10 Water Level Control
Mk8 MM Manual Page | 181
10.6 2
nd
Low Probe
10.6.1 Overview
The 2
nd
low probe is a conductivity probe, and its purpose is to act as an additional 2
nd
low water cut-off
when the water falls too low in the boiler. The conductive technology with safe electronic control has been
granted a worldwide patent for its continuous electrical and mechanical self-checking software.
If the water level in the boiler falls below the end of the probe, then a 2
nd
low water level alarm will occur.
The water level may be low due to insufficient water in the feed water tank, feed water pump failure, feed
water line isolated and/or the level controls have failed. If there is not enough water in the boiler, the heated
tubes will be left exposed and unable to cool down as there is no longer water to transfer the heat to. If the
burner were to continue firing, the temperature of the tubes would be rapidly increase, reducing the metal
strength, and could cause a collapse or explosion. On the MM system the 2
nd
low water level alarm will shut
down the burner.
10.6.2 Operation
The water level detection probes use capacitance technology, whereas the 2
nd
low safety probe uses
conductive technology. Following basic electric circuit theory, when the probe is in the water in the boiler and
an electrical voltage is applied, the current will flow; when water level drops below the probe, no current will
flow. This is the basic principle of the 2
nd
low safety probe, if the water levels drop below the cut-length of the
probe, then a 2
nd
low water alarm will occur on the MM or for standalone mode, the volt-free connection will
open to indicate this alarm. When used with an MM, the 2
nd
low water alarm requires a manual reset.
10.6.3 Specifications
Integration with external safety devices and circuits
Probe connection– quick connect
Temperature rating of housing
Maximum Allowable Pressure
Maximum Allowable Temperature
*The 2
nd
low probe can be cut to length to suit application
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