For pneumatic testing, air is used as the medium for testing and for helium testing they use helium, and the cost for helium is much higher.
![Helium Leak Test](https://usbellows.com/wp-content/uploads/2019/05/helium-leak-test.jpg)
For pneumatic testing, air is used as the medium for testing and for helium testing they use helium, and the cost for helium is much higher.