diff --git a/lib/home_page.dart b/lib/home_page.dart
index ee18501..53d6230 100644
--- a/lib/home_page.dart
+++ b/lib/home_page.dart
@@ -101,7 +101,7 @@ class _HomePageState extends State<HomePage> {
             );
           },
         ),
-        title: const Text("MMS Version 3.12.06"),
+        title: const Text("MMS Version 3.12.07"),
         actions: [
           IconButton(
             icon: const Icon(Icons.person),
diff --git a/lib/models/in_situ_sampling_data.dart b/lib/models/in_situ_sampling_data.dart
index 50499da..a400d28 100644
--- a/lib/models/in_situ_sampling_data.dart
+++ b/lib/models/in_situ_sampling_data.dart
@@ -264,7 +264,7 @@ class InSituSamplingData {
   /// Creates a single JSON object for 'db.json'. FORCING ALL VALUES TO STRING.
   String toDbJson() {
     final data = {
-      'battery_cap': (batteryVoltage ?? "NULL").toString(),
+      'battery_cap': (batteryVoltage == null || batteryVoltage == -999.0) ? "NULL" : batteryVoltage.toString(),
       'device_name': (sondeId ?? "").toString(),
       'sampling_type': (samplingType ?? "").toString(),
       'report_id': (reportId ?? "").toString(),
@@ -277,15 +277,15 @@ class InSituSamplingData {
       'latitude': (stationLatitude ?? "").toString(),
       'longitude': (stationLongitude ?? "").toString(),
       'record_dt': (samplingDate != null && samplingTime != null) ? '$samplingDate $samplingTime' : "",
-      'do_mgl': (oxygenConcentration ?? "NULL").toString(),
-      'do_sat': (oxygenSaturation ?? "NULL").toString(),
-      'ph': (ph ?? "NULL").toString(),
-      'salinity': (salinity ?? "NULL").toString(),
-      'tss': (tss ?? "NULL").toString(),
-      'temperature': (temperature ?? "NULL").toString(),
-      'turbidity': (turbidity ?? "NULL").toString(),
-      'tds': (tds ?? "NULL").toString(),
-      'electric_conductivity': (electricalConductivity ?? "NULL").toString(),
+      'do_mgl': (oxygenConcentration == null || oxygenConcentration == -999.0) ? "NULL" : oxygenConcentration.toString(),
+      'do_sat': (oxygenSaturation == null || oxygenSaturation == -999.0) ? "NULL" : oxygenSaturation.toString(),
+      'ph': (ph == null || ph == -999.0) ? "NULL" : ph.toString(),
+      'salinity': (salinity == null || salinity == -999.0) ? "NULL" : salinity.toString(),
+      'tss': (tss == null || tss == -999.0) ? "NULL" : tss.toString(),
+      'temperature': (temperature == null || temperature == -999.0) ? "NULL" : temperature.toString(),
+      'turbidity': (turbidity == null || turbidity == -999.0) ? "NULL" : turbidity.toString(),
+      'tds': (tds == null || tds == -999.0) ? "NULL" : tds.toString(),
+      'electric_conductivity': (electricalConductivity == null || electricalConductivity == -999.0) ? "NULL" : electricalConductivity.toString(),
       'sample_id': (sampleIdCode ?? "").toString(),
       'tarball': "No",
       'weather': (weather ?? "").toString(),
@@ -320,15 +320,15 @@ class InSituSamplingData {
   /// Creates a JSON object for sensor readings. FORCING ALL VALUES TO STRING.
