Managed to borrow one exactly like the red one pictured in maa2003's post. Not having any absolute reference to test against I lined up 6 rackets of different tensions according to their tones by ear from lowest to highest and then measured with the tension meter. The readings of the device agreed with the ordering by tones. Pairs of rackets that were closer in tones than other rackets also had a closer readings off the device. As for calibration, two of the rackets were recently strung at 23lbs (same model racket, same string) for these the meter showed 21 and 22 lbs - different but also the sounds from the strings were different. Anyway after paying attention to the different tensions on hand i played with the different rackets and really felt that the difference in shots. Anything that measured under 20lbs on the meter was too below par to continue with. I never really payed much attention to tension before, just took it as a given. When playing I just rotated my rackets to even out the wear and only restrung when the strings broke. After this I felt confident enough to cut the racket with the lowest tension and sent it for restringing. So I am quite pleased with the device and think it is worthwhile to have in my kit - at least to gauge tensions when there is no reference racket to compare an new/unknown racket with and to decide when to restring.