diff --git a/ConeCalorimeter/Services/ExperimentDataService.cs b/ConeCalorimeter/Services/ExperimentDataService.cs
index d379ac9..f08b20a 100644
--- a/ConeCalorimeter/Services/ExperimentDataService.cs
+++ b/ConeCalorimeter/Services/ExperimentDataService.cs
@@ -8,12 +8,14 @@ namespace ConeCalorimeter.Services;
 
 public sealed class ExperimentDataService : IExperimentDataService
 {
-    private const int MaximumRows = 1200;
     private const double TotalHeatReleaseDivisor = 1000;
+    private static readonly TimeSpan TimerInterval = TimeSpan.FromMilliseconds(250);
+    private static readonly TimeSpan DevicePollInterval = TimeSpan.FromSeconds(1);
 
     private readonly IRealtimeDataService _realtimeDataService;
     private readonly DispatcherTimer _timer;
-    private readonly DateTime _startedAt = DateTime.Now;
+    private readonly Stopwatch _testClock = new();
+    private DateTime _lastDevicePollStartedAtUtc = DateTime.MinValue;
     private double? _initialMass;
     private double? _previousMass;
     private int? _previousMassSeconds;
@@ -28,13 +30,13 @@ public sealed class ExperimentDataService : IExperimentDataService
     {
         _realtimeDataService = realtimeDataService;
         Records = [];
-        CurrentSnapshot = _realtimeDataService.GetCurrentSnapshot(TimeSpan.Zero);
+        CurrentSnapshot = BuildIdleSnapshot(_realtimeDataService.GetCurrentSnapshot(TimeSpan.Zero));
 
         _timer = new DispatcherTimer
         {
-            Interval = TimeSpan.FromSeconds(1)
+            Interval = TimerInterval
         };
-        _timer.Tick += async (_, _) => await RecordCurrentSnapshotAsync();
+        _timer.Tick += async (_, _) => await TickAsync();
         _timer.Start();
     }
 
@@ -49,21 +51,33 @@ public sealed class ExperimentDataService : IExperimentDataService
         Records.Clear();
         ResetComputedState();
         _isTestRunning = true;
+        _testClock.Restart();
 
         if (double.IsFinite(CurrentSnapshot.CurrentMass))
         {
             _initialMass = CurrentSnapshot.CurrentMass;
         }
 
+        RecordElapsedSnapshots();
+        QueueImmediateDevicePoll();
+
         Log.Information(
-            "Experiment test started. InitialMass={InitialMass}, SnapshotSeconds={TestSeconds}.",
+            "Experiment test started. InitialMass={InitialMass}, SystemSeconds={TestSeconds}.",
             _initialMass,
             CurrentSnapshot.TestSeconds);
     }
 
     public void StopTest()
     {
+        if (_isTestRunning)
+        {
+            RecordElapsedSnapshots();
+        }
+
         _isTestRunning = false;
+        _testClock.Stop();
+        PublishSnapshot(BuildIdleSnapshot(CurrentSnapshot));
+
         Log.Information(
             "Experiment test stopped. Records={RecordCount}, LastRecordedSeconds={LastRecordedSeconds}.",
             Records.Count,
@@ -76,24 +90,40 @@ public sealed class ExperimentDataService : IExperimentDataService
 
         if (!_isTestRunning)
         {
+            _testClock.Reset();
             ResetComputedState();
+            PublishSnapshot(BuildIdleSnapshot(CurrentSnapshot));
         }
     }
 
-    private async Task RecordCurrentSnapshotAsync()
+    private async Task TickAsync()
     {
-        if (_isPollingSnapshot)
+        if (_isTestRunning)
+        {
+            RecordElapsedSnapshots();
+        }
+
+        await RefreshDeviceSnapshotAsync();
+    }
+
+    private void QueueImmediateDevicePoll()
+    {
+        _lastDevicePollStartedAtUtc = DateTime.MinValue;
+        _ = RefreshDeviceSnapshotAsync();
+    }
+
+    private async Task RefreshDeviceSnapshotAsync()
+    {
+        if (!TryBeginDevicePoll())
         {
             return;
         }
 
-        _isPollingSnapshot = true;
-
         try
         {
-            var elapsed = DateTime.Now - _startedAt;
+            var elapsed = _isTestRunning ? _testClock.Elapsed : TimeSpan.Zero;
             var snapshot = await Task.Run(() => _realtimeDataService.GetCurrentSnapshot(elapsed));
-            RecordSnapshot(snapshot);
+            PublishSnapshot(BuildDisplaySnapshot(snapshot));
         }
         catch (Exception ex)
         {
@@ -106,31 +136,102 @@ public sealed class ExperimentDataService : IExperimentDataService
         }
     }
 
-    private void RecordSnapshot(RealtimeSnapshot snapshot)
+    private bool TryBeginDevicePoll()
     {
-        var massSnapshot = ApplyMassLoss(snapshot);
-        var accumulatedSnapshot = ApplyAccumulatedTotals(massSnapshot);
-
-        CurrentSnapshot = accumulatedSnapshot;
-
-        if (_isTestRunning && ShouldRecordSnapshot(accumulatedSnapshot))
+        if (_isPollingSnapshot)
         {
-            Records.Add(RealtimeDataRecord.FromSnapshot(accumulatedSnapshot));
-            _lastRecordedSeconds = accumulatedSnapshot.TestSeconds;
-
-            while (Records.Count > MaximumRows)
-            {
-                Records.RemoveAt(0);
-            }
+            return false;
         }
 
-        SnapshotUpdated?.Invoke(this, accumulatedSnapshot);
+        var now = DateTime.UtcNow;
+        if (now - _lastDevicePollStartedAtUtc < DevicePollInterval)
+        {
+            return false;
+        }
+
+        _lastDevicePollStartedAtUtc = now;
+        _isPollingSnapshot = true;
+        return true;
     }
 
