Traditional analogue gauges require a small capillary mechanism where a small pipe expands and unravels under pressure. This in turn drives a mechanical cog to rotate the gauge needle. This age old technique works quite well especially in large gauges but as this mechanism is manufactured smaller and smaller a tradeoff between accuracy and size becomes the issue. With very small gauges such as a 27mm x 27mm Mini Gauge it's difficult to make them better than 10% accuracy due to the physical constraints of the mechanical components.