Gamma do a tester http://www.ebay.co.uk/itm/Stringing...150980130306?pt=UK_Tennis&hash=item23271df602 which is cheaper at raquetdepot.com
This tension tester is for tennis racquets. I'm not sure why it states that it is for badminton, I haven't seen anything about Gamma making a tesion tester that works for badminton and everyone who tried this tester came to the conclusion that it doesn't work for badminton. Stringway makes one, but it is pretty damn expensive....
You could probably place a weight on the stringbed and use the deflection to calculate the strung tension. Not as easy as it sounds though
@yan v it says its for badminton in the title and description on the ebay listing and at raquetdepotuk. Is the reason its only for tennis because it only works at high tensions? You can also the sound or the strings make to measure the picture and compare against known values.
Yeah from what I remember, it doesn't work properly at badminton tensions. I'm not sure why both these listings say it's for badminton though :\ Maybe Gamma released one for badminton, but there is no information on that anywhere.