All alarms are bi-stable. This means that when the flow rate drops below the low alarm switch setting (or goes above the high), the alarm switch contact closes (or opens) and remains in the position until the flow rate stays in the alarm region. If the flow rate subsequently increases to the normal flow rate, the switch will then go back to the normal position as the rotameter float goes past the switch into the normal flow region.

The use of two switches can then define an acceptable band of flow rates with an electrical signal generated whenever the float is outside the acceptable band. All alarms are supplied with relays allowing wiring for:

• “Normally closed” (i.e., power on during normal flow, perhaps for a green indicator light signifying flow is OK)

• “Normally open” (i.e., no power during normal flow and power is switched on when flow drops below the low alarm or goes above high alarm, perhaps for a red warning light signifying danger)