diff --git a/app/src/main/AndroidManifest.xml b/app/src/main/AndroidManifest.xml
index 9b84410..2c0bf3b 100644
--- a/app/src/main/AndroidManifest.xml
+++ b/app/src/main/AndroidManifest.xml
@@ -3,7 +3,7 @@
     xmlns:tools="http://schemas.android.com/tools"
     package="uk.org.openseizuredetector"
     android:versionCode="115"
-    android:versionName="4.1.3n">
+    android:versionName="4.1.3m">
     <!-- android:allowBackup="false" -->
     <uses-permission android:name="android.permission.BLUETOOTH" />
     <uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />
diff --git a/app/src/main/java/uk/org/openseizuredetector/CircBuf.java b/app/src/main/java/uk/org/openseizuredetector/CircBuf.java
new file mode 100644
index 0000000..27cb597
--- /dev/null
+++ b/app/src/main/java/uk/org/openseizuredetector/CircBuf.java
@@ -0,0 +1,99 @@
+package uk.org.openseizuredetector;
+
+import android.util.Log;
+
+import java.util.ArrayList;
+
+public class CircBuf {
+    /*
+    * A circular buffer used to calculate rolling averages
+    * Based loosely on https://gist.github.com/hardik-vala/dc2d19fa7c5108536fbbff96b4fcf105
+    */
+    private final static String TAG = "CircBuf";
+
+    private double[] mBuff;
+    private double mErrVal;
+    private int mBuffLen;
+    private int mHead;
+    private int mTail;
+    private boolean mIsFull;
+
+    public CircBuf(int n, double errVal) {
+        /**
+         * Create a circular buffer of doubles, of length n members.
+         */
+        Log.i(TAG, "CircBuf Constructor");
+        mBuff = new double[n];
+        mBuffLen = n;
+        mErrVal = errVal;
+        mHead = 0;
+        mTail = 0;
+        mIsFull = false;
+    }
+
+    public void add(double val) {
+        /**
+         * Add value val to the circular buffer.
+         */
+        Log.d(TAG,"add() - before: mHead="+mHead+", mTail="+mTail);
+        //System.out.println(TAG+" add() - before: mHead="+mHead+", mTail="+mTail);
+        if (mIsFull)
+            mHead = (mHead + 1) % mBuffLen;
+
+        mBuff[mTail] = val;
+        mTail = (mTail + 1) % mBuffLen;
+        if (mTail == mHead)
+            mIsFull = true;
+        Log.d(TAG,"add() -  after: mHead="+mHead+", mTail="+mTail);
+        //System.out.println(TAG+" add() - before: mHead="+mHead+", mTail="+mTail);
+    }
+
+    public int getNumVals() {
+        int numElements;
+        if (mIsFull) {
+            numElements = mBuffLen;
+        } else {
+            // Not sure if this is necessary - why would mHead be greater than mTail if the buffer is not full?
+            if (mHead > mTail) {
+                numElements = (mTail + mBuffLen) - mHead;
+            } else {
+                numElements = mTail-mHead;
+            }
+        }
+        return numElements;
+    }
+
+    /**
+     * Returns a double array of buffer items in order of their insertion time
+     * @return double[] of buffer items
+     */
+    public double[] getVals () {
+        int numElements = getNumVals();
+        System.out.println(TAG+" getVals() - numElements=" + numElements);
+        double[] retArr = new double[numElements];
+        for (int i = 0; i < numElements; i++) {
+            retArr[i] = mBuff[(mHead + i) % mBuffLen];
+        }
+        return retArr;
+    }
+
+    public double getAverageVal() {
+        double hrSum = 0.;
+        int hrCount = 0;
+        double valArr[] = getVals();
+        double retVal;
+        for (int n=0; n<valArr.length; n++) {
+            if (valArr[n] != mErrVal) {
+                hrSum += valArr[n];
+                hrCount++;
+            }
+        }
+        if (hrCount>0) {
+            retVal = hrSum / hrCount;
+        } else {
+            retVal = -1;
+        }
+        return(retVal);
+    }
+
+}
\ No newline at end of file
diff --git a/app/src/main/java/uk/org/openseizuredetector/SdAlgHr.java b/app/src/main/java/uk/org/openseizuredetector/SdAlgHr.java
index e6b7ed9..8edf04f 100644
--- a/app/src/main/java/uk/org/openseizuredetector/SdAlgHr.java
+++ b/app/src/main/java/uk/org/openseizuredetector/SdAlgHr.java
@@ -25,13 +25,15 @@ public class SdAlgHr {
     private double mAverageHrAlarmThreshMin;
     private double mAverageHrAlarmThreshMax;
 
