thank you for your precious help. I had a look to the one you used, it's a manual kind.
I don't understand how it works, how do you do to know the weight of the traction you apply?
From what I have seen, there are two types of over-the-door traction devices:
The first type is very simple and uses a weight to create the force. They provide a plastic container which you fill with water to a certain level to create the desired weight.
The second type (the type I bought) uses a spring weighing scale (labelled "dynamometer" in your picture) and a ratchet wheel system (labeled "traction frame") to create the force. With this type, you have a string hanging down from the ratchet wheel, which you pull on, and each time you pull it, it turns the wheel through one notch on the ratchet wheel, which increases the force. There is also a second string hanging down which you can pull on, and this releases the ratchet wheel, so if you pull on this second string, all the force is released.
The only problem with this type of device is that the spring scale cannot be seen when you are using it, because the scale is located above your head. So although you can pull on the string to increase the force, you cannot see the weight reading on the scale. I had to use a webcam connected to my android tablet in order to view the scale above my head. So this is problem. But if you have someone assisting you, then they can read the scale.
Also on the model I bought, the weight units on the spring scale were not in pounds or kg, but in some weird Chinese weight unit, so I had to recalibrate the scale in kilograms. (I did this recalibration by hanging two 5 liter plastic containers filled with water to create a 10 kg force underneath the spring scale, and then marked the 10 kg point on the scale with a marker pen).
So I think the over-the-door traction device that uses a plastic container filled with water might be easier. The only disadvantage with the plastic container type is that they tend to go up to around 10 kg maximum force, whereas spring weighing scale device I bought goes up to 20 kg maximum force.
Also, note that the model I bought only has hook, it does not have a proper door attachment. So unless you have something on your ceiling you can hook onto, this device will not work for you.