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The head amplifier transmits pulses when the sensor is not oscillating (its normal state), and stops
transmitting in the alarm condition when the sensor is oscillating. Thus the normal light on the control
unit indicates a wet Hi-Sens, or dry gap sensors. When the inverting amplifier is used the sensor is in
its quiescent state for most of the time. For this reason sensor cables can be monitored to check for
cable breaks with some sensors.
Note that when an inverse acting head amplifier is used the sensor is not oscillating under normal
conditions and therefore the integrity is not as high as that of a normally acting head amplifier, which
should be used wherever possible.
Inverse acting amplifiers are only available in adjacent mounting industrial housings.
Summary of sensor head amplifiers
Type
Sensor mounting
Adjacent mounting
Inverse acting
Industrial Case
***S*PI***
MEP*AI
MEP*XI
Marine
***S*PM***
-
Sensor State
Hi-Sens Dry
Lo-Sens Wet
Hi-Sens Wet
Lo-Sens Dry
Normally acting head
amplifier *PI*, *PM*, AI
Normal State
Pulses transmitted
Green light on c.u.
Alarm State
No Pulses transmitted
Red light on c.u.
Inverse acting head
amplifier XI
Alarm State
No Pulse transmitted
Red light on c.u.
Normal state
Pulses transmitted
Green light on c.u.
Output
1.1
1.2
Sensor cable check circuitry is built into the industrial adjacent mounted head amplifiers, where the
coaxial cable linking head amplifier and sensor may be exposed on site. This is only operable where
the sensor in use has Earth continuity between the two coaxial cables, see section 2.2 Lack of screen
continuity along these cables causes the head amplifier to signal a fault condition to the control room
readout unit. To use the facility a wire link in the head amplifier must be cut during installation.
(See section 2.2)
1.5 Control Units
The control unit is available in two forms, a free standing plastic case (MEP*L), or a rack mounting
unit (MEP*R). Operation is the same in either case.
Front panel indicators show normal condition (green), alarm (red), and fault (amber). A front panel
switch to test fault and alarm circuits is provided, together with a variable time delay for the alarm
circuit.
Relay contacts are provided to indicate an alarm condition. A second relay is supplied on rack
mounting control units to indicate a fault. On both units, a fault also causes an alarm to be
indicated.
ii) Inverse acting head amplifiers
When it is required to use a Hi-Sens sensor for low level detection, or a Gap Sensor for high level
detection, an inverse acting amplifier must be used. Examples of this are given below:
• Hi-Sens can be used to detect low levels in very light liquids or foams.
• A miniature Gap Sensor 366S, may be used to detect high level when there is insufficient room for
a Hi-Sens.
• High temperature, corrosive, or aerated liquids, where Gap Sensor can be used at high level if
required.