I was doing so well eating this way. I was about four months in, fat adapted, and feeling GREAT while slowly but steadily losing weight. Every couple of days I would blow into my keto breathometer just to make sure I was still in range (since after a while I start eating lazy keto rather than tracking and checking every bite that goes into my mouth). I was always still in range. I would introduce a little more carbs and still be in range. I got curious and experimented with slowly adding more carby foods in each day and was STILL in range. I was ecstatic - I figured I must be one of those lucky people who can eat 100-150 grams of carbs a day and not get kicked out of ketosis.
Then I noticed I stopped losing and started gaining. Yet the meter would be telling me I was highly into ketosis. I couldn’t figure out what was wrong so on a whim I grabbed a second breathometer and BINGO - I was NOT in ketosis. It was the same meter from the same company (I had purchased two) but one of them was way off from the other. Do these things have a shelf life? Why would it go bad after 6 months? Yes, it was 99% my fault for adding some carbs back in, but it’s at least 1% the meter’s fault for providing false information.
So now I know urine strips and breathometers aren’t reliable. I have two blood sugar meters but I don’t think there are ketone strips available for the two I have. I’m willing to buy yet another meter that DOES let me test for ketones. Best one out there, please?
Sorry for the long post, but I warned you I was ranting. 
Sue