diff --git a/devlib/instrument/__init__.py b/devlib/instrument/__init__.py
index cceada6..6d11eda 100644
--- a/devlib/instrument/__init__.py
+++ b/devlib/instrument/__init__.py
@@ -75,19 +75,19 @@ class MeasurementType(object):
 # Standard measures
 _measurement_types = [
     MeasurementType('unknown', None),
-    MeasurementType('time', 'seconds',
+    MeasurementType('time', 'seconds', 'time',
         conversions={
             'time_us': lambda x: x * 1000000,
             'time_ms': lambda x: x * 1000,
         }
     ),
-    MeasurementType('time_us', 'microseconds',
+    MeasurementType('time_us', 'microseconds', 'time',
         conversions={
             'time': lambda x: x / 1000000,
             'time_ms': lambda x: x / 1000,
         }
     ),
-    MeasurementType('time_ms', 'milliseconds',
+    MeasurementType('time_ms', 'milliseconds', 'time',
         conversions={
             'time': lambda x: x / 1000,
             'time_us': lambda x: x * 1000,
diff --git a/doc/instrumentation.rst b/doc/instrumentation.rst
index 8aee1ce..6f381b4 100644
--- a/doc/instrumentation.rst
+++ b/doc/instrumentation.rst
@@ -229,11 +229,11 @@ defined measurement types are
 +-------------+-------------+---------------+
 | name        | units       | category      |
 +=============+=============+===============+
-| time        | seconds     |               |
+| time        | seconds     |  time         |
 +-------------+-------------+---------------+
-| time        | microseconds|               |
+| time_us     | microseconds|  time         |
 +-------------+-------------+---------------+
-| time        | milliseconds|               |
+| time_ms     | milliseconds|  time         |
 +-------------+-------------+---------------+
 | temperature | degrees     |               |
 +-------------+-------------+---------------+