
No more guessing or calculating the tension, simply MEASURE it with a dynamometer  the lineScale 2 (399 € including worldwide shipping)
lineScale is a bluetooth enabled electronic slackline dyno (tensile force meter), which we developed specially for slackline use. Its small, light, rainproof and most of all, really affordable (not to say "cheap" ;)
Key specs: 30kN working load, 80kN MBS, about the size of a hand, 22mm thin, only 600 light (incl. rechargeable battery) and of course rain proof / watertight (IP65). The bluetooth 4 interface connects it to your smartphone and the included smartphone app (iOS & Android) lets you control the lineScale and log the load values with a precision of 40Hz (one reading every 25ms, thats 40 readings per second).

Force Calculations / Tension Calculations
/ Sag Calculations
First we look at the practical aspects!
Numerous tests in practice have shown that the following formulas compute the actual values to a large degree of accuracy. Measurements of many longlines set up with the measuring equipment resulted in only very slight deviations from the calculated results. Therefore, you can rely on these formulas for the most part, but you should also keep in mind that deviations and errors are possible.
All data and formulas are given without guarantee or claim of completeness!
Formula to calculate the current tension in a rigged slackline:
Three factors will give you the basis to calculate the appoximate tension on the slackline (the formula is valid for every slackline!). First, you need to know the exact length of the slackline (the distance between the anchor points/trees) and also the sag in the middle when under a load of a certain weight (a slackliner, for example).
(Weight
in kg : Sag in m) x Length in m 

= approx. tension
in daN (kg) 

4 
For example a 100m long slackline, which has 1.2m sag in the middle under the weight of a 75kg Slackliner.
((75 : 1,2) x 100) : 4 = 6250 : 4 = approx.. 1560daN (kg) Tension
This means that this 15kN (1,5t) force is also working on every anchor point/tree! This tension is also on the line regardless of whether a slackliner is walking. Agains the widespread misconception that the tension of the line more than doubles or even increases substantially, in reality it makes hardly any difference whatsoever with such a high pretension. The load does not increase by more than the actual weight of the slackliner!
It is, however, a bit different with shorter slacklines (Tricklines) with less tension and a dynamic load. When you jump on the slackline with, for example, 500daN pretension, you can eaisily more than double the tension on the line!
Formula for the computation of sag with a specific tension:
You can calculate the sag in a slackline from three factors (this formula is valid for all slacklines!). You need to know the exact length of the line (the distance between the anchor points/trees), the desired tension in daN(kg), and the weight of the slackliner who will be walking.
Weight in
kg 

= approx. sag
in the middle in meters 

(Tension in
daN (kg) : Length in m) x 4 
For example, a 100m long slackline with 15kN tension, that is loaded with a 75kg Slackliner in the middle:
75 : ((1500 : 100) x 4) = 75 : 60 = approx. 1,25m sag in the middle
