diff --git a/Dynamic/PlantSimulator/PlantSimulator.cs b/Dynamic/PlantSimulator/PlantSimulator.cs
index 1aa229f..c62eebf 100644
--- a/Dynamic/PlantSimulator/PlantSimulator.cs
+++ b/Dynamic/PlantSimulator/PlantSimulator.cs
@@ -595,6 +595,21 @@ public bool SimulateSingle(TimeSeriesDataSet inputData, string singleModelName,
             return SimulateSingleInternalCore(inputData, singleModelName, false, out simData);
         }
 
+        /// <summary>
+        /// Simulate a single model to get the output including any additive inputs.
+        /// </summary>
+        /// <param name="inputData"></param>
+        /// <param name="model"></param>
+        /// <param name="simData"></param>
+        /// <returns></returns>
+        public static bool SimulateSingle(TimeSeriesDataSet inputData, ISimulatableModel model, out TimeSeriesDataSet simData)
+        {
+            var plant = new PlantSimulator(new List<ISimulatableModel> { model });
+
+            return plant.Simulate(inputData, out simData );
+        }
+
+
         /// <summary>
         /// Simulates a single model for a unit dataset and adds the output to unitData.Y_meas of the unitData, optionally with noise
         /// </summary>
diff --git a/TimeSeriesAnalysis.Tests/Tests/PlantSimulatorSISOTests.cs b/TimeSeriesAnalysis.Tests/Tests/PlantSimulatorSISOTests.cs
index 70996ad..f30f55b 100644
--- a/TimeSeriesAnalysis.Tests/Tests/PlantSimulatorSISOTests.cs
+++ b/TimeSeriesAnalysis.Tests/Tests/PlantSimulatorSISOTests.cs
@@ -478,6 +478,7 @@ public void BasicPID_SetpointStep_RunsAndConverges(bool delayPidOutputOneSample)
         [TestCase]
         public void BasicPID_CompareSimulateAndSimulateSingle_MustGiveSameResultForDisturbanceEstToWork()
         {
+            //var pidCopy = pidModel1.Clone();
             double newSetpoint = 51;
             int N = 100;
             var plantSim = new PlantSimulator(
@@ -492,13 +493,17 @@ public void BasicPID_CompareSimulateAndSimulateSingle_MustGiveSameResultForDistu
 
             var newSet = new TimeSeriesDataSet();
             newSet.AddSet(inputData);
-        //    var outputSignalName = SignalNamer.GetSignalName(processModel1.GetID(), SignalType.Output_Y);
-       //     newSet.Add(outputSignalName, simData.GetValues(outputSignalName));
             newSet.AddSet(simData);
             newSet.SetTimeStamps(inputData.GetTimeStamps().ToList());
 
-        //    var isOk2 = PlantSimulator.SimulateSingle(newSet, pidModel1, out var simData2);
-            var isOK2 = plantSim.SimulateSingle(newSet, pidModel1.ID,out TimeSeriesDataSet simData2);
+            // slight deviation between SimulateSingle and Simulate regardless of which version is used:
+            //v1
+            //     var isOK2 = plantSim.SimulateSingle(newSet, pidModel1.ID,out TimeSeriesDataSet simData2);
+            //v2
+            // pidCopy.SetInputIDs(pidModel1.GetModelInputIDs());
+            //  var isOk2 = PlantSimulator.SimulateSingle(newSet, pidCopy, out var simData2);
+            //v3
+            var isOk2 = PlantSimulator.SimulateSingle(newSet, pidModel1, out var simData2);
 
             if (true)
             {
diff --git a/docs/articles/processsimulator.md b/docs/articles/processsimulator.md
index 733ee79..c077f12 100644
--- a/docs/articles/processsimulator.md
+++ b/docs/articles/processsimulator.md
@@ -55,13 +55,36 @@ Each model has a number of inputs that travel through the model, each with an id
 In addition, models support adding signals directly to the output. This is a feature intended for modeling disturbances. 
 The IDs of such signals are referred to as *additive input IDs*.
 
-### Estimating disturbances and simulating their effects using PlantSimulator.SimulateSingle()
+### Estimating disturbances and simulating their effects 
 
 If the ``inputData`` given to the PlantSimulator includes measured pid-outputs ``u`` and process outputs ``y`` of feedback loops,
 then PlantSimulator uses that 
-``y_meas = y_proc+d`` 
+``y_meas[k] = y_proc[k-1]+d[k]`` 
 calculate the disturbance vector ``d`` and this disturbance is then simulated as the ``driving force`` of closed loop dynamics. 
 
 Thus when the process model is known and inputs to the model are given, the method ``PlantSimulator.SimulateSingle()`` is used to 
 simulate the disturbance vector as part of the initialization of ``PlantSimulator.Simulate()``
 
+### Closed loops : simulation order and disturbance
+
+In a closed loop the simulation order will be
+
+- *PidModel* reads ``y_meas[k]`` and ``y_setpoint[k]`` and calculates ``u_pid[k]``
+- the process model (usally a *UnitModel*) gives ``u_pid[k]`` and any other inputs ``u[k]`` and outputs an internal state ``y_proc[k]``
+- In the next iteration y_meas[k+1] = y_proc[k] + d[k+1]
+
+This is by convention, and is how the simulator avoids both reading to ymeas[k] and writing to ymeas[k] in the same iteration.
+
+This above means that implicitly all closed-loop processes in a manner of speaking have a time delay of 1. 
+
+It is somewhat challenging to define this behavior so that it is consistent when a model is for instance open-loop tested, then a pid-control is applied. 
+A static process by definition has ``y[k] = f(u[k])`` but in a closed loop that could mean that the measurement y[k] that is used to determine u_pid[k]
+is again overwritten by the output of astatic process. 
+
+It coudl also be possible to deal with this by stating that PID-controllers by convention deal with the signals of the previous step, but so that
+u_pid[k] = f(y_set[k], y_meas[k-1]), but this does not match real-world industrial data, it introduces a lag. It may be that in some cases pid-controllers
+are implemented like this, but often the analysis is done on down-sampled data, and in that case ``u_pid[k]`` appears simultanous to changes in ``y_meas[k]``
+ 
+**Implicitly the above also defines how to interpret the disturbance d[k].** To be extremely precise with how this is defined is important, as the PlantSimulator is
+used internally to back-calculte disturbances as is described in the above section, and how the distrubance is calcualted will again be important as both single simulations and co-simulations 
+are used by ClosedLoopUnitIdentifier to identify the process model including possibly time constants and time-delays. 
\ No newline at end of file