What is this?

[Wayne Edwards]

Can somebody tell me what this might be. I've never seen this before. The copper fitting in front of the thermal expansion valve.3150631507
Thanks

[Fred Weck]

If it was installed vertically it would be to prevent/lessen water hammer. Air trapped in the top of the tube acts as a cushion. As is, they may have put it in for some future appliance.

[Rick Cantrell]

Can somebody tell me what this might be. I've never seen this before. The copper fitting in front of the thermal expansion valve.3150631507
Thanks
It's a water hammer arrestor. BTW the air chamber is sealed so they work even when installed at any angle. Also, the device it's connected to is (most likely) a Pressure Relief Valve.
The need for a hammer arrestor is because the PRV can close suddenly, causing water hammer.
I can't say if it's connected properly.