diff --git a/src/libslic3r/Format/3mf.cpp b/src/libslic3r/Format/3mf.cpp
index 613b7444a..16d86ac28 100644
--- a/src/libslic3r/Format/3mf.cpp
+++ b/src/libslic3r/Format/3mf.cpp
@@ -2860,9 +2860,10 @@ namespace Slic3r {
                                     stream << prefix << SOURCE_OFFSET_Y_KEY  << "\" " << VALUE_ATTR << "=\"" << volume->source.mesh_offset(1) << "\"/>\n";
                                     stream << prefix << SOURCE_OFFSET_Z_KEY  << "\" " << VALUE_ATTR << "=\"" << volume->source.mesh_offset(2) << "\"/>\n";
                                 }
+                                assert(! volume->source.is_converted_from_inches || ! volume->source.is_converted_from_meters);
                                 if (volume->source.is_converted_from_inches)
                                     stream << prefix << SOURCE_IN_INCHES << "\" " << VALUE_ATTR << "=\"1\"/>\n";
-                                if (volume->source.is_converted_from_meters)
+                                else if (volume->source.is_converted_from_meters)
                                     stream << prefix << SOURCE_IN_METERS << "\" " << VALUE_ATTR << "=\"1\"/>\n";
                             }
 
diff --git a/src/libslic3r/Format/AMF.cpp b/src/libslic3r/Format/AMF.cpp
index d03cfd4fa..0312c7f22 100644
--- a/src/libslic3r/Format/AMF.cpp
+++ b/src/libslic3r/Format/AMF.cpp
@@ -1241,9 +1241,10 @@ bool store_amf(const char* path, Model* model, const DynamicPrintConfig* config,
                 stream << "        <metadata type=\"slic3r.source_offset_y\">" << volume->source.mesh_offset(1) << "</metadata>\n";
                 stream << "        <metadata type=\"slic3r.source_offset_z\">" << volume->source.mesh_offset(2) << "</metadata>\n";
             }
+            assert(! volume->source.is_converted_from_inches || ! volume->source.is_converted_from_meters);
             if (volume->source.is_converted_from_inches)
                 stream << "        <metadata type=\"slic3r.source_in_inches\">1</metadata>\n";
-            if (volume->source.is_converted_from_meters)
+            else if (volume->source.is_converted_from_meters)
                 stream << "        <metadata type=\"slic3r.source_in_meters\">1</metadata>\n";
 			stream << std::setprecision(std::numeric_limits<float>::max_digits10);
             const indexed_triangle_set &its = volume->mesh().its;
diff --git a/src/libslic3r/Model.cpp b/src/libslic3r/Model.cpp
index ba743f49c..c45acc048 100644
--- a/src/libslic3r/Model.cpp
+++ b/src/libslic3r/Model.cpp
@@ -460,13 +460,15 @@ void Model::convert_multipart_object(unsigned int max_extruders)
     this->objects.push_back(object);
 }
 
+static constexpr const double volume_threshold_inches = 9.0; // 9 = 3*3*3;
+
 bool Model::looks_like_imperial_units() const
 {
     if (this->objects.size() == 0)
         return false;
 
     for (ModelObject* obj : this->objects)
-        if (obj->get_object_stl_stats().volume < 9.0) // 9 = 3*3*3;
+        if (obj->get_object_stl_stats().volume < volume_threshold_inches)
             return true;
 
     return false;
@@ -474,22 +476,26 @@ bool Model::looks_like_imperial_units() const
 
 void Model::convert_from_imperial_units(bool only_small_volumes)
 {
-    double in_to_mm = 25.4;
+    static constexpr const in_to_mm = 25.4;
     for (ModelObject* obj : this->objects)
-        if (! only_small_volumes || obj->get_object_stl_stats().volume < 9.0) { // 9 = 3*3*3;
+        if (! only_small_volumes || obj->get_object_stl_stats().volume < volume_threshold_inches) {
             obj->scale_mesh_after_creation(Vec3d(in_to_mm, in_to_mm, in_to_mm));
-            for (ModelVolume* v : obj->volumes)
+            for (ModelVolume* v : obj->volumes) {
+                assert(! v->source.is_converted_from_meters);
                 v->source.is_converted_from_inches = true;
+            }
         }
 }
 
+static constexpr const double volume_threshold_meters = 0.001; // 0.001 = 0.1*0.1*0.1
+
 bool Model::looks_like_saved_in_meters() const
 {
     if (this->objects.size() == 0)
         return false;
 
     for (ModelObject* obj : this->objects)
-        if (obj->get_object_stl_stats().volume < 0.001) // 0.001 = 0.1*0.1*0.1;
+        if (obj->get_object_stl_stats().volume < volume_threshold_meters)
             return true;
 
     return false;
@@ -497,12 +503,14 @@ bool Model::looks_like_saved_in_meters() const
 
 void Model::convert_from_meters(bool only_small_volumes)
 {
-    double m_to_mm = 1000;
+    static constexpr const double m_to_mm = 1000;
     for (ModelObject* obj : this->objects)
-        if (! only_small_volumes || obj->get_object_stl_stats().volume < 0.001) { // 0.001 = 0.1*0.1*0.1;
+        if (! only_small_volumes || obj->get_object_stl_stats().volume < volume_threshold_meters) {
             obj->scale_mesh_after_creation(Vec3d(m_to_mm, m_to_mm, m_to_mm));
-            for (ModelVolume* v : obj->volumes)
+            for (ModelVolume* v : obj->volumes) {
+                assert(! v->source.is_converted_from_inches);
                 v->source.is_converted_from_meters = true;
+            }
         }
 }
 
