The Johnson T-3100 proportional action, pneumatic thermostat is designed to maintain space temperature in accordance with the return air temperature in a terminal air conditioning unit. Direct or reverse acting models with liquid-filled bulb element are available with external set point adjustment. The thermostat mechanism is protected by a snap-on cover. The external set point adjusting dial marked “Cool-Warm” and a pointer are located on top of the cover. The pointer has tabs that permit restricting the temperature adjustment range to as little as ±2.5F° (±1.4C°). It is factory set for a nominal adjustment of ±7.5F° (±4C°), from a set point of 75F° (24C). Linear expansion and contraction of the liquid in the element permits uniform dial graduations and provides a high degree of accuracy over the entire range of operation.