How accurate are polygraphs/lie detectors?

I have a friend who was talking about lie detector tests or polygraph machines? What’s your opinion or thoughts on giving someone a polygraph test? My friend said that the secret to “cheating” or “beating” a lie detector test is to have complete control of your emotions. Do not react anyway to any of the questions, positively or negatively? He said no movements, sharp breathing, twitching. Or No High blood pressure, or increase or decrease in heart rate spikes? What’s your thoughts?

If you were given a polygraph test, how accurate would you think your results would be? 70%? 75%? 80%?? Or more than that?

 

They are not able to be used in courts of law, so I suppose they reflect nervousness in addition to supposed guilt. Which would mean it would be dangerous to a defendant that was nervous instead of being guilty.