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Dewar evaporation test method
Date: Jun 17, 2020
Dewar vacuum failure is a rare phenomenon. Usually the vacuum system of the gas cylinder can be maintained for several years, during which the vacuum of the gas cylinder is gradually (at a very slow rate) gradually decreasing. If the vacuum plug of the gas cylinder is not abnormal, but the vacuum is suspected to be invalid, the dewar should be tested for evaporation rate (NER), and the test result is compared with the evaporation rate value in the product specification table.
1. First, fill the cylinder with liquid nitrogen, the filling quality should be 1/2 to 2/3 of the total filling quality;
2. Close the inlet and outlet valves of the pressure vessel, the booster valve, the gas use valve, and open the discharge valve to discharge the gas;
3. Leave the gas cylinder for 24 hours, then weigh the gas cylinder accurately and record the weight, time and date;
4. Continue to keep the Dewar still for 48 hours, then accurately weigh the gas cylinder again, and record the second weight, time and date;
Compare the test result with the evaporation rate value in the product specification table. The evaporation rate value of the new cylinder with the evaporation rate value of less than twice is normal, and any test result greater than twice indicates that the interlayer vacuum has been or is being lost. Please replace the new pressure vessel or repair the Dewar in time.
It should be noted that the first filling of hot bottles or overfilling of gas cylinders can also cause excessive evaporation rates. The pressure of the liner of a bottle with interlayer vacuum lost will rise rapidly. The change of this pressure is very obvious.
The temperature of the outer shell of the pressure vessel, especially where the pipeline is concentrated and near the support is lower than the temperature of other parts. The frost or "sweating" of the outer shell depends on the temperature and humidity of the surrounding environment. The existence of this condensation phenomenon can not be used as a reliable indicator to judge the existence of a cylinder vacuum, and the test of the daily evaporation rate of a cylinder is a reliable and effective judgment method.
1. First, fill the cylinder with liquid nitrogen, the filling quality should be 1/2 to 2/3 of the total filling quality;
2. Close the inlet and outlet valves of the pressure vessel, the booster valve, the gas use valve, and open the discharge valve to discharge the gas;
3. Leave the gas cylinder for 24 hours, then weigh the gas cylinder accurately and record the weight, time and date;
4. Continue to keep the Dewar still for 48 hours, then accurately weigh the gas cylinder again, and record the second weight, time and date;
It should be noted that the first filling of hot bottles or overfilling of gas cylinders can also cause excessive evaporation rates. The pressure of the liner of a bottle with interlayer vacuum lost will rise rapidly. The change of this pressure is very obvious.
The temperature of the outer shell of the pressure vessel, especially where the pipeline is concentrated and near the support is lower than the temperature of other parts. The frost or "sweating" of the outer shell depends on the temperature and humidity of the surrounding environment. The existence of this condensation phenomenon can not be used as a reliable indicator to judge the existence of a cylinder vacuum, and the test of the daily evaporation rate of a cylinder is a reliable and effective judgment method.
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