   String toReadingJson() {
     final data = {
-      'do_mgl': (oxygenConcentration ?? "NULL").toString(),
-      'do_sat': (oxygenSaturation ?? "NULL").toString(),
-      'ph': (ph ?? "NULL").toString(),
-      'salinity': (salinity ?? "NULL").toString(),
-      'tds': (tds ?? "NULL").toString(),
-      'tss': (tss ?? "NULL").toString(),
-      'temperature': (temperature ?? "NULL").toString(),
-      'turbidity': (turbidity ?? "NULL").toString(),
-      'electric_conductivity': (electricalConductivity ?? "NULL").toString(),
+      'do_mgl': (oxygenConcentration == null || oxygenConcentration == -999.0) ? "NULL" : oxygenConcentration.toString(),
+      'do_sat': (oxygenSaturation == null || oxygenSaturation == -999.0) ? "NULL" : oxygenSaturation.toString(),
+      'ph': (ph == null || ph == -999.0) ? "NULL" : ph.toString(),
+      'salinity': (salinity == null || salinity == -999.0) ? "NULL" : salinity.toString(),
+      'tds': (tds == null || tds == -999.0) ? "NULL" : tds.toString(),
+      'tss': (tss == null || tss == -999.0) ? "NULL" : tss.toString(),
+      'temperature': (temperature == null || temperature == -999.0) ? "NULL" : temperature.toString(),
+      'turbidity': (turbidity == null || turbidity == -999.0) ? "NULL" : turbidity.toString(),
+      'electric_conductivity': (electricalConductivity == null || electricalConductivity == -999.0) ? "NULL" : electricalConductivity.toString(),
       'date_sampling_reading': (dataCaptureDate ?? "").toString(),
       'time_sampling_reading': (dataCaptureTime ?? "").toString(),
     };
@@ -352,7 +352,7 @@ class InSituSamplingData {
     final Map<String, String> map = {};
     void add(String key, dynamic value) {
       if (value != null) {
-        String stringValue = (value is double) ? ((value == -999.0) ? 'NULL' : value.toStringAsFixed(5)) : value.toString();
+        String stringValue = (value is double) ? ((value == -999.0) ? '-999' : value.toStringAsFixed(5)) : value.toString();
         if (stringValue.isNotEmpty) map[key] = stringValue;
       }
     }
diff --git a/lib/models/marine_inves_manual_sampling_data.dart b/lib/models/marine_inves_manual_sampling_data.dart
index 2f01433..a815cdb 100644
--- a/lib/models/marine_inves_manual_sampling_data.dart
+++ b/lib/models/marine_inves_manual_sampling_data.dart
@@ -88,6 +88,7 @@ class MarineInvesManualSamplingData {
   });
 
   /// Creates a single JSON object with all submission data for offline storage.
+  /// Logic updated to replace -999.0 with "NULL" for external server compatibility.
   Map<String, dynamic> toDbJson() {
     return {
       // Step 1
@@ -126,16 +127,17 @@ class MarineInvesManualSamplingData {
       'sonde_id': sondeId,
       'data_capture_date': dataCaptureDate,
       'data_capture_time': dataCaptureTime,
-      'oxygen_concentration': oxygenConcentration,
-      'oxygen_saturation': oxygenSaturation,
-      'ph': ph,
-      'salinity': salinity,
-      'electrical_conductivity': electricalConductivity,
-      'temperature': temperature,
-      'tds': tds,
-      'turbidity': turbidity,
-      'tss': tss,
-      'battery_voltage': batteryVoltage,
+      // Handle NULL values for JSON output
+      'oxygen_concentration': (oxygenConcentration == null || oxygenConcentration == -999.0) ? "NULL" : oxygenConcentration,
+      'oxygen_saturation': (oxygenSaturation == null || oxygenSaturation == -999.0) ? "NULL" : oxygenSaturation,
+      'ph': (ph == null || ph == -999.0) ? "NULL" : ph,
+      'salinity': (salinity == null || salinity == -999.0) ? "NULL" : salinity,
+      'electrical_conductivity': (electricalConductivity == null || electricalConductivity == -999.0) ? "NULL" : electricalConductivity,
+      'temperature': (temperature == null || temperature == -999.0) ? "NULL" : temperature,
+      'tds': (tds == null || tds == -999.0) ? "NULL" : tds,
+      'turbidity': (turbidity == null || turbidity == -999.0) ? "NULL" : turbidity,
+      'tss': (tss == null || tss == -999.0) ? "NULL" : tss,
+      'battery_voltage': (batteryVoltage == null || batteryVoltage == -999.0) ? "NULL" : batteryVoltage,
       // Status
       'submission_status': submissionStatus,
       'submission_message': submissionMessage,
@@ -158,7 +160,10 @@ class MarineInvesManualSamplingData {
   factory MarineInvesManualSamplingData.fromJson(Map<String, dynamic> json) {
     double? doubleFromJson(dynamic value) {
       if (value is num) return value.toDouble();
-      if (value is String) return double.tryParse(value);
+      if (value is String) {
+        if (value == "NULL") return -999.0;
+        return double.tryParse(value);
+      }
       return null;
     }
 
@@ -243,7 +248,7 @@ class MarineInvesManualSamplingData {
   }
 
 
-  /// Maps data to keys for the API submission.
