air check valves

which way do these air check valves go?

the way the arrow tells you to put on. lol haha

what if there is no arrow? not all of them have arrows.

blow in it and see which way the air flows through and if it goes both ways throw it in the trash

http://www.petsmart.com/product/zoom/index.jsp?productId=2752253

in this picture the cone inside the check valve goes from big on the left to small on the right. the airline that connects to the airpump should be attached to the bigger/left side. and the line that goes into the tank/airstone/whatever should attach to the smaller right side.

I like the idea that if you blow through it.... and air doesnt flow from either direction then it is probably trash.

Thanks I was wondering about that
So I will embellish on your question.
What are check valves used for?

they are used for preventing water from escaping the tank via the air hose. sometimes when power goes out or if the air pump is unplugged or something, water can rise up the tube much higher than the tanks water level. that can result in a siphoning of the whole tank. so the check valves will only allow flow in one direction, that way water cannot get out via the air hose. really... check valves are great investments.... they are real cheap (unless you get the co2 resistant type) and give you peace of mind.

hmmm... good comeback