diff --git a/TBF/Properties/AssemblyInfo.cs b/TBF/Properties/AssemblyInfo.cs
index bc6ef45b1..7c19c9359 100644
--- a/TBF/Properties/AssemblyInfo.cs
+++ b/TBF/Properties/AssemblyInfo.cs
@@ -32,5 +32,5 @@ using System.Runtime.InteropServices;
 //      Build Number
 //      Revision
 //
-[assembly: AssemblyVersion("3.9.3146.118")]
-[assembly: AssemblyFileVersion("3.9.3146.118")]
+[assembly: AssemblyVersion("3.9.3146.120")]
+[assembly: AssemblyFileVersion("3.9.3146.120")]
diff --git a/TBF/Rig/MettlerToledo/Standard/ReadStableMassOp.cs b/TBF/Rig/MettlerToledo/Standard/ReadStableMassOp.cs
index 2e6c950eb..963666c80 100644
--- a/TBF/Rig/MettlerToledo/Standard/ReadStableMassOp.cs
+++ b/TBF/Rig/MettlerToledo/Standard/ReadStableMassOp.cs
@@ -79,6 +79,17 @@ namespace TBF.Rig.MettlerToledo.Standard
         /// </summary>
         void StartMeasurement()
         {
+            if (scale.MsrmntState == MsrmntState.Underload)
+            {
+                log.Warn("ReadStableMassOp ... underload before measurement, zeroing scale");
+                currentReadingsCount = 0;
+                Array.Clear(massReadings, 0, massReadings.Length);
+                Array.Clear(sortedMassReadings, 0, sortedMassReadings.Length);
+                zeroingAfterUnderload = true;
+                scale.SendZeroWhenStableCmd();
+                return;
+            }
+
             if (method == MassMethod.Scale)
                 scale.SendStableMeasurementCmd();
             else
@@ -95,9 +106,9 @@ namespace TBF.Rig.MettlerToledo.Standard
 
             if (delayBefore == 0 && scale.Activity == Activity.Idle)
             {
-                StartMeasurement();
-                scale.Activity = Activity.RunningOperation;
                 opState = OpState.MassMeasurement;
+                scale.Activity = Activity.RunningOperation;
+                StartMeasurement();
             }
             else
             {
@@ -117,9 +128,9 @@ namespace TBF.Rig.MettlerToledo.Standard
             {
                 if (StateMachine.Time >= startTime + delayBefore && scale.Activity == Activity.Idle)
                 {
-                    StartMeasurement();
-                    scale.Activity = Activity.RunningOperation;
                     opState = OpState.MassMeasurement;
+                    scale.Activity = Activity.RunningOperation;
+                    StartMeasurement();
                 }
 
                 return Event.None;
@@ -132,6 +143,29 @@ namespace TBF.Rig.MettlerToledo.Standard
                 return Event.ScaleDone;
             }
 
+            // A zero response is not a mass sample. Never fall through to measurement
+            // retries after failed zeroing, or the next mass could look like zero success.
+            if (zeroingAfterUnderload)
+            {
+                if (scale.MsrmntState == MsrmntState.Busy)
+                {
+                    // Give stable zeroing its own timeout, independent of the mass method.
+                    if (scale.MsrmntTime < 60) return Event.None;
+                    log.Error("ReadStableMassOp ... zeroing after underload timed out");
+                    scale.MsrmntState = MsrmntState.Failed;
+                    return Event.Error;
+                }
+                if (scale.MsrmntState != MsrmntState.Valid)
+                {
+                    log.ErrorFormat("ReadStableMassOp ... zeroing after underload failed: {0}", scale.MsrmntState);
+                    return Event.Error;
+                }
+                log.Info("ReadStableMassOp ... zeroing after underload completed, restarting measurement");
+                zeroingAfterUnderload = false;
+                StartMeasurement();
+                return Event.None;
+            }
+
 			if (scale.MsrmntState == MsrmntState.Busy)
 			{
                 /// Measurement is in progress - resend command if timeout occurs               
@@ -141,28 +175,7 @@ namespace TBF.Rig.MettlerToledo.Standard
 
             if (scale.MsrmntState == MsrmntState.Underload)
             {
-                log.WarnFormat(
-                    "ReadStableMassOp.Run() ... scale underload detected, zeroing scale");
-
-                currentReadingsCount = 0;
-                Array.Clear(massReadings, 0, massReadings.Length);
-                Array.Clear(sortedMassReadings, 0, sortedMassReadings.Length);
-
-                zeroingAfterUnderload = true;
-                scale.SendZeroWhenStableCmd();
-                scale.Activity = Activity.RunningOperation;
-
-                return Event.None;
-            }
-
-            if (zeroingAfterUnderload && scale.MsrmntState == MsrmntState.Valid)
-            {
-                log.InfoFormat(
-                    "ReadStableMassOp.Run() ... scale zeroing after underload completed, restarting measurement");
-
-                zeroingAfterUnderload = false;
                 StartMeasurement();
-
                 return Event.None;
             }
 
