The gas pictured in the first post is C25 as I'm sure the OP knows. That is ideal for 99% of automotive purposes. The left gauge is flow rate measured in CF/PHR. Set your rate with the trigger depressed as previously stated. The right gauge is tank pressure.
20 cf/PHR is a good starting point. If all is going well you can dial it back till you see porosity. Going too high will lead to problems too. A high flow rate can create turbulence that introduces atmosphere into the pool. 15-30 will cover almost any situation.
As also stated, check your polarity.