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3.4.3. Interface detection between two immiscible similar
liquids
When liquids are ultrasonically very similar - as happens for
example with paraffin and water - the procedure in section
3.4.2. produces very little difference between the two “false
dry” points. In this case the “reflection” method of interface
detection is used.
If an ultrasonic beam is transmitted from one liquid to another
at a suitable angle (10%) it is split at the interface into a
reflected and a refracted beam, so that it does not reach the
receiver crystal. If there is no interference in the gap, but
only one liquid, the beam is received and the sensor oscillates.
The gain adjustment is made so that the gain is 3 divisions
higher than the highest false dry position obtained, as in
section 3.4.2. Performance at the interface should then be
checked.
Note that the non oscillating state of the sensor, at the
interface, also occurs throughout any emulsion layer at the
interface, and also when the sensor is in air.
Fig 16 Interface detector by reflection method
a) For a gap sensor giving false dry indication, this could
possibly be due to aeration or solids in the liquid. This can
be overcome by increasing the gain slightly, to a maximum
of X-2. This increases the sensitivity and is appropriate for
high level alarms.
b) For a gap sensor giving false indication, this could be
due to cross talk between cables - check that all junctions
use coax connectors with the outer casings isolated.
Seperate the two coax cables for long cable runs. False wet
can also be caused by viscous liquids clinging to the
sensor: sensitivity can be decreased slightly by reducing the
gain to X-6 minimum - check for reliable operation in the
liquid.
c) For a Hisens sensor the gain adjustment is slightly more
critical, as quoted in 3.2.3. Sensitivity is increased by
reducing the gain to X+2. Decreasing sensitivity, sometimes
necessary to avoid condensation, is achieved by increasing
the gain to perhaps X+4.
The Hisens sensor can take 30 seconds or more to drain
off surface coatings and re-instate a dry signal in viscous
liquids.
d) Check for correct sensor operation whenever the gain is
adjusted away from the normal set point. Assume an
overlap of 2 divisions between the "LO" & "HI" gain ranges.
(iv) Check the incorrect operation has not been produced
by incorrect setting of the frequency selection switch or an
external short circuit on the auxiliary input terminals.
(v) The circuit board can be checked by linking the sensor
terminals 1 and 2 with a wire, to simulate an oscillating
sensor.
4. Maintenance
5.1 The following parts are suitable for replacements on
the MCU201.
Main pcb complete K2641
LED indicator pcb K2643
LED pcb spacers K2623
LED pcb connector K2624/50
Box assembly K2662
Gland assembly K746/K747/K748
There are no consumable items such as fuses.
5.2 Fault Finding
(i) At least one LED should be illuminated. If not check
the power supply to the unit.
(ii) If the “Fault” LED is on and the sensor is a standard
Mobrey 300 series or a 402/422/433, check the coax
cable to the sensor for incorrect wiring or damage.
Particularly check continuity of extension cables,
connection of crimped connectors on cable ends. For
other types of sensor switch the cable check circuit “OUT”
- (see section 2). The pcb board can be checked by
linking the two terminals labelled E on the sensor
terminals - this should cancel the fault indication LED.
(iii) If the sensor is giving incorrect indications check the
gain adjustment (see section 3).
5. Spares and fault finding
Safety maintenance: This is limited to periodic inspection
by a qualified person to ensure that the installation
including wiring and equipment housing is safe.
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International:
Emerson Process Management
Mobrey Measurement
158 Edinburgh Avenue,
Slough, Berks, SL1 4UE, UK
T +44 1753 756600
F +44 1753 823589
www.mobrey.com
The Emerson logo is a trade mark and service mark of Emerson Electric Co.
Mobrey is a registered trademark of Mobrey Ltd.
All other marks are the property of their respective owners.
We reserve the right to modify or improve the designs or specifications of product and services at any time without notice.
Americas:
Emerson Process Management
Rosemount Inc.
8200 Market Boulevard
Chanhassen, MN USA 55317
T (US) (800) 999-9307
T (International) (952) 906-8888
F (International) (952) 949-7001
Manual reference : IP227 Rev AA, July 2007