si5servo: Update control voltage limits

si5servo.py

@@ -69,14 +69,17 @@ def main(setpoint, params, frequency, amplitude, source, datadir, dataname):
     args = {name: float(value) for name, value in (param.split('=') for param in params)}
     args.setdefault('o', output.attr('output'))
 
-    # 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
+    # When the cryofan speed is set to zero, there is no gas flown and the
+    # helium in the closed circuit warms up. When the helium starts
+    # circulating again, the system temperature increases instead of
+    # decreasing, making a fool of the temperature control loop. Maintaining
+    # a minimum gas flow prevents this effect.
+    #
+    # Experimentally it has been found that the minimum control voltage
+    # below corresponds to a cryofan rotation speed that results in an
+    # helium flow that does not contribute significant cooling while not
+    # allowing warm helium to accumulate. See measurements on 30-04-2024.
+    vmin, vmax = 2.75, 10.0
 
     # PI control loop with anti-windup.
     servo = control.AntiWindupPI(setpoint=setpoint, vmin=vmin, vmax=vmax, **args)