HERA Transverse Polarimeter absolute scale and error by rise-time calibration

We give the results of an analysis of some 18 rise-time calibrations which are based on data collected in 1996/97. Such measurements are used to determine the absolute polarization scale of the transverse electron beam polarimeter (TPOL) at HERA. The results of the 1996/97 calibrations are found to be in good agreement with earlier calibrations of the TPOL performed in 1994 with errors of 1.2% and 1.1%. Based on these calibrations and a comparison with measurements from the longitudinal polarimeter (LPOL) at HERA carried out over a two-months period in 2000, we obtain a mean LPOL/TPOL ratio of 1.018. Both polarimeters are found to agree with each other within their overall errors of about 2% each.