-    private bool ShouldRecordSnapshot(RealtimeSnapshot snapshot)
+    private void RecordElapsedSnapshots()
     {
-        return snapshot.TestSeconds >= 0
-            && (!_lastRecordedSeconds.HasValue || snapshot.TestSeconds > _lastRecordedSeconds.Value);
+        if (!_isTestRunning)
+        {
+            return;
+        }
+
+        var currentSecond = GetElapsedTestSeconds();
+        var nextSecond = _lastRecordedSeconds.HasValue ? _lastRecordedSeconds.Value + 1 : 0;
+
+        for (var second = nextSecond; second <= currentSecond; second++)
+        {
+            var timedSnapshot = CurrentSnapshot with
+            {
+                TestSeconds = second
+            };
+            var massSnapshot = ApplyMassLoss(timedSnapshot);
+            var accumulatedSnapshot = ApplyAccumulatedTotals(massSnapshot);
+
+            PublishSnapshot(accumulatedSnapshot);
+            Records.Add(RealtimeDataRecord.FromSnapshot(accumulatedSnapshot));
+            _lastRecordedSeconds = second;
+        }
+    }
+
+    private RealtimeSnapshot BuildDisplaySnapshot(RealtimeSnapshot snapshot)
+    {
+        if (!_isTestRunning)
+        {
+            return BuildIdleSnapshot(snapshot);
+        }
+
+        var timedSnapshot = snapshot with
+        {
+            TestSeconds = GetElapsedTestSeconds()
+        };
+
+        if (!_initialMass.HasValue && double.IsFinite(timedSnapshot.CurrentMass))
+        {
+            _initialMass = timedSnapshot.CurrentMass;
+        }
+
+        var massLoss = _initialMass.HasValue && double.IsFinite(timedSnapshot.CurrentMass)
+            ? _initialMass.Value - timedSnapshot.CurrentMass
+            : double.NaN;
+
+        return timedSnapshot with
+        {
+            InitialMass = _initialMass ?? double.NaN,
+            MassLoss = massLoss,
+            MassLossRate = CurrentSnapshot.MassLossRate,
+            TotalHeatRelease = _accumulatedTotalHeatRelease,
+            TotalSmoke = _accumulatedTotalSmoke
+        };
+    }
+
+    private RealtimeSnapshot BuildIdleSnapshot(RealtimeSnapshot snapshot)
+    {
+        return snapshot with
+        {
+            TestSeconds = -1,
+            InitialMass = _initialMass ?? double.NaN,
+            MassLoss = double.NaN,
+            MassLossRate = double.NaN,
+            TotalHeatRelease = double.NaN,
+            TotalSmoke = double.NaN
+        };
+    }
+
+    private int GetElapsedTestSeconds()
+    {
+        return Math.Max(0, (int)Math.Floor(_testClock.Elapsed.TotalSeconds));
+    }
+
+    private void PublishSnapshot(RealtimeSnapshot snapshot)
+    {
+        CurrentSnapshot = snapshot;
+        SnapshotUpdated?.Invoke(this, snapshot);
     }
 
     private RealtimeSnapshot ApplyAccumulatedTotals(RealtimeSnapshot snapshot)
diff --git a/ConeCalorimeter/Services/ModbusRealtimeDataService.cs b/ConeCalorimeter/Services/ModbusRealtimeDataService.cs
index b8892bc..d81a8ca 100644
--- a/ConeCalorimeter/Services/ModbusRealtimeDataService.cs
+++ b/ConeCalorimeter/Services/ModbusRealtimeDataService.cs
@@ -26,7 +26,6 @@ public sealed class ModbusRealtimeDataService : IRealtimeDataService
     private const ushort SmokeProductionRegister = 392;
     private const ushort IrradianceRegister = 410;
     private const ushort IgnitionSecondsRegister = 1014;
-    private const ushort TestSecondsRegister = 1015;
     private const ushort M3FlameMonitorBit = 3;
     private static readonly ModbusFloatByteOrder[] RealtimeFloatByteOrders =
     [
@@ -85,10 +84,17 @@ public sealed class ModbusRealtimeDataService : IRealtimeDataService
             MassLoss: double.NaN,
             MassLossRate: double.NaN,
             IgnitionSeconds: ReadInt16OrEmpty(IgnitionSecondsRegister),
-            TestSeconds: ReadInt16OrEmpty(TestSecondsRegister),
+            TestSeconds: ToElapsedSeconds(elapsed),
             TotalSmoke: double.NaN);
     }
 
+    private static int ToElapsedSeconds(TimeSpan elapsed)
+    {
+        return elapsed >= TimeSpan.Zero
+            ? (int)Math.Floor(elapsed.TotalSeconds)
+            : -1;
+    }
+
     private double ReadRangedFloatOrEmpty(string label, ushort registerAddress, double minimum, double maximum)
     {
         if (!_tcpDeviceConnectionService.TryReadFloatValues(registerAddress, out var result))
diff --git a/ConeCalorimeter/ViewModels/TestPageViewModel.cs b/ConeCalorimeter/ViewModels/TestPageViewModel.cs
index a3ae483..2b916da 100644
--- a/ConeCalorimeter/ViewModels/TestPageViewModel.cs
+++ b/ConeCalorimeter/ViewModels/TestPageViewModel.cs
@@ -431,8 +431,8 @@ public sealed class TestPageViewModel : PageViewModel
         {
             if (isStarting)
             {
-                _experimentDataService.StartTest();
                 ClearPlotSeries();
+                _experimentDataService.StartTest();
             }
             else
             {