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Deadband is also known as hysteresis and is the amount of a measured variable (pressure, temperature, etc.) between the point where a switch closes and then re-opens. It can be implemented is software if you have an analog input (i.e., open at 1.02 PSI, close at 0.98 PSI) or designed into a switching mechanism. Often the deadband of a switch is fixed and cannot be adjusted. A typical example is in a wall thermostat in your house.
To find lost valves and buried wires, the Model 521a Valve and Wire Locator operates on the null principle. When the receiver is immediately to the left or right of a buried wire, it emits a strong beeping signal. Directly over the wire, the signal falls off. This is the null, or absence of signal.