Armfield Instruction Manual
132
Procedure
SAFETY NOTE: During this exercise the apparatus may reach temperatures hot
enough to cause burns or scalds. Do not touch the process vessel during the
exercise, or remove the coil or sensors from within the vessel. Allow the apparatus to
cool before draining the water or disconnecting any of the tubes from the process
vessel.
Setting the On/Off Controller
In the PCT40 software, select ‘Control’ to open the PID controller window.
Set the Proportional Band (P) to 0%, the Integral Time (I) to 0s and the Derivative
Time (D) to 0s. The controller is now set as a simple On/Off controller with fixed
hysteresis of 0.5% of full scale.
Set the temperature at which the on/off controller will operate. A recommended
starting value is 30 degrees Celsius. Check that the temperature reading T1 is a
sensible value (usually between 5 and 30 degrees Celsius depending on mains water
temperature). A higher Set Point temperature will be required if the mains water inlet
supply temperature is close to 30°C: a value of at least 10 degrees higher than the
inlet supply is recommended.
Apply the settings and close the controller window.
From the Sample menu at the top of the screen, select Configure… and set data
logging to automatic at intervals of thirty seconds.
Select the
icon to begin data logging.
Note the behaviour of the heating element as the temperature approaches the set
point. It is possible to observe heated water rising from the heating element when the
power is on. The heater also responds audibly when power is switched on or off.
Additionally, the switch for the heater is displayed on the mimic diagram screen
within the software, allowing observation of the heater action.
The change in temperature may be most easily observed using the graph screen in
the software. If the graph does not appear automatically when the graph screen is
selected, select ‘T1’ for the Y axis and ‘Time’ for the x-axis, then plot the graph. New
points will be added as they are logged.
Continue logging for 10 minutes. Select the
icon to finish data logging.
Disturbances
Within the software it is possible to introduce disturbances in the temperature inside
the process vessel by changing the flowrate of water through the vessel:
In the software, select the
icon to create a new results sheet.
Select the
icon to begin data logging.
In the software, increase the PSV setting by 20%. Open the drain valve at the base
of the process vessel enough to balance the flow in and out of the vessel, so that the
fluid level in the vessel remains constant.
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