Measured and theoretical values for the line ratio (I_17.05 + I_17.10)/I_15.01 as a function of temperature. The lines indicate the values predicted by the currently available spectral codes: AtomDB v1.3.1 (grey), SPEX (black) updated with Doron & Behar (2002), and AtomDB v2.0.1 (dashed). The black triangle indicates the value found through laboratory measurements (Beiersdorfer et al. 2004). The grey triangles show measurements of other ellipticals performed by Werner et al. (2009) and Werner et al. (2006) (M87). For M87, the temperature determining the line ratio is assumed to be the coolest temperature in the two-temperature model. The black dots show our measurements for NGC 5044 and NGC 5813.