si5servo: Adjust minimum cryofan speed

si5servo.py

@@ -65,18 +65,21 @@ def main(setpoint, params, frequency, amplitude, source, datadir, dataname):
     # Get output device.
     output = rpc.Client(cryo124k.ADDRESS, cryo124k.TIMEOUT)
 
-    # Control loop parameters.
+    # Parse control loop parameters.
     args = {name: float(value) for name, value in (param.split('=') for param in params)}
     args.setdefault('o', output.attr('output'))
 
-    # PI control loop with anti-windup. With the cryofan speed set to
-    # zero, there is no He circulation and the gas in the He circuit
-    # warms up to a temperature above the system temperature. When the
-    # He starts circulating again, the system temperature increases
-    # quickly instead of decreasing. To avoid this effect, always keep
-    # a minimum He flow by setting the minimum cryofan control voltage
-    # to 0.1 V.
-    servo = control.AntiWindupPI(setpoint=setpoint, vmin=0.1, vmax=10.0, **args)
+    # When the cryofan speed is set to zero, there is no He circulation and
+    # the He in the closed circuit warms up. When the He starts circulating
+    # again, the system temperature increases instead of decreasing.
+    # Maintaining a minimum He flow prevents this effect. Experimentally it
+    # has been found that for this setup the flow obtained with a cryofan
+    # control voltage of 2.2 V does not contribute significant cooling while
+    # not allowing warm He to accumulate.
+    vmin, vmax = 2.2, 10.0
+
+    # PI control loop with anti-windup.
+    servo = control.AntiWindupPI(setpoint=setpoint, vmin=vmin, vmax=vmax, **args)
 
     # Log loop parameters.
     data = datalogger.open_data_file(datadir, dataname, None)