It's good to make sure your gauges are reading correctly. Here's how! 1. Get 2 heavy duty zip lock baggies. 2. table salt 3. small container for salt( 2 liter soda bottle top for example) Put the salt in the container. Put a few drops of water onto the salt, just to moisten the salt. Then place the container inside the baggie. Now place the gauge in the baggie. Make sure you have this in a spot somewhere that no small kids can get to or that it can be knocked over. Wait between 4-6 hours. When you check it after that amount of time, look in the bag without opening it.Your gauge should be reading 75%. If it IS NOT, then thats how much your gauge is off by. To fix this you can get a small screwdriver and turn the screw on the gauge. If it's off by 7%(68%) then turn it so it reads 7% higher.