I was working with full regulator and about 40 per cent cut-off and we were going quite hard when André called my attention to the pyrometer. This was showing a comparatively low temperature (about 250 deg C.) which meant that unless I eased up, the boiler might start to prime. So we eased her a shade, let the water come down to a third of a glass and then we were away again in perfect order with the pyrometer showing about 375 deg C.
Source: wiktionary