-    private ArrayList<Double> mHrHist;
+    private CircBuf mAdaptiveHrBuff;
+    private CircBuf mAverageHrBuff;
 
     public SdAlgHr(Context context) {
         Log.i(TAG, "SdAlgHr Constructor");
         mContext = context;
-        mHrHist = new ArrayList<Double>();
         updatePrefs();
+        mAdaptiveHrBuff = new CircBuf(mAdaptiveHrAlarmWindowDp, -1.0);
+        mAverageHrBuff = new CircBuf(mAverageHrAlarmWindowDp, -1.0);
     }
 
     public void close() {
@@ -93,15 +95,7 @@ public class SdAlgHr {
 
     }
 
-    private void addToHist(double hrVal) {
-        /**
-         * Add value hrVal to the heart rate history list, truncating the list if it is
-         * longer than the required length.
-         */
-        Log.d(TAG,"addToHist() - length before="+mHrHist.size());
-        mHrHist.add(hrVal);
-        Log.d(TAG,"addToHist() - length before="+mHrHist.size());
-    }
+
 
     private boolean checkSimpleHr(double hrVal) {
         /**
@@ -118,33 +112,42 @@ public class SdAlgHr {
     }
 
     private boolean checkAdaptiveHr(double hrVal) {
-        // FIXME Make this do something
-        return(false);
+        boolean retVal;
+        double hrThreshMin;
+        double hrThreshMax;
+        double avHr = mAdaptiveHrBuff.getAverageVal();
+        hrThreshMin = avHr - mAdaptiveHrAlarmThresh;
+        hrThreshMax = avHr + mAdaptiveHrAlarmThresh;
+
+        retVal = false;
+        if (hrVal < hrThreshMin) {
+            retVal = true;
+        }
+        if (hrVal > hrThreshMax) {
+            retVal = true;
+        }
+        Log.d(TAG, "checkAdaptiveHr() - hrVal="+hrVal+", avHr="+avHr+", thresholds=("+hrThreshMin+", "+hrThreshMax+"): Alarm="+retVal);
+
+        return(retVal);
     }
 
     private boolean checkAverageHr(double hrVal) {
-        // FIXME Make this do something
-        return(false);
-    }
+        boolean retVal;
+        double avHr = mAdaptiveHrBuff.getAverageVal();
 
-    public double getAverageHrVal() {
-        double hrSum = 0.;
-        int hrCount = 0;
-        double retVal;
-        for (int n=0; n<mHrHist.size(); n++) {
-            if (mHrHist.get(n) > -1) {
-                hrSum += mHrHist.get(n);
-                hrCount++;
-            }
+        retVal = false;
+        if (hrVal < mAverageHrAlarmThreshMin) {
+            retVal = true;
         }
-        if (hrCount>0) {
-            retVal = hrSum / hrCount;
-        } else {
-            retVal = -1;
+        if (hrVal > mAverageHrAlarmThreshMax) {
+            retVal = true;
         }
+        Log.d(TAG, "checkAverageHr() - hrVal="+hrVal+", avHr="+avHr+", thresholds=("+mAverageHrAlarmThreshMin+", "+mAverageHrAlarmThreshMin+"): Alarm="+retVal);
         return(retVal);
     }
 
