(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).
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.
4. Maintenance
Safety maintenance: This is limited to periodic inspection by a qualified person to ensure that the installation including wiring and equipment housing is safe.
5. Spares and fault finding
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
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.
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Mobrey Measurement | Rosemount Inc. |
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Manual reference : IP227 Rev AA, July 2007
