms at 0°C and changes by 38.50 Ohms from 0 to 100°C. Platinum resistors provide highly stable and accurate temperature measurements. RTDs are usually 2-wire devices but can be extended to 3 or 4 wires for enhanced accuracy. Thermocouples: Thermocouples consist of a junction between two different alloys and a two-wire extension. The EMF generated between the hot junction and a reference junction creates a stable and repeatable temperature measurement. Various thermocouple types (e.g., J, K, T, N) are defined by the alloys used, each displaying distinct EMF characteristics.   RTD’s are, generally:  • More expensive  • More accurate  • Highly stable (if used carefully)  • Capable of better resolution  • Restricted in their range of temperature  • Stem, not tip sensitive  • Rarely available in small diameters (below 3mm)   Thermocouples are, generally:  • Relatively inexpensive  • More rugged  • Less accurate  • More prone to drift  • More sensitive  • Tip sensing  • Available in smaller diameters  • Available with a wider temperature range  • More versatile   Conclusion Each sensor type has its strengths and is chosen based on the specific requirements of the application, such as accuracy, temperature range, response time, and cost.