This paper identifies and investigates potential sources of measurement error using a prototype of a device for measuring tongue-palate distance, contact and pressure across the whole of the hard palate. The Optopalatograph (OPG) is similar in principle to the Glossometer and similar in configuration to the Electropalatograph. It uses optical fibres to relay light to and from the palate and distance sensing is achieved by measuring the amount of light reflected from the surface of the tongue. A high power halogen light source is currently used to compensate for light attenuation and losses in the system. This source is not readily switchable and we evaluate the error in the measured light intensity when all the sources are on simultaneously. We conclude that the halogen-based OPG is a practical device with a worst case error of 10% in estimated distance values below 5mm.