SWG w/ Hi TDS?

[Poolsean]

Swing check valves are also called gravity check valves simply because there's a weighted flapper that relies on gravity to keep the check valve closed. This is mostly used for backflow prevention with low backpressures...again, this is gravity.
A spring check valve is mostly used when a more positive seal is necessary, or in instances where a controlled flow on a bypass line is needed, such as with the Pool Pilot manifold assembly.
Spring check valves are rated at different lbs of backpressure. Check valves for spa blowers are usually 1/4lb. For chemical backflow prevention are usually 2 lbs. For hydraulic flow control is usually 5 lbs or more.

What application are you using the check valve for?

[chem geek]

Since you are talking about an SWG pool, then you'll also want to measure the salt level which can be done with the Taylor K-1766 or with an Aquacheck test strip (one of the few tests that actually works well with test strips). This will let you know how much additional salt you need to add to get to the 3000 ppm or whatever recommended level for the SWG.