from *XMM-Newton* observations of the REXCESS cluster sample. Recently [@Bulbul:2010] have developed an analytical cluster model based on assuming a polytropic relationship between the gas density and temperature, and parameterise the total mass using a generalised NFW model. In order to account for cool-core behaviour the resulting radial expressions are modified by the same core taper used by [@Vikhlinin:2006].

The CBI2 array has relatively low spatial resolution, and a large field-of-view, and hence a simple parameterized model is required that is accurate over the bulk of the volume of the cluster. We could choose any function of gas density and temperature as the basis for our gas model, e.g. pressure ($\propto Tn_\rmn{e}$) or entropy ($\propto Tn_\rmn{e}^{-2/3}$). For example [@Nagai:2007] choose to use a four-parameter model for the cluster pressure profile. We choose to parameterize the ICM based on the entropy because there is evidence from both observations and simulations that it can be modelled as a simple power-law over most of the cluster volume (see Section\[section:entropy\_model\] below). We combine this with an NFW parametrization of the dark matter halo component. By modelling the gas in this way, the underlying physics of cluster gas in hydrostatic equilibrium is encoded into the 