There are several different methods used in the design of electric tension heads to achieve and maintain the set tension. One of these designs uses a spring and several lbs. of tension has to be lost once the set tension is reached for the machine to begin pulling again(this is what an electric constant pull machine does). It looks like this machine uses a spring(there is a knob on the right side at the bottom of the tension head, which I understand is a tell tale sign of such a machine). The bottom line is that this design is not nearly as accurate as other electric machines that use a load cell(s) in their design. You might want to check with the dealer to get more information on how the constant pull mechanism on this machine works.
