Frank, do you mean you've used the ones I linked to earlier?
http://www.andymark.com/product-p/am-2304.htm
If I understand things correctly, the problem is that everyone is rating the same basic concept but different parts of it and in different units. The valve I've linked to for example has M7 ports which are internally threaded with a major diameter of about 0.4" which accept 1/4" fittings by which it is really meant that it will accept a 1/4" OD tube and that tube has a ~0.16" ID which is compliant with the rules. If I understand it correctly, NPT refers to the OD of the tubing that can be accepted. The port will be larger and the ID will be smaller. Or something. Honestly, this all seems needlessly confusing to someone who doesn't already have an industrial understanding of the topic. How are high school students supposed to pick a valve correctly? It has been days and I still don't feel confident enough to pick a valve that I'm reasonably sure will be within the rules.