+
+
     public ArrayList<Boolean> checkHr(double hrVal) {
         /**
          * Checks the current Heart Rate reading hrVal against the
@@ -153,7 +156,8 @@ public class SdAlgHr {
          * true=ALARM, false=OK.
          */
         Log.v(TAG, "checkHr("+hrVal+")");
-        addToHist(hrVal);
+        mAdaptiveHrBuff.add(hrVal);
+        mAverageHrBuff.add(hrVal);
         ArrayList<Boolean> retVal = new ArrayList<Boolean>();
         retVal.add(checkSimpleHr(hrVal));
         retVal.add(checkAdaptiveHr(hrVal));
diff --git a/app/src/test/java/uk/org/openseizuredetector/CircBufTest.java b/app/src/test/java/uk/org/openseizuredetector/CircBufTest.java
new file mode 100644
index 0000000..32573d7
--- /dev/null
+++ b/app/src/test/java/uk/org/openseizuredetector/CircBufTest.java
@@ -0,0 +1,103 @@
+package uk.org.openseizuredetector;
+
+import junit.framework.TestCase;
+
+public class CircBufTest extends TestCase {
+    private CircBuf mCb;
+
+    public void setUp() throws Exception {
+        super.setUp();
+        mCb = new CircBuf(10, -1);
+    }
+
+    public void tearDown() throws Exception {
+    }
+
+    public void printArr(double[] arr) {
+        for (int n=0; n<arr.length; n++) {
+            System.out.println("arr["+n+"] = "+arr[n]);
+        }
+    }
+
+    public void testAdd() {
+        double[] retArr;
+
+        // Add a single value = we should get a single element array back.
+        mCb.add(1);
+        retArr = mCb.getVals();
+        assertEquals(1,retArr.length);
+
+        // Add 9 more elements - we should get a 10 element array back.
+        for (int i=0; i<9;i++) {
+            mCb.add(i);
+        }
+        retArr = mCb.getVals();
+        assertEquals(10,retArr.length);
+
+
+    }
+
+    public void testGetVals() {
+        double[] retArr;
+        // Add 10 more elements - we should get a 10 element array back.
+        for (int i=0; i<10;i++) {
+            mCb.add(i);
+        }
+        retArr = mCb.getVals();
+        assertEquals(0.0, retArr[0]);
+        assertEquals(9.0, retArr[9]);
+
+        //add one more element
+        mCb.add(10.0);
+        retArr = mCb.getVals();
+        printArr(retArr);
+        assertEquals(1.0, retArr[0]);
+        assertEquals(10.0, retArr[9]);
+
+        //add one more element
+        mCb.add(10.0);
+        retArr = mCb.getVals();
+        printArr(retArr);
+        assertEquals(2.0, retArr[0]);
+        assertEquals(10.0, retArr[9]);
+
+    }
+
+    public void testGetAverageVal() {
+        double[] retArr;
+        double avVal;
+
+        // empty array;
+        avVal = mCb.getAverageVal();
+        assertEquals(-1.0, avVal, 1e-5);
+
+
+
+        // A trivial example
+        for (int i=0; i<10;i++) {
+            mCb.add(1.0);
+        }
+        retArr = mCb.getVals();
+        avVal = mCb.getAverageVal();
+        assertEquals(1.0, avVal,1e-5);
+
+        // Real calculation
+        double sum = 0.0;
+        for (int i=0; i<10;i++) {
+            mCb.add(i);
+            sum += i;
+        }
+        avVal = mCb.getAverageVal();
+        assertEquals(sum/10, avVal, 1e-5);
+
+        // Error value in array - should now be average of 9 values between 1 and 9 (because zero value is removed when -1 is added to end).
+        sum = 0.0;
+        for(int i=1;i<10;i++)
+            sum+= i;
+
+        mCb.add(-1.0);
+        avVal = mCb.getAverageVal();
+        assertEquals(sum/9, avVal, 1e-5);
+
+    }
+}
\ No newline at end of file