The highest capacity scale you will need is for the pinch roller tension. For that the 2000 gram model should probably do for most machines like the TSR-8, 38, etc. But check your service manual for other machines, as this will vary. Reel tension measurement is a different story because we’re measuring torque and we want finer gradations on the scale. The 40-gram increments on the 2000-gram model aren’t quite as accurate as I would like, so having at least two scales is recommended. Plus the closer the scale capacity to your max measurement requirements, the more accurate it will be.
Pinch roller tension is just a straight pressure measurement, but reel tension is a calculation… gram-centimeters (g-cm). We calculate this by multiplying the spring scale reading by the radius of the reel hub in centimeters. Radius is measured from the center of the hub to the outer rim of the hub (the hub not the reel flange). This is about 5.6cm for a NAB hub. What this means is that your spring scale for this measurement can be a smaller capacity with finer gradations for more precise readings. To measure 1100 g-cm for the TASCAM 38 back tension we only need a 250-gram spring scale… 1100 divided by 5.6 = 196.4.
So to put this in practice for measuring back tension on the TASCAM 38 in PLAY mode we wrap a string a few times around the supply hub, enough so it won’t slip. Next tie a loop with the free end of the string for the scale hook and attach the spring scale. Hold the right tension arm up with a rubber band or something and press PLAY. The scale should read between 160 and 196 grams. Now multiply that by your hub radius (5.6cm) and you have your torque figure, which should fall between 900 and 1100 g-cm per TASCAM Spec. Brake tension measurements will be higher, but a 500 g scale will have you covered for the TSR-8 and 38.
Of the scales in the link below, I would get the model 10-G6 for brake and reel torque, and the model 10-G3 for pinch roller pressures.
http://www.sciencelab.com/page/S/PVAR/10535/10-G4
Other brands and models of tape decks use different methods, like taking measurements with a full reel, or both full and empty. So pay attention to the service guide. Some require a special $500.00 gauge for tape tension.
Hope that helps…
