The International Open Source Hydrogen On Demand Builders Network.
Take a large plastic bottle with a screw on lid such as an empty milk gallon. With a metric measuring cup add 250ml (mililiters) of water and screw the lid on. Invert the bottle and with a magic marker make a horizontal line at the top of the water and write next to the line "250ml". Add another 250ml, and with the lid on invert the bottle again and write "500ml". Add another 250ml and write "750ml". Add another 250ml and write "1l" (one liter). Keep adding 250ml at a time until the milk jug is completely full of water.
Next fill a bucket, tub, or other fairly large vessel with water. Take the milk jug with the lid on and turn it upside down and put it in the large vessel of water so that the lid is underwater. Remove the lid and put the output hose of your electrolizer inside the milk jug and start timing as the bottle fills with hydroxy gas. This should give you a reading of liters per minute as the gas displaces the water in the milk jug.