diff --git a/frappy_psi/SR.py b/frappy_psi/SR.py
index d07a6f3..e3f0026 100644
--- a/frappy_psi/SR.py
+++ b/frappy_psi/SR.py
@@ -45,13 +45,13 @@ class XY(HasIO, Readable):
     autorange = Parameter('autorange_on', EnumType('autorange', off=0, soft=1, hard=2),
                           readonly=False, default=0)
 
-    sen_range = {name: value + 1 for value, name in enumerate(
+    sen_range = {index + 1: name for index, name in enumerate(
         ['2nV', '5nV', '10nV', '20nV', '50nV', '100nV', '200nV', '500nV', '1uV',
          '2uV', '5uV', '10uV', '20uV', '50uV', '100uV', '200uV', '500uV', '1mV',
          '2mV', '5mV', '10mV', '20mV', '50mV', '100mV', '200mV', '500mV', '1V']
     )}
 
-    irange = Parameter('sensitivity index', EnumType('sensitivity index range', sen_range), readonly=False)
+    irange = Parameter('sensitivity index', EnumType('sensitivity index range', sen_range))
     nm = Parameter('noise mode on', BoolType(), readonly=False)
     phase = Parameter('reference phase control', FloatRange(-360, 360), unit='deg', readonly=False)
     vmode = Parameter('control mode', EnumType(both_grounded=0, A=1, B=2, A_B_diff=3), readonly=False)
@@ -76,7 +76,7 @@ class XY(HasIO, Readable):
         """
         value_with_unit = re.compile(r'(\d+)([pnumkMG]?)')
         value, pfx = value_with_unit.match(value).groups()
-        pfx_dict = {'p': 1e-12, 'n': 1e-9, 'u': 1e-6, 'm': 1e-3, 'k': 1e3, 'M': 1e6, 'G':1e9}
+        pfx_dict = {'p': 1e-12, 'n': 1e-9, 'u': 1e-6, 'm': 1e-3, 'k': 1e3, 'M': 1e6, 'G': 1e9}
         if pfx in pfx_dict:
             value = round(float(value) * pfx_dict[pfx], 12)
         return float(value)
@@ -140,12 +140,15 @@ class XY(HasIO, Readable):
     def write_range(self, target):
         cl_idx = None
 
-        for name, idx in self.sen_range.items():
-            value = self.string_to_value(name)
-            if value < target < self.sen_range.get(idx + 1, float('inf')):
-                cl_idx = idx + 1
-            elif value == target:
-                cl_idx = idx
+        for idx, name in self.sen_range.items():
+
+            if idx < len(self.sen_range):
+                value_l = self.string_to_value(self.sen_range.get(idx))
+                value_r = self.string_to_value(self.sen_range.get(idx + 1))
+                if value_l < target <= value_r:
+                    cl_idx = idx + 1
+                    break
+            cl_idx = idx
         self.write_irange(cl_idx)
         return self.read_range()