Precautions in the calibration of mechanical thermometers and hygrometers
Verification of temperature indication
During the calibration of temperature values, a platinum resistance thermometer should be selected as the standard. During the specific operation, note that the probe needs to be placed near the center of the verification box, on the same level as the thermometer and hygrometer. Turn on the switch of the platinum resistance thermometer and preheat it in advance. Start the power switch of the test box, enter the specified parameters on the panel of the temperature and humidity test box, and press the confirmation key. The verification box will enter in a stable state after running for a period of time. At this time, the calibration personnel can carefully observe whether the reading entered by the thermometer is in a stable state. Since the thermal response time of the thermometer and hygrometer is much longer than that of the platinum thermal resistance, it is necessary to wait for the thermometer and hygrometer to enter a steady state after the thermometer and hygrometer enter a steady state. If the thermometer and hygrometer enter in a stable state, the indication values of the thermometer and hygrometer can be read after stabilization for 10 minutes.
Verification of two humidity indication values
During the calibration of humidity indication, a precision dew point meter is used as a standard device. The dew point probe and temperature probe of the precision dew point meter should be placed near the center of the temperature and humidity test chamber, enter the corresponding temperature and humidity values on the panel of the temperature and humidity test chamber, and press the confirmation key. After the humidity value in the calibration box reaches a stable state, observe whether the humidity indication value of the precision dew point meter is stable. If the indication value of the humidity dew point meter is in a stable state, it will take about 10 minutes to read the indication value of the temperature and humidity meter and the precision dew point meter.
Three standard equipment are ventilation psychrometer
When the ventilation psychrometer is used as a standard device for metrological calibration, when the ventilation psychrometer is verified by a higher standard and meets the specified requirements, the following should be noted during use: First, the wet bulb yarn cover needs to be replaced regularly due to contamination. When replacing, please use the same type of yarn cover provided by the manufacturer, because the manufacturer usually performs humidity correction based on the wet bulb sensor and wet bulb yarn cover adopted by the manufacturer. When using different wet bulb yarn covers, the diameter of the wet bulb will change to a certain extent due to different thicknesses, and the temperature gradient in the cross-sectional direction of the yarn sleeve will increase, which will become a source of error. The second is to add water. Generally speaking, the water container of the ventilated hygrometer should not be too large. It can only be used for one day under continuous use and needs to be filled with water in time. If you do not use it for a long time, or if the wet-bulb yarn sleeve has dried up, you should turn on the power every half an hour or more after adding water. Otherwise, when the fan is ventilated after power-on, the wet-bulb yarn sleeve will not easily wet the sensor position. In addition, distilled water should be used to ensure that the surface layer of the yarn is not contaminated. Third, the location of the sensor. According to the principle of the ventilated hygrometer, its hygrometer should be located in the center of the steel pipe to ensure uniform ventilation. However, due to the negligence of the installer or the loosening of the screws over time, the sensor will tilt or even contact the pipe wall, which will cause a large error in temperature measurement. So observe frequently.
Effective working area of Wensheng constant humidity chamber and ventilation problems of hygrometer
According to the provisions of JJG205-2005 "Verification Regulations for Mechanical Thermometers and Hygrometers", the thermometer to be inspected should be placed in the effective working area of the constant temperature and humidity chamber. Although this description is only a simple sentence, it has a great impact on the verification results. During the verification process, if too many thermohygrometers are placed in the constant temperature and humidity cabinet in order to increase the number of thermohygrometers to be tested, the thermohygrometers around the box may have a large deviation in the indicated value. In addition, placing too many thermohygrometers at one time will also affect the ventilation wind speed in the constant temperature and humidity cabinet. If the ventilation wind speed cannot be guaranteed, the thermohygrometer to be tested will have a large indication error. In the actual verification work, at least 3 to 4 ventilation distances should be reserved for mercury medium dry and wet thermometers (such as type 272) (the verification distance of organic liquid medium dry and wet thermometers can be appropriately shortened). In addition, for layered constant temperature and humidity chambers with larger spaces, the middle layer should be selected as much as possible to place the dry and wet thermometer for testing.