diff --git a/TBF/Rig/Scales/MettlerToledo/ReadStableMassOp.cs b/TBF/Rig/Scales/MettlerToledo/ReadStableMassOp.cs
index 739fe6515..24a711c37 100644
--- a/TBF/Rig/Scales/MettlerToledo/ReadStableMassOp.cs
+++ b/TBF/Rig/Scales/MettlerToledo/ReadStableMassOp.cs
@@ -78,6 +78,17 @@ namespace TBF.Rig.Scales.MettlerToledo
         /// </summary>
         void StartMeasurement()
         {
+            if (scale.MsrmntState == MsrmntState.Underload)
+            {
+                log.Warn("ReadStableMassOp ... underload before measurement, zeroing scale");
+                currentReadingsCount = 0;
+                Array.Clear(massReadings, 0, massReadings.Length);
+                Array.Clear(sortedMassReadings, 0, sortedMassReadings.Length);
+                opState = OpState.ZeroingAfterUnderload;
+                scale.SendZeroWhenStableCmd();
+                return;
+            }
+
             if (method == MassMethod.Scale)
                 scale.SendStableMeasurementCmd();
             else
@@ -93,9 +104,9 @@ namespace TBF.Rig.Scales.MettlerToledo
 
             if (delayBefore == 0 && scale.Activity == Activity.Idle)
             {
-                StartMeasurement();
-                scale.Activity = Activity.RunningOperation;
                 opState = OpState.MassMeasurement;
+                scale.Activity = Activity.RunningOperation;
+                StartMeasurement();
             }
             else
             {
@@ -115,9 +126,9 @@ namespace TBF.Rig.Scales.MettlerToledo
             {
                 if (StateMachine.Time >= startTime + delayBefore && scale.Activity == Activity.Idle)
                 {
-                    StartMeasurement();
-                    scale.Activity = Activity.RunningOperation;
                     opState = OpState.MassMeasurement;
+                    scale.Activity = Activity.RunningOperation;
+                    StartMeasurement();
                 }
 
                 if (scale.DebugLevel == DebugMode.Simulate)
@@ -143,15 +154,35 @@ namespace TBF.Rig.Scales.MettlerToledo
                 return Event.ScaleDone;
             }
 
+            // A zero response is not a mass sample. Never fall through to measurement
+            // retries after failed zeroing, or the next mass could look like zero success.
+            if (opState == OpState.ZeroingAfterUnderload)
+            {
+                if (scale.MsrmntState == MsrmntState.Busy)
+                {
+                    // Give stable zeroing its own timeout, independent of the mass method.
+                    if (scale.MsrmntTime < 60) return Event.None;
+                    log.Error("ReadStableMassOp ... zeroing after underload timed out");
+                    scale.MsrmntState = MsrmntState.Failed;
+                    return Event.Error;
+                }
+                if (scale.MsrmntState != MsrmntState.Valid)
+                {
+                    log.ErrorFormat("ReadStableMassOp ... zeroing after underload failed: {0}", scale.MsrmntState);
+                    return Event.Error;
+                }
+                log.Info("ReadStableMassOp ... zeroing after underload completed, restarting measurement");
+                opState = OpState.MassMeasurement;
+                StartMeasurement();
+                return Event.None;
+            }
+
             if (scale.MsrmntState == MsrmntState.Busy)
             {
                 /// The command is still being processed - resend it after timeout.
                 if (scale.MsrmntTime >= timeout)
                 {
-                    if (opState == OpState.ZeroingAfterUnderload)
-                        scale.SendZeroWhenStableCmd();
-                    else
-                        StartMeasurement();
+                    StartMeasurement();
                 }
 
                 return Event.None;
@@ -159,25 +190,7 @@ namespace TBF.Rig.Scales.MettlerToledo
 
             if (scale.MsrmntState == MsrmntState.Underload)
             {
-                log.Warn("ReadStableMassOp.Run() ... scale underload detected, zeroing scale");
-
-                currentReadingsCount = 0;
-                Array.Clear(massReadings, 0, massReadings.Length);
-                Array.Clear(sortedMassReadings, 0, sortedMassReadings.Length);
-
-                opState = OpState.ZeroingAfterUnderload;
-                scale.SendZeroWhenStableCmd();
-
-                return Event.None;
-            }
-
-            if (opState == OpState.ZeroingAfterUnderload && scale.MsrmntState == MsrmntState.Valid)
-            {
-                log.Info("ReadStableMassOp.Run() ... scale zeroing completed, restarting mass measurement");
-
-                opState = OpState.MassMeasurement;
                 StartMeasurement();
-
                 return Event.None;
             }