@@ -1075,6 +1083,7 @@ void ModelObject::convert_units(ModelObjectPtrs& new_objects, ConversionType con
                     vol->source.is_converted_from_inches = conv_type == ConversionType::CONV_FROM_INCH;
                 if (conv_type == ConversionType::CONV_FROM_METER || conv_type == ConversionType::CONV_TO_METER)
                     vol->source.is_converted_from_meters = conv_type == ConversionType::CONV_FROM_METER;
+                assert(! vol->source.is_converted_from_inches || ! vol->source.is_converted_from_meters);
             }
             else
                 vol->set_offset(volume->get_offset());
@@ -1827,6 +1836,7 @@ void ModelVolume::transform_this_mesh(const Matrix3d &matrix, bool fix_left_hand
 
 void ModelVolume::convert_from_imperial_units()
 {
+    assert(! this->source.is_converted_from_meters);
     double in_to_mm = 25.4;
     this->scale_geometry_after_creation(Vec3d(in_to_mm, in_to_mm, in_to_mm));
     this->set_offset(Vec3d(0, 0, 0));
@@ -1835,6 +1845,7 @@ void ModelVolume::convert_from_imperial_units()
 
 void ModelVolume::convert_from_meters()
 {
+    assert(! this->source.is_converted_from_inches);
     double m_to_mm = 1000;
     this->scale_geometry_after_creation(Vec3d(m_to_mm, m_to_mm, m_to_mm));
     this->set_offset(Vec3d(0, 0, 0));
diff --git a/src/slic3r/GUI/Plater.cpp b/src/slic3r/GUI/Plater.cpp
index 665ae8088..4da29afdf 100644
--- a/src/slic3r/GUI/Plater.cpp
+++ b/src/slic3r/GUI/Plater.cpp
@@ -2388,8 +2388,8 @@ std::vector<size_t> Plater::priv::load_files(const std::vector<fs::path>& input_
                     //wxMessageDialog msg_dlg(q, _L(
                     MessageDialog msg_dlg(q, _L(
                         "This file contains several objects positioned at multiple heights.\n"
-                        "Instead of considering them as multiple objects, should I consider\n"
-                        "this file as a single object having multiple parts?") + "\n",
+                        "Instead of considering them as multiple objects, should \n"
+                        "should the file be loaded as a single object having multiple parts?") + "\n",
                         _L("Multi-part object detected"), wxICON_WARNING | wxYES | wxNO);
                     if (msg_dlg.ShowModal() == wxID_YES) {
                         model.convert_multipart_object(nozzle_dmrs->values.size());
@@ -3211,9 +3211,10 @@ void Plater::priv::replace_with_stl()
     new_volume->set_material_id(old_volume->material_id());
     new_volume->set_transformation(old_volume->get_transformation());
     new_volume->translate(new_volume->get_transformation().get_matrix(true) * (new_volume->source.mesh_offset - old_volume->source.mesh_offset));
+    assert(! old_volume->source.is_converted_from_inches || ! old_volume->source.is_converted_from_meters);
     if (old_volume->source.is_converted_from_inches)
         new_volume->convert_from_imperial_units();
-    if (old_volume->source.is_converted_from_meters)
+    else if (old_volume->source.is_converted_from_meters)
         new_volume->convert_from_meters();
     new_volume->supported_facets.assign(old_volume->supported_facets);
     new_volume->seam_facets.assign(old_volume->seam_facets);
@@ -3419,9 +3420,10 @@ void Plater::priv::reload_from_disk()
                     new_volume->set_material_id(old_volume->material_id());
                     new_volume->set_transformation(old_volume->get_transformation());
                     new_volume->translate(new_volume->get_transformation().get_matrix(true) * (new_volume->source.mesh_offset - old_volume->source.mesh_offset));
+                    assert(! old_volume->source.is_converted_from_inches || ! old_volume->source.is_converted_from_meters);
                     if (old_volume->source.is_converted_from_inches)
                         new_volume->convert_from_imperial_units();
-                    if (old_volume->source.is_converted_from_meters)
+                    else if (old_volume->source.is_converted_from_meters)
                         new_volume->convert_from_meters();
                     std::swap(old_model_object->volumes[sel_v.volume_idx], old_model_object->volumes.back());
                     old_model_object->delete_volume(old_model_object->volumes.size() - 1);