The lvdt versus wavelength relationship has proven to vary with time, not in a continuous manner. The cause of this grating behaviour is not yet understood and is under study. The variations are largest for the extreme values of lvdt (around 1000 or lower and around 3000 and higher; i.e. close to the overlap regions of the detectors), where in the worse case they have reached 50 but there is no variation for the mid-values of lvdt, around 2000 (i.e. close to the center of the detector range).
In particular there has been a sudden change in the grating behaviour between rev 342 and rev 349, followed by a relatively stable state (as up to rev 524). This has the consequence that the present calibration, which has been derived with observations taken before rev 342, is not optimum for present observations and can yield to errors of up to half a resolution element for the extreme parts of the detectors.
The grating behaviour is monitored on a weekly basis to be prepared to deliver a time-dependant calibration in post-mission times.