Check Valve Instructions
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Check valves are simple devices, simply connect in-line of your tubing to prevent water siphoning from your aquarium to your CO2 system. The only thing to ensure is you correctly identify the input and output.
Stainless Steel Check Valve
Pretty obvious in this case; simply ensure your CO2 flows in the direction of the arrow.
Plastic Check Valve
Looking at the valve, you'll see the red plastic is molded to one end of the unit - that is the input. The other non-molded end is the output.
To verify; you should be able to blow down what you think is the input and you should not be able to blow down the output.
Glass Check Valve
This unit uses gravity to operate, it must therefore be aligned in an upright/vertical position - as shown in the adjacent picture.
Looking at the valve, you will notice that one end is slightly more tapered (narrower) than the other. It may also have a faint line running about around the tapered section - this is the inlet and should be at the bottom.
To verify; while holding the valve in a vertical position, you should not be able to blow down the outlet. If you can, you've got it the wrong way around.