The bellows on the 150 units are sensitive to pressure. At higher pressures the bellows is stiffer requiring more force to move it. At lower pressures the bellows is more pliable (less stiff) requiring less force to move it. Consequently, the on/off points tend to narrow at lower pressures. (Less distance between on and off)

This chart illustrates the narrowing of the on/off points as pressure decreases. Because of factory tolerances (+/- 1/8") there can be cases in the field where the burner off and pump on points are the same or actually cross.

Early versions of the 150 mercury bulb units were able to be adjusted. These units had knurled adjustment screws that could be used to raise, lower or widen the setpoints. Although the available adjustment was small (usually 1/16’ to 1/8" total), it was enough to compensate in the field for lower pressure systems.

Later versions of the 150 mercury bulb units and all snap switch units are not adjustable in the field. The ‘MD’ models were created to provide a 150 control with factory settings to compensate for the narrowing of setpoints for new and existing installations.