+  /// Maps data to keys for the API submission. Preserved exactly as originally provided.
   Map<String, String> toApiFormData() {
     final Map<String, String> map = {};
 
@@ -279,7 +284,7 @@ class MarineInvesManualSamplingData {
 
     add('inves_station_type', stationTypeSelection);
     if (stationTypeSelection == 'Existing Manual Station') {
-      // ✅ FIX: Ensure 'station_id' is added correctly for limit validation
+      // FIX: Ensure 'station_id' is added correctly for limit validation
       add('station_id', selectedStation?['station_id'] ?? selectedStation?['man_station_id']);
       add('inves_station_code', selectedStation?['man_station_code']);
       add('inves_station_name', selectedStation?['man_station_name']);
diff --git a/lib/models/river_in_situ_sampling_data.dart b/lib/models/river_in_situ_sampling_data.dart
index d0ab728..4639433 100644
--- a/lib/models/river_in_situ_sampling_data.dart
+++ b/lib/models/river_in_situ_sampling_data.dart
@@ -121,7 +121,7 @@ class RiverInSituSamplingData {
       ..labRemarks = json['labRemarks'] ?? json['r_man_lab_remark']
       ..sondeId = json['sondeId'] ?? json['r_man_sondeID']
       ..dataCaptureDate = json['dataCaptureDate'] ?? json['data_capture_date']
-      ..dataCaptureTime = json['dataCaptureTime'] ?? json['data_capture_time']
+      ..dataCaptureTime = json['data_capture_time'] ?? json['data_capture_time']
       ..oxygenConcentration = doubleFromJson(json['oxygenConcentration'] ?? json['r_man_oxygen_conc'])
       ..oxygenSaturation = doubleFromJson(json['oxygenSaturation'] ?? json['r_man_oxygen_sat'])
       ..ph = doubleFromJson(json['ph'] ?? json['r_man_ph'])
@@ -311,7 +311,7 @@ class RiverInSituSamplingData {
   /// FORCING ALL VALUES TO STRING.
   String toDbJson() {
     final data = {
-      'battery_cap': (batteryVoltage ?? "NULL").toString(),
+      'battery_cap': (batteryVoltage == null || batteryVoltage == -999.0) ? "NULL" : batteryVoltage.toString(),
       'device_name': (sondeId ?? "").toString(),
       'sampling_type': (samplingType ?? "").toString(),
       'report_id': (reportId ?? "").toString(),
@@ -326,15 +326,15 @@ class RiverInSituSamplingData {
       'record_dt': (samplingDate != null && samplingTime != null)
           ? '$samplingDate $samplingTime'
           : "",
-      'do_mgl': (oxygenConcentration ?? "NULL").toString(),
-      'do_sat': (oxygenSaturation ?? "NULL").toString(),
-      'ph': (ph ?? "NULL").toString(),
-      'salinity': (salinity ?? "NULL").toString(),
-      'temperature': (temperature ?? "NULL").toString(),
-      'turbidity': (turbidity ?? "NULL").toString(),
-      'tds': (tds ?? "NULL").toString(),
-      'electric_conductivity': (electricalConductivity ?? "NULL").toString(),
-      'flowrate': (flowrateValue ?? "NULL").toString(),
+      'do_mgl': (oxygenConcentration == null || oxygenConcentration == -999.0) ? "NULL" : oxygenConcentration.toString(),
+      'do_sat': (oxygenSaturation == null || oxygenSaturation == -999.0) ? "NULL" : oxygenSaturation.toString(),
+      'ph': (ph == null || ph == -999.0) ? "NULL" : ph.toString(),
+      'salinity': (salinity == null || salinity == -999.0) ? "NULL" : salinity.toString(),
+      'temperature': (temperature == null || temperature == -999.0) ? "NULL" : temperature.toString(),
+      'turbidity': (turbidity == null || turbidity == -999.0) ? "NULL" : turbidity.toString(),
+      'tds': (tds == null || tds == -999.0) ? "NULL" : tds.toString(),
+      'electric_conductivity': (electricalConductivity == null || electricalConductivity == -999.0) ? "NULL" : electricalConductivity.toString(),
+      'flowrate': (flowrateValue == null || flowrateValue == -999.0) ? "NULL" : flowrateValue.toString(),
       'odour': "",
       'floatable': "",
       'sample_id': (sampleIdCode ?? "").toString(),
@@ -367,15 +367,15 @@ class RiverInSituSamplingData {
   /// Creates a JSON object for sensor readings. FORCING ALL VALUES TO STRING.
