diff --git a/frappy_psi/ahcapbridge.py b/frappy_psi/ahcapbridge.py
index 84df767b..6853e38a 100644
--- a/frappy_psi/ahcapbridge.py
+++ b/frappy_psi/ahcapbridge.py
@@ -52,7 +52,7 @@ class IO(StringIO):
     timeout = 5
     sent_command = False  # used to detect that communicate was called directly
     ECHO = re.compile('>|AV |VO |FR |SI |SH ')  # this is recognized as an echo
-    MEAS = re.compile(' *([FC]=|NO DATA)')  # overriden by the module 
+    MEAS = None  # overriden by the module
 
     @Command(StringType(), result=StringType())
     def communicate(self, command, noreply=False):
@@ -63,7 +63,7 @@ class IO(StringIO):
             reply = super().communicate(command, noreply)
             reply = reply and reply.strip()
             if self.check_echo_off(reply):
-                 return reply
+                return reply
         raise CommunicationFailedError('detected echo but can not switch off')
 
     def check_echo_off(self, reply):
@@ -74,6 +74,8 @@ class IO(StringIO):
                 if self.MEAS.match(reply or ''):
                     # this is a meas reply
                     break
+                if reply == 'NO DATA FOUND':
+                    break
             return False
         return True
 
@@ -158,7 +160,13 @@ class AHBase(HasIO, Pinata, Acquisition):
         reply = self.communicate('UN 2')
         # this should be a measurement reply
         mdict = self.get_meas_reply(reply)
-        self._lossunit = mdict.get('lossunit', 'undefined')
+        unit = mdict.get('lossunit', 'undefined')
+        if unit == self.UNIT:
+            if self._lossunit != self.UNIT:
+                self.log.warn('changed loss unit from %r to %r', self._lossunit, self.UNIT)
+        else:
+            self.log.warn('bad reply for UN 2: %r', reply)
+        self._lossunit = unit
 
     def change_param(self, short, value, param):
         if self._mode == RUNNING: