In this video we will discuss the ideal gas law and its effects on leak testing.
During leak testing, changes in pressure can occur due to changes in temperature, volume, and number of moles of the gas. Since parameters like pressure, volume, and amount of air are involved in this process, it is governed by the ideal gas law.
PV = nRT
P = the Pressure of the air (or gas) enclosed in the container, measured in atmospheres
V = the Volume of the container occupied by the gas, measured in Liters
n = the Number of moles of the gas
R is the Ideal gas constant with a value of 8.314
& T = the Absolute temperature of the gas, measured in Kelvin
Changes in pressure, temperature, and volume are related and a change in any single parameter can affect other parameters.
When the temperature of air rises, the average kinetic energy and velocity of the particles increases. This raises both the pressure and temperature.
When the test part is pressurized, the walls of the part can undergo expansion. This causes a change in the volume of the part. With an increase in area, the overall pressure drops. Similarly, a decrease in volume causes an increase in pressure, so long as the temperature and number of moles are constant. If the number of particles increases, this exerted force per unit area also increases.
Using the ideal gas law, a suitable relation can be derived that allows determination of leak rates through pressure loss in the part. A leak rate can be seen as the volume of gas that escapes through the part per second.
During a leak test, if the separation of the results, or Delta P, between leaking and non-leaking parts is not great enough, the repeatability of the process will be compromised. This is due to environmental factors influencing the pressure and temperature of the part under test.
The selection of Fill and Settle times during the test influence the part’s ability to come to equilibrium, which directly affects the test outcome. The greatest contributor to successful test outcomes is the Fill step. At higher test pressures, a longer fill step will allow the thermodynamic process to reach equilibrium. Parts that have more compliance, or capability to experience expansion, also need to reach a point of equilibrium. The ideal gas law illustrates how reducing or nullifying pressure or thermodynamic change will influence test repeatability by causing greater separation in the ΔP, thus improving the test.
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