We use our King portable testers on a daily basis in 100 degree heat and freezing cold temperatures at many different angles, even upside down, and the quality and reliability is unmatched.
I find it amazing that a device originally designed and patented in 1936 is still the mainstay of hardness testing for most shops. Not many products that are in existence in this day and age like that. A true testimony of what a well-thought-out product it is.