   String toReadingJson() {
     final data = {
-      'do_mgl': (oxygenConcentration ?? "NULL").toString(),
-      'do_sat': (oxygenSaturation ?? "NULL").toString(),
-      'ph': (ph ?? "NULL").toString(),
-      'salinity': (salinity ?? "NULL").toString(),
-      'temperature': (temperature ?? "NULL").toString(),
-      'turbidity': (turbidity ?? "NULL").toString(),
-      'tds': (tds ?? "NULL").toString(),
-      'electric_conductivity': (electricalConductivity ?? "NULL").toString(),
-      'flowrate': (flowrateValue ?? "NULL").toString(),
+      'do_mgl': (oxygenConcentration == null || oxygenConcentration == -999.0) ? "NULL" : oxygenConcentration.toString(),
+      'do_sat': (oxygenSaturation == null || oxygenSaturation == -999.0) ? "NULL" : oxygenSaturation.toString(),
+      'ph': (ph == null || ph == -999.0) ? "NULL" : ph.toString(),
+      'salinity': (salinity == null || salinity == -999.0) ? "NULL" : salinity.toString(),
+      'temperature': (temperature == null || temperature == -999.0) ? "NULL" : temperature.toString(),
+      'turbidity': (turbidity == null || turbidity == -999.0) ? "NULL" : turbidity.toString(),
+      'tds': (tds == null || tds == -999.0) ? "NULL" : tds.toString(),
+      'electric_conductivity': (electricalConductivity == null || electricalConductivity == -999.0) ? "NULL" : electricalConductivity.toString(),
+      'flowrate': (flowrateValue == null || flowrateValue == -999.0) ? "NULL" : flowrateValue.toString(),
       'date_sampling_reading': (dataCaptureDate ?? "").toString(),
       'time_sampling_reading': (dataCaptureTime ?? "").toString(),
     };
diff --git a/lib/models/river_inves_manual_sampling_data.dart b/lib/models/river_inves_manual_sampling_data.dart
index dd2850e..6ef821e 100644
--- a/lib/models/river_inves_manual_sampling_data.dart
+++ b/lib/models/river_inves_manual_sampling_data.dart
@@ -329,7 +329,6 @@ class RiverInvesManualSamplingData {
 
     // Sampler & Time Info (Assuming same API keys as manual)
     add('first_sampler_user_id', firstSamplerUserId);
-    // *** FIX: Changed 'r_inv_' to 'r_inves_' to match API ***
     add('r_inves_second_sampler_id', secondSampler?['user_id']);
     add('r_inves_date', samplingDate);
     add('r_inves_time', samplingTime);
@@ -409,7 +408,6 @@ class RiverInvesManualSamplingData {
   /// Converts the image properties into a Map<String, File?> for the multipart API request.
   /// Keys should match the expected API endpoint fields for Investigative images.
