Telescope [ACT, @Kosowsky:2003] and the Arcminute Microkelvin Imager [AMI, @Zwart:2009]. These surveys will make use of the redshift independence of the SZ surface brightness to map the distribution and evolution of large scale structure throughout the Universe.

In order to interpret the data from SZ surveys we require well-calibrated scaling relations between the SZ signal and physical properties and a good physical understanding of the intrinsic scatter therein. Pointed SZ observations with interferometer and bolometer arrays coupled with X-ray imaging and spectra allow us to probe the intra-cluster medium, while weak and strong gravitational lensing observations directly measure the projected gravitational potential and hence total mass of the cluster. Recently, there has been concerted effort in developing physically motivated analytic models that account for the observed non-isothermality in cluster gas \[see e.g. @Komatsu:2001 [@Ostriker:2005; @Atrio-Barandela:2008; @Bulbul:2010] (polytropic equation of state), @Pointecouteau:2004 [@Vikhlinin:2006; @Mahdavi:2007] (component separation and modified $\beta$ models), and @Nagai:2007 [@Mroczkowski:2009; @Arnaud:2010] (explicit pressure parametrization)\].

We present a model in this work that avoids using an explicit temperature and electron density parametrization that may lead to non-physical cluster properties on large scales. We instead choose to base our ICM model on a simple parametrization of the cluster entropy, consistent 