The automatic calibration system of humidity fixed points at CMS

The automatic hygrostat is designed to calibrate hygrometers by employing several humidity fixed points under thermostat conditions. The set incorporates a blowing fan, a box-like measuring vessel, a control system, and an assembly of five acrylic resin trays which contain stainless steel covers, labyrinth baffle-boards at the bottom and are all set in a reciprocating motion. The trays are successively turned on or off by operating solenoid valves to give a closed circulation with the measuring vessel, establishing the humidity at a specific value selected by the user. The desired relative humidities are attained by adding lithium chloride, potassium carbonate, sodium chloride, potassium chloride, and potassium sulfate to pure water inside the five trays and can generate levels of approximately 12, 43, 75, 86, and 97% RH respectively, which have a maximum difference value, compared to reference values of ±1% RH in the range around room temperature. After switching to another tray for the control setting, the time required to reach a new equilibrium should be no longer than about 20 minutes.