【20090806 Key Topic】Issues with vacuum level testing in the new system
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This post was last edited by jothan on 2009-8-5 19:37. Dear everyone: While conducting vacuum tests on the negative pressure system for a relocation project, I observed a strange phenomenon. Specifically: When the negative pressure system is maintained at an absolute pressure of 20 KPa, why does the absolute pressure increase as the ambient temperature rises, while it decreases as the temperature falls? Isn’t this phenomenon quite strange? It’s exactly the opposite of the results of the system’s pressure test! It also does not conform to the ideal gas law. I would like to ask everyone why this phenomenon occurs
This post was last edited by jothan on 2009-8-5 20:11. In my opinion, the phenomena observed under negative pressure are incorrect. Vacuum level = Atmospheric pressure – Absolute pressure. As the temperature rises, atmospheric pressure increases, while the absolute pressure is determined by the equipment used to create vacuum; it should remain constant. Therefore, as the temperature rises, the vacuum level increases ； As the temperature drops, atmospheric pressure decreases, and of course, the vacuum level also drops. Have you mixed up the concepts of vacuum degree and absolute pressure?
There’s no contradiction – it’s a gauge pressure meter. As the temperature rises, the negative pressure certainly decreases, while the gauge pressure increases accordingly; and vice versa. In a positive-pressure experiment, as the temperature rises, the volume expands and the pressure increases; vice versa.
3rd floor. Thank you for your participation. Our pressure testing results are the exact opposite of what you mentioned: the absolute pressure is lowest during the day when the temperature is high (meaning the vacuum level is better), while it is higher at night when the temperature is low (meaning the vacuum level is relatively worse). I’m not sure what explains this phenomenon
2nd floor: Your idea has really enlightened me; thank you. I’m correct about the concept of vacuum level, haha. I’ve also been working in the chemical industry for several years now! Your formula, once I have it, will allow me to reach a conclusion. Thank you! ! ! !
Gauge pressure? Is it absolute pressure? Hehe. . . . Learn* it. For pressure testing, it can be done under positive pressure, at atmospheric pressure, or under negative pressure. Generally, we first conduct pressure testing under positive pressure and then under negative pressure; in both cases, it is necessary to maintain the pressure.
The phenomenon you observe is caused by changes in temperature, but as long as the pressure change during testing is less than 8 mmHg per 2 hours, it meets the requirements
Generally, before testing for negative pressure in a negative-pressure system, it is necessary to first check for leaks using 2 kilograms of positive pressure (the exact amount of positive pressure can be determined based on actual conditions; it’s best not to exceed 2 kilograms of pressure). After that, a negative-pressure retention test is carried out (it isn’t absolutely necessary to conduct a positive-pressure test as well; if the quality of the work is good, such a test may not be needed. Positive pressure is neither the goal nor the outcome – what’s important is that the results of the negative-pressure retention test meet the requirements of relevant standards). I’m not sure which standard the person on floor 7 is referring to; we follow the SH3501 standard for the petrochemical industry, which is quite strict. A pass mark is achieved when the absolute pressure increase over 24 hours does not exceed 5%. Is what the person on floor 7 said too lenient? Is it acceptable if the pressure change within two hours is within 1.06 KPa? I don’t know if it’s related to the standards
What you said is completely correct; the results of the experiment are fine. Vacuum level is absolute pressure minus atmospheric pressure; therefore, if your absolute pressure is high, it means your vacuum level has decreased! That’s indeed the case; as environmental issues worsen, if all other conditions remain unchanged, your vacuum level will definitely decrease!
This post was last edited by dongliangsir on 2009-8-9 at 12:14. As the temperature rises, atmospheric pressure decreases. The atmospheric pressure in winter is higher than that in summer. If you don’t believe me, check the weather data. The effect of ambient temperature on vacuum level or residual pressure, I think, should be considered based on the specific equipment, with the underlying reasons being investigated. For example, when a steam ejector is used in combination with a condenser to create a vacuum, as the ambient temperature rises, the temperature after cooling also increases, which inevitably leads to a decrease in vacuum level and an increase in residual pressure. During your negative pressure test, can the container be kept 100% isolated from the outside world?