diff --git a/firmware/lib/session/SessionChunk.h b/firmware/lib/session/SessionChunk.h
index 4c82607..7602f59 100644
--- a/firmware/lib/session/SessionChunk.h
+++ b/firmware/lib/session/SessionChunk.h
@@ -3,14 +3,14 @@
 #include "StreamingMsgPackEncoder.h"
 #include <cstdint>
 
-
-template<typename Measurement_T, uint32_t SIZE>
+template <typename Measurement_T, uint32_t SIZE>
 class SessionChunk
 {
 public:
     SessionChunk()
         : nextFree_(0), sessionStartTime(0), startIndex(0)
-    {}
+    {
+    }
 
     void init(uint32_t epochStartTime, uint32_t startIdx)
     {
@@ -19,21 +19,24 @@ public:
         startIndex = startIdx;
     }
 
-    uint32_t getStartTime() const {
+    uint32_t getStartTime() const
+    {
         return sessionStartTime;
     }
 
-    uint32_t getStartIndex() const {
+    uint32_t getStartIndex() const
+    {
         return startIndex;
     }
 
-    uint32_t numMeasurements() const {
+    uint32_t numMeasurements() const
+    {
         return nextFree_;
     }
 
     bool addPoint(Measurement_T measurement)
     {
-        if( nextFree_ >= SIZE)
+        if (nextFree_ >= SIZE)
             return false;
         values[nextFree_] = measurement;
         nextFree_++;
@@ -41,8 +44,8 @@ public:
         return true;
     }
 
-    template<typename T>
-    void serialize(StreamingMsgPackEncoder<T> & encoder) const
+    template <typename T>
+    void serialize(StreamingMsgPackEncoder<T> &encoder) const
     {
         sendHeader(encoder, sessionStartTime, startIndex);
         encoder.sendArray(values, nextFree_);
@@ -57,16 +60,26 @@ public:
         return encoder.getContentLength();
     }
 
-    Measurement_T * getDataPointer() {
+    static uint32_t arrayHeaderOffset()
+    {
+        StreamingMsgPackEncoder<DummyWriter> encoder(nullptr);
+        encoder.setSizeCountMode(true);
+        sendHeader(encoder, 0, 0);
+        return encoder.getContentLength();
+    }
+
+    Measurement_T *getDataPointer()
+    {
         return values;
     }
 
-    const Measurement_T * getDataPointer() const {
+    const Measurement_T *getDataPointer() const
+    {
         return values;
     }
 
-    template<typename Encoder_T>
-    static void sendHeader(Encoder_T & encoder, uint32_t sessionStartTime, uint32_t startIndex)
+    template <typename Encoder_T>
+    static void sendHeader(Encoder_T &encoder, uint32_t sessionStartTime, uint32_t startIndex)
     {
         encoder.sendMap16(3);
 
diff --git a/firmware/lib/session/SessionManager.h b/firmware/lib/session/SessionManager.h
index e9f812c..e6f95b3 100644
--- a/firmware/lib/session/SessionManager.h
+++ b/firmware/lib/session/SessionManager.h
@@ -93,8 +93,10 @@ void SessionManager<SessionT>::stopMeasurements()
 template <typename SessionT>
 void SessionManager<SessionT>::iteration()
 {
-    if (!measuring_)
-        return;
+    if (!measuring_) {
+        delay(1);
+        return; // give control to HTTP server thread
+    }
 
     uint16_t measurement = -1;
     bool measurementDone = false;
@@ -106,6 +108,7 @@ void SessionManager<SessionT>::iteration()
     {
         Serial.println("Maximum time of session reached - stopping");
         stopMeasurements();
+        delay(1); // give control to HTTP server thread
         return;
     }
     if (lastCallTime_ != 0)
diff --git a/firmware/lib/session/SimpleMeasurementSession.h b/firmware/lib/session/SimpleMeasurementSession.h
index 8d9fbd8..4dcf3fb 100644
--- a/firmware/lib/session/SimpleMeasurementSession.h
+++ b/firmware/lib/session/SimpleMeasurementSession.h
@@ -9,7 +9,7 @@ public:
     using ChunkT = SessionChunk<Measurement_T, MAX_SIZE>;
     using MeasurementType = Measurement_T;
     // save interval in number of measurements (by default every minute)
-    SimpleMeasurementSession(uint32_t saveInterval = 10 * 60)
+    SimpleMeasurementSession(uint32_t saveInterval = 10 * 20)
         : chunk(nullptr), saveInterval_(saveInterval)
     {
     }
@@ -76,22 +76,25 @@ private:
     {
         // 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());
         deleteUntilBytesFree(CONFIG_SESSION_MAX_SIZE);
+        Serial.printf("%ld after deleteUntilBytesFree()\n", millis());
 
         String filename = String(CONFIG_DATA_PATH) + "/" + String(chunk->getStartTime());
         if (portablefs::exists(filename.c_str()))
         {
             auto file = portablefs::open(filename.c_str(), "a");
-            file.seek(0, SeekSet);
-            StreamingMsgPackEncoder<portablefs::File> encoder(&file);
-            chunk->sendHeader(encoder, chunk->getStartTime(), 0);
-
             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;
             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);
+            StreamingMsgPackEncoder<portablefs::File> encoder(&file);
+            encoder.template sendArrayHeader<Measurement_T>(numMeasurements());
         }
         else
         {
@@ -100,6 +103,7 @@ private:
             StreamingMsgPackEncoder<portablefs::File> encoder(&file);
             chunk->serialize(encoder);
         }
+        Serial.printf("%ld saveToFileSystem done\n", millis());
     }
 
     void deleteUntilBytesFree(size_t requiredSpace)
diff --git a/firmware/src/firmware_main.cpp b/firmware/src/firmware_main.cpp
index db22030..32736a9 100644
--- a/firmware/src/firmware_main.cpp
+++ b/firmware/src/firmware_main.cpp
@@ -189,33 +189,4 @@ void loop()
 {
     sessionManager.iteration();
     webSocketServer.iteration();
-    /*
-    if (webSocketServer.poll())
-    {
-        websocketClients[nextFreeWebsocketClient] = webSocketServer.accept();
-        websocketClients[nextFreeWebsocketClient].onMessage(onMessage);
-        Serial.println("Websocket connection");
-        nextFreeWebsocketClient = (nextFreeWebsocketClient + 1) % MAX_WEBSOCKET_CONNECTIONS;
-    }
-    for (int i = 0; i < MAX_WEBSOCKET_CONNECTIONS; ++i)
-        //if (websocketClients[i].available()) {
-            //Serial.printf("Polling client %d\n", i);
-            websocketClients[i].poll();
-        //}
-
-    auto &session = sessionManager.session();
-    if (session.numMeasurements() < measurementsSent)
-        measurementsSent = 0;
-    else if (session.numMeasurements() > measurementsSent)
-    {
-        for (int i = 0; i < MAX_WEBSOCKET_CONNECTIONS; ++i)
-            if (websocketClients[i].available())
-            {
-                auto dataToSend = (const char*)(session.getDataPointer() + measurementsSent);
-                auto numBytes = (session.numMeasurements() - measurementsSent) * sizeof(MeasurementT);
-                Serial.printf("Sent %d bytes via websocket\n", numBytes);
-                websocketClients[i].sendBinary(dataToSend, numBytes);
-                measurementsSent = session.numMeasurements();
-            }
-    }*/
 }