Difference Between C-Seal and W-Seal Gas Fittings

This is a C-Seal fitting:


Note that the sealing surface (that's the flange around the hole in the middle) is flat. This is the bottom fitting on a Celerity Unit Mass Flow Controller (MFC).

These are MKS's take on W-Seal fittings:


Those are on the bottom of an MKS MFC. Notice there is something poking up in the middle of the well in what would be the C-Seal sealing surface. That's the "W"

When SEMI went to set a standard for gas fittings for the semiconductor industry, they evaluated several and ended up with FIVE standards in the standard. Oh well. Toolmaker/integration giant Applied Materials (in America) pushes the C-seal, and Fujikin (overseas) likes the W-seal.


Why is it called "C-seal?" Here's why:

(from Vacuum Physics and Technology by G.L. Weissler, Robert Warner Colson)


When properly tightened, the top and bottom of the "C" in the C-Seal will smash flat against the flat sealing face, and make a seal.

And here is a W-seal cross-section:

(from www.technofittings.com)

Here is Fujikin's description of how the W-Seal works:



Now you understand. But you really came here for pictures. Here you go:

Single-port C-Seal fittings on the bottom of an MFC:


Double-port C-Seal fitting on a CKD shutoff valve:


Triple-ported C-Seal fitting on a different CKD valve:


Modular C-Seal fitting blocks from an Applied Materials gas stick:


W-sealed stuff is harder for me to find where I work, so here is an image from www.tksct.com:


The take-away from this exercise is that a C-seal has flat glands and a W-seal has a ring poking out in the middle. I have seen Mykrolis MFCs with W-Seal glands with complex geometry like that in the drawings above, but apparently some manufacturers call the round-ring-in-the-middle 'close enough'.

********

Those are the sealing surfaces. Here is a seal. It's technically a MicroSeal, which is better than a standard C-Seal but they look similar


The main reason I prefer VCR is that a Swagelok Ni-VCR-4 will set you back about a dollar for a silver-plated nickel seal. If you want a reusable gasket, that'll be $5. The C-Seal and W-Seal will run to middle double-digits or better, and most of them can't be reused.

In the semiconductor industry, there are several types of fittings used for the gas delivery systems ("gas sticks") on-tool. Recently I had occasion to list for sale some flow controllers with downmount fittings. It had been a while since I was in the industry, so I wanted to do a quick check online, to be sure of what C Seal and W Seal fittings looked like. I Couldn't find one. Here you go.