Switch no. 1
Switch no. 2
Switch no. 3
(iv) Relay output and LED logic selection
The bank of six slide switches towards the top of the pcb
sets the relay output state logic relative to the sensor
state, associated time delays and the LEDs. These are
slide switches, best adjusted with a pencil, and the ex
factory wetting is with all switches to the right.
Each switch is numbered as shown in gure 7, and the
pc board labels give brief function information.
Fig 7 * Note that these times are approximate only
Set the switches in the following order, starting at the
bottom and working upwards.
Switch 6: If the MCU200 relay is to be energised (E)
when the sensor is oscillating (OSC) then set the No 6
blue switch to the right (OSC=E). This is the preferred
setting, to give a de-energised relay in the ALARM state
for a gap sensor as a low level alarm or for Hi-Sens as
a high level alarm. The opposite setting might be used
for a sludge blanket detector, when an oscillating sensor
(OSC), which occurs in clear liquids, might preferably
cause the relay to de-energise (OSC=NE)
Switch 5: This selects the relay change which is subject
to the time delay selected on the top switches. When
the No.5 green switch is set to the right, the delay
occurs between the sensor changing state and the relay
de-energising or becoming “not energised” (NE). This
time delay is a minimum of 0.5 seconds, (achieved by
switching the top BROWN switch to the right) and is
used to prevent relay chatter at the changeover point.
Longer time delays are selected on the top three slide
switches as follows:
The relay change in the opposite direction is immediate
(within 50 milliseconds).
Switch 4: Only one of the GREEN or RED LEDs will be
illuminated at any one time. These LEDs show the state
of the MCU200 output relay. The RED LED is labelled
“ALARM” and the GREEN LED is labelled “NORMAL”.
The Switch 4 slide switch (Number 4) determines which
LED will be illuminated when the relay is energised (E).
It is usual to have the GREEN/NORMAL condition occur
with the relay energised, ie with switch Number 4 to the
right (E=GREEN).
3. Applications
3.1 Gain adjustment
Correct adjustment of the gain (HI/LO switch and
potentiometer) is essential for proper operation of any
ultrasonic sensor system. This adjusts the gain of the
feedback amplier in the control unit, which produces
oscillation of the sensor when the coupling between the
ultrasonic crystals is sufcient. Therefore the higher the
gain setting, the lower the coupling needed to produce
an oscillating sensor.
The universal control unit of the MCU200 operates
with many sensors, so the correct setting for the
particular sensor and application should be found on
site by experiment, if possible. This will take account
of particular site conditions like RF coupling between
extension cables, which can affect the maximum allowed
gain.
Other liquid characteristics, such as presence of
suspended solids, or air bubbles, can mean that for
reliable operation the MCU200 gain must be set as high
as possible, to overcome future solids build up, but at
least one potentiometer division below the maximum
allowed level, to ensure temperature and component
ageing stability. With Hisens sensors, condensation
on the sensor may be overcome by increasing the gain
as high as possible. With sludge blanket sensors, the
gain adjustment changes the density of sludge at which
the system will switch, increased gain giving increased
solids levels.
The particular procedures outlined below for gain
adjustments give the mid point gain settings, which may
need to be adjusted to meet specic site/sensor future
requirements as indicated above.
3.2 Level Alarm
3.2.1. Low level alarm, gap type sensor
The normal gap sensor application. Relay de-energises
for alarm immediately (after 50 milliseconds). Most
sensors of this type operate at 1 MHz.
(i) Check that sensor is “dry”, cables are connected
correctly and “FAULT” LED is not illuminated. Put
gain switch to “HI” and rotate the gain potentiometer
to “MAX”. In most cases the green LED will illuminate,
this is known as the “false wet”. Rotate the gain
potentiometer until this LED extinguishes. Note the
setting (X).
(ii) Reduce the gain potentiometer by 4 divisions from X,
to X-4. If necessary switch to “LO” gain. If no “false wet”
was possible set gain to “6” on the “HI” gain range.
(iii) Check that the green LED illuminates when the
sensor gap is lled with the liquid to be monitored.
3.2.2. High level alarm, gap type sensor
This typical application has relay de-energising for alarm.
Cable check here is important to provide a sensor check
2 seconds* 8 seconds* 30 seconds*
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