Calibration of thermometers is standardized using the constant-volume gas thermometer with the triple point of water as a critical reference. Calibration typically utilizes the freezing and boiling points of water under standard atmospheric pressure.
The standard used for calibrating thermometers is based on the constant-volume gas thermometer, which metrologists—experts in measurement science—regard as the most accurate. This standard involves a vessel of constant volume filled with gas that changes pressure in response to temperature changes, correlating directly to temperature variations. The most critical point for this standard is the triple point of water, defined as 0.01 degrees Celsius or 273.16 Kelvin, where water exists as a solid, liquid, and gas in equilibrium. For calibration, two easily reproducible temperatures, often the freezing and boiling points of water at standard atmospheric pressure, are used as reference points. When the volume of gas in the thermometer is extrapolated to zero density (meaning no particulate interference), different gases yield very similar results, providing a standardized measure for temperature calibration.
The complete question probably:
What is the standard used for calibrating thermometers?