Emerson MCU200 installation instructions High level alarm, Hisens sensor, Special site conditions

3.2.3. High level alarm, Hisens sensor

The Hi-Sens sensor oscillates when the sensor is dry, so producing a fail safe high level alarm. Normally such systems have the relay de-energising for alarm. This example has time delay of 30 seconds to Normal to allow the operator to identify any tank giving an occasional alarm.

Note that all Hisens sensors are 1 MHz and that cable check circuit is out - firstly because it does not function with Hisens but secondly that a cable fault with Hisens will produce an alarm signal anyway.

Gain adjustment:

(i)With sensor dry, reduce time delays to 2 seconds for simpler adjustment. Set gain switch to “LO” and reduce pot to “MIN”. Red LED will illuminate - this is the “false wet”. Rotate the gain potentiometer clockwise slowly until the green LED illuminates: note the setting (X).

(ii)Increase the gain setting X + 3 if it is necessary to switch to the “HI” gain range, re-check for a “false wet” position on the “HI” range, or assume an overlap between “LO” and “HI” of 2 divisions of gain.

(iii)Check that the sensor when immersed in water gives an alarm output. Recheck in the liquid to be monitored.

(iv)Reset the time delays as needed.

Special site conditions:

In the above application the time delay can be used to prevent high level alarms caused by splashing of the sensor, by setting the green switch to “Delay to NE”. Avoiding severe splashing or condensation effects may require a slight increase in the gain setting to X + 4. To detect splashing or for use on light oils the gain can be reduced to X + 2.

Fig. 11

3.3 Pump control and latching alarms

The output relay of the MCU200 can be latched into the de- energised state. This latch is achieved by short circuiting the two terminals of the Auxiliary input, labelled as terminals 3 & 4 on the pc board (see figure 5). The relay will remain de- energised while the latching short circuit is applied. Only after this circuit is broken can the relay re-energise under the control of the sensor.

A latching alarm can therefore be achieved by connecting one pole of the output relay into the auxiliary input, through a push to break “reset” button. NC1 and C1 should be used to create this latch, with NC2, C2 and NO2 being used for the external alarm circuit.

For a pump control application, switching a pump to control a liquid between two sensors in a tank, this auxiliary input can be used to monitor the second sensor. The control system must be designed as follows:

(i)The pump to be latched “on” must be driven by the MCU200 output relay.

(ii)The sensor attached directly to the MCU200 must initiate the pump action to be latched.

(iii)The latched pump action occurs when the MCU200 relay is de-energised.

(iv)The separate sensor must be used to detect the liquid presence at the switch off point.

(v)The signal from the separate sensor to switch off the pump must be an open circuit, and is connected to the auxiliary input in the MCU200.

Fig. 12 Tank filling pump control

In this example the number 6 switch (blue) on the MCU200 would be set to OSC = E, so that when the lower gap sensor sees air it de-energises the relay to bring on the pump. With switch number 4 (yellow) set to E = GREEN, the red LED illuminates when the pump is running. With “Delay to NE” set at 2 seconds, the sensor will ignore occasional bubbles or surface turbulence which could trigger a pump start.

The high level sensor, illustrated as a Mobrey MCU200 plus sensor, could alternatively be a Mobrey 005, or another MCU200 or a float operated level switch: all will give a volt free contact output suitable for the auxiliary input on the lower MCU200.