7037/7038/7040/7041
User's Guide
50
Cooling Control
To control the refrigeration power with the digital interface the front panel
refrigeration switch must be off. The refrigeration power function is controlled with
the “F2" command. Setting the “F2" value to 0 turns the refrigeration off and
setting it to 1 turns it on. “F2" alone returns 0 or 1 showing the state of the
refrigeration power control.
The “F3" command controls the cooling temperature (expansion valve). This
valve adjusts the cooling temperature which sets the cooling capacity. Setting
”F3" to 1 opens the valve (on or high) and 0 closes the valve (off or low). A
command with no value returns the current setting. With the valve closed, the
cooling temperature is set with the cooling temperature knob labeled “LOW”. This
should be adjusted for approximately 0–1 psi if the bath is to be used as low as –
40°C. The knob labeled “HIGH” controls the cooling temperature when the
expansion valve is on. This should be adjusted for approximately 65 psi. These
values are typical but may be adjusted as required for specific applications. The
low range must be set to a lower pressure (temperature) than the high range
valve.
The “F4" command controls the back pressure bypass valve. When the valve is
open the cooling capacity is significantly reduced. Cooling capacity may need to
be reduced at higher temperatures. Adjust the back pressure valve manually with
the bypass valve closed until the duty cycle at the desired bath temperature is
within 10–30 %. This setting may be bypassed automatically by opening the
bypass valve. Table 5 summarizes the control functions for heating and cooling.
Table 5. Serial Power Control Functions
Function
Command
0
1
Heater F1
low
high
Refrigeration F2
off
on
Expansion valve
F3
off
on
Back pressure bypass
F4
off
on
Full Remote Operation
The back pressure valve knob is used to control the cooling power of the bath at
higher temperatures. Normally, it is enabled for manual operation and it is
adjusted to its full counterclockwise range for maximum cooling power.
For full remote operation, the following steps need to be followed.
1. Set the temperature of the bath to 25
°
C.
2. Set the low cooling power to its maximum psi but no more than 90 psi.
3. When the bath stabilizes, adjust the back pressure valve so that the duty
cycle is about 8 %.
4. Set the cooling power back to 2 psi.
You are now ready to control the bath remotely according to the ranges
recommended in the following table. The suggested ranges overlap and are not
absolute values.
