diff --git a/app/src/main/java/de/fhws/indoor/sensorreadout/MainActivity.java b/app/src/main/java/de/fhws/indoor/sensorreadout/MainActivity.java
index 4d1ead3..ae127b1 100644
--- a/app/src/main/java/de/fhws/indoor/sensorreadout/MainActivity.java
+++ b/app/src/main/java/de/fhws/indoor/sensorreadout/MainActivity.java
@@ -259,10 +259,10 @@ public class MainActivity extends WearableActivity {
                 }
 
                 // dump buffer stats every x entries
-                if (++loadCounterAny % 100 == 0) {
+                if (++loadCounterAny % 250 == 0) {
                     final TextView txt = (TextView) findViewById(R.id.txtBuffer);
-                    final int kbPerMin = (int) (logger.getSize() / 1024 * 1000 * 60 / (System.currentTimeMillis() - logger.getStartTS()));
-                    txt.setText( (logger.getSize() / 1024) + "kb, " + logger.getNumEntries() + ", " + kbPerMin + "kb/min");
+                    final int kbPerMin = (int) (logger.getTotalSize() / 1024 * 1000 * 60 / (System.currentTimeMillis() - logger.getStartTS()));
+                    txt.setText( (logger.getCurrentSize() / 1024) + "k, " + logger.getNumEntries() + ", " + kbPerMin + "k/m");
                 }
 
             }
diff --git a/app/src/main/java/de/fhws/indoor/sensorreadout/sensors/Logger.java b/app/src/main/java/de/fhws/indoor/sensorreadout/sensors/Logger.java
index 3f4b133..4b363ae 100644
--- a/app/src/main/java/de/fhws/indoor/sensorreadout/sensors/Logger.java
+++ b/app/src/main/java/de/fhws/indoor/sensorreadout/sensors/Logger.java
@@ -1,13 +1,11 @@
 package de.fhws.indoor.sensorreadout.sensors;
 
 import android.content.Context;
-import android.os.Environment;
+import android.os.AsyncTask;
 import android.util.Log;
 
-import java.io.ByteArrayOutputStream;
 import java.io.File;
 import java.io.FileOutputStream;
-import java.io.OutputStreamWriter;
 
 /**
  * log sensor data to file
@@ -15,11 +13,16 @@ import java.io.OutputStreamWriter;
  */
 public final class Logger {
 
+	private static final int FLUSH_LIMIT = 2*1024*1024;
+
 	private StringBuilder sb = new StringBuilder();
 	private File file;
 	private FileOutputStream fos;
 	private Context context;
+
 	private int entries = 0;
+	private int sizeCurrent = 0;
+	private int sizeTotal = 0;
 
 	/** timestamp of logging start. all entries are relative to this one */
 	private long startTS = 0;
@@ -34,6 +37,8 @@ public final class Logger {
 		// start empty
 		sb.setLength(0);
 		entries = 0;
+		sizeTotal = 0;
+		sizeCurrent = 0;
 
 		// starting timestamp
 		startTS = System.currentTimeMillis();
@@ -55,12 +60,8 @@ public final class Logger {
 	/** stop logging and flush RAM-data to the flash-chip */
 	public final void stop() {
 		synchronized (this) {
-			try {
-				fos.write(sb.toString().getBytes());
-				fos.close();
-			} catch (final Exception e) {
-				throw new MyException("error while writing log-file", e);
-			}
+			flush(true);
+			close();
 		}
 	}
 
@@ -68,7 +69,8 @@ public final class Logger {
 		return file;
 	}
 
-	public int getSize() {return sb.length();}
+	public int getCurrentSize() {return sizeCurrent;}
+	public int getTotalSize() {return sizeTotal;}
 
 	public int getNumEntries() {return entries;}
 
@@ -83,18 +85,75 @@ public final class Logger {
 			sb.append(csv);
 			sb.append('\n');
 			++entries;
+			sizeTotal += csv.length() + 10; // approx!
+			sizeCurrent = sb.length();
+			if (sb.length() > FLUSH_LIMIT) {flush(false);}
 		}
+
 		debug();
 	}
 
+
+
+	/** helper method for exception-less writing. DO NOT CALL DIRECTLY! */
+	private final void _write(final byte[] data) {
+		try {
+			fos.write(data);
+			Log.d("logger", "flushed " + data.length + " bytes to disk");
+		} catch (final Exception e) {
+			throw new RuntimeException("error while writing log-file", e);
+		}
+	}
+
+	/** helper-class for background writing */
+	class FlushAsync extends AsyncTask<byte[], Integer, Integer> {
+		@Override
+		protected final Integer doInBackground(byte[][] data) {
+			_write(data[0]);
+			return null;
+		}
+	};
+
+	/** flush current buffer-contents to disk */
+	private final void flush(boolean sync) {
+
+		// fetch current buffer contents to write and hereafter empty the buffer
+		// this action MUST be atomic, just like the add-method
+		byte[] data = null;
+		synchronized (this) {
+			data = sb.toString().getBytes();		// fetch data to write
+			sb.setLength(0);						// reset the buffer
+			sizeCurrent = 0;
+		}
+
+		// write
+		if (sync) {
+			// write to disk using the current thread
+			_write(data);
+		} else {
+			// write to disk using a background-thread
+			new FlushAsync().execute(new byte[][] {data});
+		}
+
+
+	}
+
+	private final void close() {
+		try {
+			fos.close();
+		} catch (final Exception e) {
+			throw new MyException("error while writing log-file", e);
+		}
+	}
+
 	public final long getStartTS() {
 		return startTS;
 	}
 
 	int cnt = 0;
 	private final void debug() {
-		if (++cnt % 500 == 0) {
-			Log.d("buffer", "size: " + sb.length());
+		if (++cnt % 1000 == 0) {
+			Log.d("buffer", "size: " + sizeCurrent);
 		}
 	}