Gas needs a pressure difference to flow through a flow meter. With the same supply conditions, the greater the flow meter pressure drop, the more the pressurizing flow is hindered, and the slower the tested volume usually is to build pressure and settle.
A low-pressure-drop instrument reduces this resistance and helps shorten the wait before testing; how much faster it really is also depends on the tested volume, the whole gas line and the thermal settling process.
First pressurize, then stabilize, finally read the leak
At the start of a test, gas enters the part under test through the gas line and its internal pressure rises. What the flow meter reads at this point is mainly pressurizing flow and cannot be taken as leakage.
As the pressure of the part approaches the supply pressure, the pressure difference driving gas in becomes smaller and the make-up flow gradually decreases. The greater the gas line resistance, the slower this approach to equilibrium; a large volume needs more gas to be added, so the effect is more pronounced.
Only once pressure and temperature have stabilized can the small make-up flow that continues be used to judge leakage. If the wait is too short, make-up flow from residual pressurizing and gas cooling is easily mixed into the reading.
Why low pressure drop can speed this up
Low pressure drop is equivalent to less resistance in the gas line: with the same available pressure difference, gas enters the tested volume more easily, and near equilibrium it is also easier to complete the remaining make-up so that the pressure settles faster.
A simplified analogy helps: the lower the resistance and the smaller the volume, the faster the pressure usually builds. This explains only the trend and cannot be used to calculate the actual waiting time.
If fast pressurizing uses a separate bypass, the benefit of low pressure drop shows mainly in the re-equilibration after switching to the measuring branch, and faster pressurizing cannot simply be equated with faster testing of the whole machine.
When choosing low pressure drop, also look at the whole gas line
- Choose the low-pressure-drop series according to the actual gas, leak flow and pressure, and also check range, working pressure and allowable differential pressure.
- Reduce the resistance from unnecessary pipe length, narrow bores and fittings, while keeping the necessary filtering and protection.
- Low pressure drop does not eliminate temperature disturbance. Keep the gas source and the workpiece at a stable temperature, then confirm the waiting time from the actual gas line.
Frequently asked questions
Does low pressure drop mean higher accuracy?
Not necessarily. Pressure drop affects gas line resistance, and accuracy has to be evaluated separately; accuracy is quoted after flow zeroing, with temperature, pressure and humidity in equilibrium.
Why are large volumes slower?
The same pressure change requires exchanging more gas, and both gas line resistance and heat exchange can lengthen the settling time.
After changing to a low-pressure-drop instrument, can I read the leak at once?
You still need to wait for pressure and temperature to stabilize. Low pressure drop helps shorten gas line equilibration but cannot replace the judgment that the system is stable.