CO2 supplimentation

I have no idea what I am doing here but from what I have summed up this is what I came up with. I hava tiny grow room that will support about 6 plants(my veggie room). I run the CO2 tank for 15 minutes at like 10 PSI. I keep the exhaust off during this time for about 2 hours and have it set for about 1 hour right before the next cycle. THe exhaust runs all night with no CO2. I know the correct PPM but don't know how to achieve it without a CO2 guage with I would never spend that kind of money on.

You should not post the same thing in different forums. I answered this already in the Introduce Yourself forum.

My bad, First time here or in any forum for that matter

So I got the tank for free, the regulator for under 100$. Very cheap setup. Now I know the plants will benefit from extra CO2 as ther is 300-500 PPM in the air and they could use 1500PPM. It's already setup and running. I'm simply asking for suggestions on how not to kill my plants.