Exercise N
113
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.
Type in the Set Point value at which the on/off controller will operate. A
recommended starting value is 30 degrees Celsius. 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 (30) seconds. Check that the reading L1 is
varying with the rising water levels.
Check that the temperature reading T1 is a sensible value (usually between 5 and 30
degrees Celsius depending on mains water temperature).
Select the
icon to begin data logging.
Note the behaviour of the heating element as the temperature approaches the
temperature 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 30 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 passing cold water through the coil inside the vessel:
Create a new results sheet by selecting the
icon in the tool bar of the software.
Select the
icon to begin data logging.
Log for two minutes (with the software taking readings every 10 seconds), then set
the PSV to 50%.
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