Our Friend the pressurestat.
Purpose: Pressure operated relay.
In order to be useful and safe, the steam boiler needs a mechanism to regulate the steam pressure. The most common way of setting the steam boiler pressure is with a pressurestat. A pressurestat is a mechanical devise that turns a switch, or set of switches, on and off as the unit reaches a given pressure. Think of a switch that is connected to a balloon that is inflated by the steam pressure. When the device is cold, the balloon is not inflated, and the switch is held “on” by a spring. As the pressure gets higher, the balloon inflates until it presses the switch and turns the switch “off”. If that switch is connected to the steam boiler heater element, then the element will have power when the machine has just been turned on. When enough steam has been generated, the balloon turns the switch off and no more steam is created. When the steam is used, or the steam cools down, the spring pulls the switch back on and the cycle continues.
In practice, the balloon is a metal diaphragm, and the switch may be a single switch that activates a larger switch called a contactor (see contactor), or it may have a set of heavy switches activated by a larger diaphragm. Either way will work much the same, and some machines have changed from one.
Next newsletter we will review how this piece work with the contactor, how to test and how it fails
Next style to the other between models with little or no change to the structure of the machine.