The unit I received has a defective sensor. It's impossible to know unless you have another to compare. Relying on this piece of junk has caused me to damage my Lexus air conditioner and now I have to pay hundreds or thousands of dollars for repairs. AVOID THIS SIGN. Following the instructions down to the last detail, I pulled the trigger in short bursts and twisted the gas cylinder. Every time I released the trigger, the dial would jump to almost 50 and then back to 40. Since it was reading 40 before I hit the gas, this wasn't the result I was expecting. So, again following her instructions, I did this over and over again, just for a second or two each time. Each time the scale jumped up to 50 and then back down to 40. Until she jumped up to about 80 and stayed at that level. That's MORE than enough to break the seal on your system as it seems on mine. DON'T BELIEVE IT!