   Map<String, File?> toApiImageFiles() {
-    // *** FIX: Updated keys to 'r_inves_' to match DB/Controller ***
     return {
       'r_inves_background_station': backgroundStationImage,
       'r_inves_upstream_river': upstreamRiverImage,
@@ -425,7 +423,7 @@ class RiverInvesManualSamplingData {
   /// Creates a single JSON object for FTP 'db.json', forcing every value to String.
   String toDbJson() {
     final data = {
-      'battery_cap': (batteryVoltage ?? "").toString(),
+      'battery_cap': (batteryVoltage == null || batteryVoltage == -999.0) ? "NULL" : batteryVoltage.toString(),
       'device_name': (sondeId ?? "").toString(),
       'sampling_type': (samplingType ?? "Investigative").toString(),
       'report_id': (reportId ?? "").toString(),
@@ -438,16 +436,16 @@ class RiverInvesManualSamplingData {
       'latitude': (stationLatitude ?? "").toString(),
       'longitude': (stationLongitude ?? "").toString(),
       'record_dt': '$samplingDate $samplingTime',
-      'do_mgl': (oxygenConcentration ?? -999.0).toString(),
-      'do_sat': (oxygenSaturation ?? -999.0).toString(),
-      'ph': (ph ?? -999.0).toString(),
-      'salinity': (salinity ?? -999.0).toString(),
-      'temperature': (temperature ?? -999.0).toString(),
-      'turbidity': (turbidity ?? -999.0).toString(),
-      'tds': (tds ?? -999.0).toString(),
-      'electric_conductivity': (electricalConductivity ?? -999.0).toString(),
-      'tss': (ammonia ?? 0.0).toString(), // Mapped ammonia to 'tss' key for FTP consistency
-      'flowrate': (flowrateValue ?? -999.0).toString(),
+      'do_mgl': (oxygenConcentration == null || oxygenConcentration == -999.0) ? "NULL" : oxygenConcentration.toString(),
+      'do_sat': (oxygenSaturation == null || oxygenSaturation == -999.0) ? "NULL" : oxygenSaturation.toString(),
+      'ph': (ph == null || ph == -999.0) ? "NULL" : ph.toString(),
+      'salinity': (salinity == null || salinity == -999.0) ? "NULL" : salinity.toString(),
+      'temperature': (temperature == null || temperature == -999.0) ? "NULL" : temperature.toString(),
+      'turbidity': (turbidity == null || turbidity == -999.0) ? "NULL" : turbidity.toString(),
+      'tds': (tds == null || tds == -999.0) ? "NULL" : tds.toString(),
+      'electric_conductivity': (electricalConductivity == null || electricalConductivity == -999.0) ? "NULL" : electricalConductivity.toString(),
+      'tss': (ammonia == null || ammonia == -999.0) ? "NULL" : ammonia.toString(),
+      'flowrate': (flowrateValue == null || flowrateValue == -999.0) ? "NULL" : flowrateValue.toString(),
       'odour': '',
       'floatable': '',
       'sample_id': (sampleIdCode ?? "").toString(),
@@ -491,16 +489,16 @@ class RiverInvesManualSamplingData {
   /// Creates JSON for FTP 'river_inves_reading.json', forcing every value to String.
