Boilers that provide hot water and steam for heating and processing of manufactured goods must be carefully monitored to ensure the presence of adequate water. The failure to maintain an adequate water supply in a boiler can be hazardous. If the water level in a boiler drops below a safe level the boiler can be damaged, and even more seriously it can explode. In view of this, the monitoring of the water level in boilers is a rather critical control function.
In a typical boiler there are two boiler water sensing devices. Normally one device monitors the level of the boiler water and controls the burner that fires the boiler. The other sensing device normally monitors the water level and controls a feedwater pump or valve to add water to the boiler when necessary. These two functions cannot normally be combined due to the spontaneous variations in boiler water level due to the heating of the water, which causes an erradic flow or change in the water level. If a typical single probe is used to control the burner and the feedwater source, the moment the sensing mechanism determines that more water is necessary, the burner is turned off. If the water at the sensing means or probe is quickly restored, the feedwater is turned off and the burner is reignited. This function can oscillate causing a very undesirable cycling of the entire system.
In prior art systems thermal types of delays have been used for control with one of the two normal sensing probes. The thermal type delays have a relatively long delay time in operation, and further have the undesirable function of requiring a recycled time due to the thermal mass of the delaying element. Due to the nature of a thermal delay and its cycling rate, the application of a thermal time delay device to the probes used in a boiler limit their application.