The work represents a next step in studies of longitudinal energy losses from linear magnetic plasma confinement devices. The paper describes the experimental technique that has been developed and used to measure the absolute values of longitudinal particle and energy density fluxes from an open magnetic mirror trap. Using this technique, it was possible to measure and account for the secondary electron emission coefficient from plasma-facing end plate, and perform a comparison with the existing theory of energy confinement in open traps. It is shown that the average energy transported by an ion-electron pair leaving the trap is close to theoretically predicted minimum value of 6-8 electron temperatures. As confirmed by the measured dependencies, this conclusion is valid in a broad range of parameters and the threshold value of the magnetic expansion ratio does not exceed its theoretically predicted minimum of √m//me.