   String toReadingJson() {
     final data = {
-      'do_mgl': (oxygenConcentration ?? -999.0).toString(),
-      'do_sat': (oxygenSaturation ?? -999.0).toString(),
-      'ph': (ph ?? -999.0).toString(),
-      'salinity': (salinity ?? -999.0).toString(),
-      'temperature': (temperature ?? -999.0).toString(),
-      'turbidity': (turbidity ?? -999.0).toString(),
-      'tds': (tds ?? -999.0).toString(),
-      'electric_conductivity': (electricalConductivity ?? -999.0).toString(),
-      'tss': (ammonia ?? -999.0).toString(),
-      'flowrate': (flowrateValue ?? -999.0).toString(),
+      'do_mgl': (oxygenConcentration == null || oxygenConcentration == -999.0) ? "NULL" : oxygenConcentration.toString(),
+      'do_sat': (oxygenSaturation == null || oxygenSaturation == -999.0) ? "NULL" : oxygenSaturation.toString(),
+      'ph': (ph == null || ph == -999.0) ? "NULL" : ph.toString(),
+      'salinity': (salinity == null || salinity == -999.0) ? "NULL" : salinity.toString(),
+      'temperature': (temperature == null || temperature == -999.0) ? "NULL" : temperature.toString(),
+      'turbidity': (turbidity == null || turbidity == -999.0) ? "NULL" : turbidity.toString(),
+      'tds': (tds == null || tds == -999.0) ? "NULL" : tds.toString(),
+      'electric_conductivity': (electricalConductivity == null || electricalConductivity == -999.0) ? "NULL" : electricalConductivity.toString(),
+      'tss': (ammonia == null || ammonia == -999.0) ? "NULL" : ammonia.toString(),
+      'flowrate': (flowrateValue == null || flowrateValue == -999.0) ? "NULL" : flowrateValue.toString(),
       'date_sampling_reading': (dataCaptureDate ?? "").toString(),
       'time_sampling_reading': (dataCaptureTime ?? "").toString(),
     };
diff --git a/lib/models/river_manual_triennial_sampling_data.dart b/lib/models/river_manual_triennial_sampling_data.dart
index 38b44f0..7f47458 100644
--- a/lib/models/river_manual_triennial_sampling_data.dart
+++ b/lib/models/river_manual_triennial_sampling_data.dart
@@ -160,7 +160,7 @@ class RiverManualTriennialSamplingData {
         String stringValue;
         if (value is double) {
           if (value == -999.0) {
-            stringValue = 'NULL';
+            stringValue = '-999';
           } else {
             stringValue = value.toStringAsFixed(5);
           }
@@ -180,7 +180,7 @@ class RiverManualTriennialSamplingData {
     add('r_tri_time', samplingTime);
     add('r_tri_type', samplingType);
     add('r_tri_sample_id_code', sampleIdCode);
-    add('station_id', selectedStation?['station_id']); // Ensure this is the correct foreign key
+    add('station_id', selectedStation?['station_id']);
     add('r_tri_current_latitude', currentLatitude);
     add('r_tri_current_longitude', currentLongitude);
     add('r_tri_distance_difference', distanceDifferenceInKm);
@@ -199,7 +199,7 @@ class RiverManualTriennialSamplingData {
 
     // Step 3 Data
     add('r_tri_sondeID', sondeId);
-    add('data_capture_date', dataCaptureDate); // Note: Keys likely shared with in-situ for capture time
+    add('data_capture_date', dataCaptureDate);
     add('data_capture_time', dataCaptureTime);
     add('r_tri_oxygen_conc', oxygenConcentration);
     add('r_tri_oxygen_sat', oxygenSaturation);
@@ -303,7 +303,7 @@ class RiverManualTriennialSamplingData {
   /// Every value is explicitly converted to a String for the API.
   String toDbJson() {
     final data = {
-      'battery_cap': (batteryVoltage ?? "NULL").toString(),
+      'battery_cap': (batteryVoltage == null || batteryVoltage == -999.0) ? "NULL" : batteryVoltage.toString(),
       'device_name': (sondeId ?? "").toString(),
       'sampling_type': (samplingType ?? "").toString(),
       'report_id': (reportId ?? "").toString(),
@@ -316,16 +316,16 @@ class RiverManualTriennialSamplingData {
       'latitude': (stationLatitude ?? "").toString(),
       'longitude': (stationLongitude ?? "").toString(),
       'record_dt': '$samplingDate $samplingTime',
-      'do_mgl': (oxygenConcentration ?? "NULL").toString(),
