MASTERCOOL 52220 1 Analog Thermometer Review

I recently added a can of Freon and wanted to see if it would help cool my car. I stuck a thermometer in the air conditioner hood. I average around 70-75 degrees. I added another jar to the system and it raised my temp to 40-42 degrees at the vent. The manometer was a good tool to check how much freon I needed to put in my system. I've been told that you can put too much into it and it will lock up your system.