diff --git a/libraries/AP_Compass/AP_Compass.h b/libraries/AP_Compass/AP_Compass.h
index 40d204eef7..693ff374c1 100644
--- a/libraries/AP_Compass/AP_Compass.h
+++ b/libraries/AP_Compass/AP_Compass.h
@@ -454,6 +454,10 @@ private:
 
         // board specific orientation
         enum Rotation rotation;
+
+        // accumulated samples, protected by _sem, used by AP_Compass_Backend
+        Vector3f accum;
+        uint32_t accum_count;
     } _state[COMPASS_MAX_INSTANCES];
 
     AP_Int16 _offset_max;
diff --git a/libraries/AP_Compass/AP_Compass_Backend.cpp b/libraries/AP_Compass/AP_Compass_Backend.cpp
index 37226e330a..94d71f2252 100644
--- a/libraries/AP_Compass/AP_Compass_Backend.cpp
+++ b/libraries/AP_Compass/AP_Compass_Backend.cpp
@@ -114,11 +114,12 @@ void AP_Compass_Backend::accumulate_sample(Vector3f &field, uint8_t instance,
 
     WITH_SEMAPHORE(_sem);
 
-    _accum += field;
-    _accum_count++;
-    if (max_samples && _accum_count >= max_samples) {
-        _accum_count /= 2;
-        _accum /= 2;
+    Compass::mag_state &state = _compass._state[instance];
+    state.accum += field;
+    state.accum_count++;
+    if (max_samples && state.accum_count >= max_samples) {
+        state.accum_count /= 2;
+        state.accum /= 2;
     }
 }
 
@@ -127,19 +128,21 @@ void AP_Compass_Backend::drain_accumulated_samples(uint8_t instance,
 {
     WITH_SEMAPHORE(_sem);
 
-    if (_accum_count == 0) {
+    Compass::mag_state &state = _compass._state[instance];
+
+    if (state.accum_count == 0) {
         return;
     }
 
     if (scaling) {
-        _accum *= *scaling;
+        state.accum *= *scaling;
     }
-    _accum /= _accum_count;
+    state.accum /= state.accum_count;
 
-    publish_filtered_field(_accum, instance);
+    publish_filtered_field(state.accum, instance);
 
-    _accum.zero();
-    _accum_count = 0;
+    state.accum.zero();
+    state.accum_count = 0;
 }
 
 /*
diff --git a/libraries/AP_Compass/AP_Compass_Backend.h b/libraries/AP_Compass/AP_Compass_Backend.h
index 4685c245ee..2cb343bdd7 100644
--- a/libraries/AP_Compass/AP_Compass_Backend.h
+++ b/libraries/AP_Compass/AP_Compass_Backend.h
@@ -112,10 +112,6 @@ protected:
     // semaphore for access to shared frontend data
     HAL_Semaphore_Recursive _sem;
 
-    // accumulated samples, protected by _sem
-    Vector3f    _accum;
-    uint32_t    _accum_count;
-
     // Check that the compass field is valid by using a mean filter on the vector length
     bool field_ok(const Vector3f &field);