-      'do_sat': (oxygenSaturation ?? "NULL").toString(),
-      'ph': (ph ?? "NULL").toString(),
-      'salinity': (salinity ?? "NULL").toString(),
-      'temperature': (temperature ?? "NULL").toString(),
-      'turbidity': (turbidity ?? "NULL").toString(),
-      'tds': (tds ?? "NULL").toString(),
-      'electric_conductivity': (electricalConductivity ?? "NULL").toString(),
-      'tss': (ammonia ?? "NULL").toString(),
-      'flowrate': (flowrateValue ?? "NULL").toString(),
+      'do_mgl': (oxygenConcentration == null || oxygenConcentration == -999.0) ? "NULL" : oxygenConcentration.toString(),
+      'do_sat': (oxygenSaturation == null || oxygenSaturation == -999.0) ? "NULL" : oxygenSaturation.toString(),
+      'ph': (ph == null || ph == -999.0) ? "NULL" : ph.toString(),
+      'salinity': (salinity == null || salinity == -999.0) ? "NULL" : salinity.toString(),
+      'temperature': (temperature == null || temperature == -999.0) ? "NULL" : temperature.toString(),
+      'turbidity': (turbidity == null || turbidity == -999.0) ? "NULL" : turbidity.toString(),
+      'tds': (tds == null || tds == -999.0) ? "NULL" : tds.toString(),
+      'electric_conductivity': (electricalConductivity == null || electricalConductivity == -999.0) ? "NULL" : electricalConductivity.toString(),
+      'tss': (ammonia == null || ammonia == -999.0) ? "NULL" : ammonia.toString(),
+      'flowrate': (flowrateValue == null || flowrateValue == -999.0) ? "NULL" : flowrateValue.toString(),
       'odour': '',
       'floatable': '',
       'sample_id': (sampleIdCode ?? "").toString(),
@@ -364,16 +364,16 @@ class RiverManualTriennialSamplingData {
   /// Every value is explicitly converted to a String.
   String toReadingJson() {
     final data = {
-      'do_mgl': (oxygenConcentration ?? "NULL").toString(),
-      'do_sat': (oxygenSaturation ?? "NULL").toString(),
-      'ph': (ph ?? "NULL").toString(),
-      'salinity': (salinity ?? "NULL").toString(),
-      'temperature': (temperature ?? "NULL").toString(),
-      'turbidity': (turbidity ?? "NULL").toString(),
-      'tds': (tds ?? "NULL").toString(),
-      'electric_conductivity': (electricalConductivity ?? "NULL").toString(),
-      'tss': (ammonia ?? "NULL").toString(),
-      'flowrate': (flowrateValue ?? "NULL").toString(),
+      'do_mgl': (oxygenConcentration == null || oxygenConcentration == -999.0) ? "NULL" : oxygenConcentration.toString(),
+      'do_sat': (oxygenSaturation == null || oxygenSaturation == -999.0) ? "NULL" : oxygenSaturation.toString(),
+      'ph': (ph == null || ph == -999.0) ? "NULL" : ph.toString(),
+      'salinity': (salinity == null || salinity == -999.0) ? "NULL" : salinity.toString(),
+      'temperature': (temperature == null || temperature == -999.0) ? "NULL" : temperature.toString(),
+      'turbidity': (turbidity == null || turbidity == -999.0) ? "NULL" : turbidity.toString(),
+      'tds': (tds == null || tds == -999.0) ? "NULL" : tds.toString(),
+      'electric_conductivity': (electricalConductivity == null || electricalConductivity == -999.0) ? "NULL" : electricalConductivity.toString(),
+      'tss': (ammonia == null || ammonia == -999.0) ? "NULL" : ammonia.toString(),
+      'flowrate': (flowrateValue == null || flowrateValue == -999.0) ? "NULL" : flowrateValue.toString(),
       'date_sampling_reading': (dataCaptureDate ?? "").toString(),
       'time_sampling_reading': (dataCaptureTime ?? "").toString(),
     };
diff --git a/lib/screens/settings.dart b/lib/screens/settings.dart
index f486ac0..c892e25 100644
--- a/lib/screens/settings.dart
+++ b/lib/screens/settings.dart
@@ -221,7 +221,7 @@ class _SettingsScreenState extends State<SettingsScreen> {
                   ListTile(
                     leading: const Icon(Icons.info_outline),
                     title: const Text('App Version'),
-                    subtitle: const Text('MMS Version 3.12.06'),
+                    subtitle: const Text('MMS Version 3.12.07'),
                     dense: true,
                   ),
                   ListTile(