181-122018_THCD-101 Instruction Manual
Page 11 of 26
3. Operation
Features
This section describes the principles used in the THCD-100. It details the various settings for the inputs and the
outputs including setpoint control. It does not detail any servicing or calibration procedures.
3.1.1. Analog Input
The analog input signal sent by the process sensor and received by the THCD-101 is scaled via front panel
controls to indicate a process value on the front panel in user selectable units. The user defines what the full-
scale process value is and what analog signal would be received from the sensor to indicate this full-scale
process value. The full-scale analog signal input value is configurable via front panel menu for any value from
0 to 10 Vdc (20 mA for 4 – 20 mA units). The THCD-101 will show the over-range error (“RANGE”) when the
analog input is more than 15% over the full-scale analog value.
The secondary input is used to accept an external command setpoint and has a non-configurable full scale of
10V. This input is used when the setpoint is set to the slave source.
3.1.2. Setpoint Control
The THCD-101 has an analog output that is used for command setpoint to control a mass flow controller or
similar device. The setpoint command output has a voltage full scale that matches the input channel
configuration.
Example - If the input channel is setup for a 100 slm, 5V full-scale device, and the setpoint value is 10.0 slm
(assuming setpoint is in Auto mode and not a slave source), then the output voltage of the setpoint would be
0.5V.
The setpoint does have some configuration parameters that can be used to alter the function of the setpoint
control and they are detailed as follows:
(a) Setpoint Mode - The setpoint can be configured in one of three modes – Auto, Open and Closed.
In Auto mode the setpoint output is dependent on the setpoint source and value settings (see below).
In Open mode the setpoint outputs a voltage greater than the full scale of the device. For a setpoint
command full scale of 5V or less, the output in this mode is 7V. For any other setpoint command full scale
(e.g. 10V), the output in this mode is 12V.
In Closed mode the setpoint outputs a voltage less than the minimum output voltage of most devices, the
setpoint output voltage is –0.25V.
(b) Setpoint Source - The setpoint source dictates where the setpoint value comes from, assuming the mode is
set to Auto (see above). This can be one of two possibilities – Internal or Slave.
Internal source
–
the setpoint uses the value set internally via the
‘▲’
&
‘▼’
switches or via the
externals comms.
Slave source – in this case the setpoint uses a percentage of an externally produced value. The percentage
is set via the
‘▲’
&
‘▼’
switches and the external value comes from the secondary input channel (as
mentioned above)
3.1.3. User Rezero
Over time, it is possible that the input will ‘drift’ slightly due to various conditions (temperature changes,
etc.). As such, it may be necessary for the channel to be re-zeroed by the user.
The user re-zero function is provided for this task. It simply takes the current reading (sampled and averaged
over 3 seconds) and uses that as an additional offset for the channel in question, subtracting the value from all
