diff --git a/secop_psi/ips_mercury.py b/secop_psi/ips_mercury.py
index ebd07e4..f74972a 100644
--- a/secop_psi/ips_mercury.py
+++ b/secop_psi/ips_mercury.py
@@ -20,6 +20,7 @@
 # *****************************************************************************
 """oxford instruments mercury IPS power supply"""
 
+import time
 from secop.core import Parameter, EnumType, FloatRange, BoolType
 from secop.lib.enum import Enum
 from secop.errors import BadValueError, HardwareError
@@ -51,6 +52,7 @@ class Field(MercuryChannel, Magfield):
     nslaves = 3
     slave_currents = None
     __init = True
+    __reset_switch_time = False
 
     def doPoll(self):
         super().doPoll()
@@ -62,7 +64,7 @@ class Field(MercuryChannel, Magfield):
         if self.__init:
             self.__init = False
             self.persistent_field = pf
-        if self.switch_heater != 0 or self._field_mismatch is None:
+        if self.switch_heater == self.switch_heater.on or self._field_mismatch is None:
             self.forced_persistent_field = False
             self._field_mismatch = False
             return self.current
@@ -94,7 +96,18 @@ class Field(MercuryChannel, Magfield):
         return self.change('PSU:ACTN', value, hold_rtoz_rtos_clmp)
 
     def read_switch_heater(self):
-        return self.query('PSU:SIG:SWHT', off_on)
+        value = self.query('PSU:SIG:SWHT', off_on)
+        now = time.time()
+        switch_time = self.switch_time[self.switch_heater]
+        if value != self.switch_heater:
+            self.__reset_switch_time = True
+            if now < (switch_time or 0) + 10:
+                # probably switch heater was changed, but IPS reply is not yet updated
+                return self.switch_heater
+        elif self.__reset_switch_time:
+            self.__reset_switch_time = False
+            self.switch_time = [None, None]
+        return value
 
     def write_switch_heater(self, value):
         return self.change('PSU:SIG:SWHT', value, off_on)
diff --git a/secop_psi/magfield.py b/secop_psi/magfield.py
index 1d800b2..ef90bc2 100644
--- a/secop_psi/magfield.py
+++ b/secop_psi/magfield.py
@@ -44,6 +44,9 @@ Status = Enum(
     FINALIZING=390,
 )
 
+OFF = 0
+ON = 1
+
 
 class Magfield(HasLimits, Drivable):
     value = Parameter('magnetic field', datatype=FloatRange(unit='T'))
@@ -52,7 +55,7 @@ class Magfield(HasLimits, Drivable):
         'persistent mode', EnumType(Mode), readonly=False, default=Mode.PERSISTENT)
     tolerance = Parameter(
         'tolerance', FloatRange(0, unit='$'), readonly=False, default=0.0002)
-    switch_heater = Parameter('switch heater', EnumType(off=0, on=1),
+    switch_heater = Parameter('switch heater', EnumType(off=OFF, on=ON),
                               readonly=False, default=0)
     persistent_field = Parameter(
         'persistent field', FloatRange(unit='$'), readonly=False)
@@ -87,8 +90,7 @@ class Magfield(HasLimits, Drivable):
     _state = None
     __init = True
     _last_target = None
-    switch_on_time = None
-    switch_off_time = None
+    switch_time = None, None
 
     def doPoll(self):
         if self.__init:
@@ -185,14 +187,11 @@ class Magfield(HasLimits, Drivable):
 
     def update_switch_heater(self, value):
         """is called whenever switch heater was changed"""
-        if value != 0:
-            self.switch_off_time = None
-            if self.switch_on_time is None:
-                self.switch_on_time = time.time()
-        else:
-            self.switch_on_time = None
-            if self.switch_off_time is None:
-                self.switch_off_time = time.time()
+        switch_time = self.switch_time[value]
+        if switch_time is None:
+            switch_time = time.time()
+        self.switch_time = [None, None]
+        self.switch_time[value] = switch_time
 
     def start_switch_on(self, state):
         """switch heater on"""
@@ -213,12 +212,10 @@ class Magfield(HasLimits, Drivable):
                 abs(self.target - self.persistent_field) <= self.tolerance):  # short cut
             return self.check_switch_off
         self.read_switch_heater()
-        if self.switch_on_time is None:
-            if state.now - self.switch_off_time > 10:
-                 self.log.warning('switch turned off manually?')
-                 return self.start_switch_on
-            return Retry()
-        if state.now - self.switch_on_time < self.wait_switch_on:
+        if self.switch_time[ON] is None:
+            self.log.warning('switch turned off manually?')
+            return self.start_switch_on
+        if state.now - self.switch_time[ON] < self.wait_switch_on:
             return Retry()
         self._last_target = self.target
         return self.start_ramp_to_target
@@ -279,12 +276,10 @@ class Magfield(HasLimits, Drivable):
             return self.start_switch_on
         self.persistent_field = self.value
         self.read_switch_heater()
-        if self.switch_off_time is None:
-            if state.now - self.switch_on_time > 10:
-                 self.log.warning('switch turned on manually?')
-                 return self.start_switch_off
-            return Retry()
-        if state.now - self.switch_off_time < self.wait_switch_off:
+        if self.switch_time[OFF] is None:
+            self.log.warning('switch turned on manually?')
+            return self.start_switch_off
+        if state.now - self.switch_time[OFF] < self.wait_switch_off:
             return Retry()
         if abs(self.value) > self.persistent_limit:
             self.status = Status.IDLE, 'leads current at field, switch off'