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QTI thermistors are manufactured utilizing standards with calibrations that are either traceable to the National Institute of Standards and Technology (NIST) or are derived from the International Temperature Scale of 1990 (ITS-90) fixed points: Triple Point of Mercury (TPHg), Triple Point of Water (TPW), Melting Point of Gallium (MPGa), Freezing Point of Tin (FPSn), and/or Freezing Point of Zinc (FPZn). The QTI calibration system is described below.
Triple Point of Water Cell (Jarrett Style), which is the same type of temperature standard used by NIST to calibrate Standard Platinum Resistance Thermometers (SPRTs) and other thermometers requiring extremely low uncertainties. The TPW is a defining temperature point for the ITS-90 with a value of 0.01°C and an uncertainty of + 0.00000°C / 0.00015°C. Melting Point of Gallium System, an ITS-90 defining temperature point with a value of 29.7646°C and an uncertainty of ± 0.001°C.
Standard Platinum Resistance Thermometer (SPRT), considered by NIST and other national laboratories to be a defining instrument for the ITS-90. QTI’s SPRT is calibrated by a nationally recognized metrology laboratory and verified by in-house TPH2O and MPGa standards. The expanded uncertainty*, U, of QTI’s primary SPRT calibration system is ± 0.003°C with a coverage factor k = 2. Super stable temperature probes, which are secondary temperature standards used to calibrate the production temperature controlled baths to an expanded uncertainty of ± 0.01°C. Resistance standards with expanded uncertainties of ± 2 µΩ/Ω are used to calibrate resistance measuring instruments to an expanded uncertainty of ± 0.02 %, which corresponds to an equivalent temperature expanded uncertainty of ± 0.007°C.
Therefore, the QTI test and measurement system used to manufacture thermistors and temperature sensors has a total expanded uncertainty of ± 0.012°C and is traceable to NIST.
* Note: Each expanded uncertainty listed in this document was calculated by multiplying the combined standard uncertainty by the coverage factor k = 2, as recommended by NIST.