Help with Co2 gauge problems...

My dad went out and bought a new CO2 regulator for my kegerator and I didn't look at it closely before using it.  He bought it from a hardware store that stocks welding supplies, but didn't mention that to me until now. 

I just pressurized a new batch of beer and after thinking I was putting 15psi into it, I realized it wasn't psi. 

The gauge reads 0-40 litre/min.  I set it at 15 litre/min.  Can anyone tell me whether this can be converted to psi?  And if not...is there any way of knowing what I carbonated my beer to?  At this point, I'm taking a "wait and see" stance until we drink it tomorrow, but I'd like to know whether I need to buy a new gauge or not. 

Thanks everyone. 

It can be converted, but it is beyond me. I googgled for a converter but did not find anything usefull at a quick glance.

Cheers
Preston