diff --git a/firmware/lib/session/SessionManager.h b/firmware/lib/session/SessionManager.h
index e6f95b3..790658e 100644
--- a/firmware/lib/session/SessionManager.h
+++ b/firmware/lib/session/SessionManager.h
@@ -103,7 +103,7 @@ void SessionManager<SessionT>::iteration()
     while(!measurementDone)
         measurementDone = scale_.measure(measurement);
     bool addPointSuccessful = session_.addPoint(measurement);
-    Serial.printf("Measured: %d\n", measurement);
+    //Serial.printf("Measured: %d\n", measurement);
     if (!addPointSuccessful)
     {
         Serial.println("Maximum time of session reached - stopping");
@@ -121,7 +121,7 @@ void SessionManager<SessionT>::iteration()
         else
         {
             const long skipped = (cycleDuration / CONFIG_MEASURE_DELAY);
-            Serial.printf("Warning: measurements skipped: %ld, cycleDuration %ld", skipped, cycleDuration);
+            Serial.printf("Measurements skipped: %ld, cycleDuration %ld\n", skipped, cycleDuration);
 
             for (int i = 0; i < skipped; ++i)
                 session_.addPoint(measurement);
diff --git a/firmware/lib/session/SimpleMeasurementSession.h b/firmware/lib/session/SimpleMeasurementSession.h
index 4dcf3fb..950f61d 100644
--- a/firmware/lib/session/SimpleMeasurementSession.h
+++ b/firmware/lib/session/SimpleMeasurementSession.h
@@ -74,6 +74,10 @@ public:
 private:
     void saveToFileSystem()
     {
+        static const uint32_t arrayHeaderOffset = ChunkT::arrayHeaderOffset();
+        Serial.printf(" --------  Array header offset ---- %u\n", arrayHeaderOffset);
+        const uint32_t numMeasurements = chunk->numMeasurements();
+
         // todo: check this! free doesn't mean that the file writing actually works ok
         // use error codes of write instead? anyway: test it!
         Serial.printf("%ld saveToFileSystem start()\n", millis());
@@ -83,18 +87,19 @@ private:
         String filename = String(CONFIG_DATA_PATH) + "/" + String(chunk->getStartTime());
         if (portablefs::exists(filename.c_str()))
         {
-            auto file = portablefs::open(filename.c_str(), "a");
+            auto file = portablefs::open(filename.c_str(), "r+");
             file.seek(0, SeekEnd);
             size_t existingMeasurements = (file.size() - ChunkT::valueOffset()) / sizeof(Measurement_T);
-            Serial.printf("Incremental save, existing %d\n", existingMeasurements);
-            size_t measurementsToWrite = chunk->numMeasurements() - existingMeasurements;
+            size_t measurementsToWrite = numMeasurements - existingMeasurements;
             Measurement_T *startPtr = chunk->getDataPointer() + existingMeasurements;
             file.write((uint8_t *)(startPtr), measurementsToWrite * sizeof(Measurement_T));
             
             Serial.printf("%ld Incr Save: before header patch\n", millis());
-            file.seek(ChunkT::arrayHeaderOffset(), SeekSet);
+            file.seek(arrayHeaderOffset);
+
             StreamingMsgPackEncoder<portablefs::File> encoder(&file);
-            encoder.template sendArrayHeader<Measurement_T>(numMeasurements());
+            encoder.template sendArrayHeader<Measurement_T>(numMeasurements);
+            Serial.printf("%ld total measurements up to now %u\n", millis(), numMeasurements);
         }
         else
         {
@@ -103,7 +108,7 @@ private:
             StreamingMsgPackEncoder<portablefs::File> encoder(&file);
             chunk->serialize(encoder);
         }
-        Serial.printf("%ld saveToFileSystem done\n", millis());
+        Serial.printf("%ld saveToFileSystem done-------------\n", millis());
     }
 
     void deleteUntilBytesFree(size_t requiredSpace)