diff --git a/tests/BasicSimulator.cpp b/tests/BasicSimulator.cpp
index fd08eff2..4c290104 100644
--- a/tests/BasicSimulator.cpp
+++ b/tests/BasicSimulator.cpp
@@ -495,7 +495,10 @@ int main(int argc, char **argv)
 		FlipID(id,Nx*Ny*Nz);
 	}
 */
-	if (! pBC)	FlipID(id,Nx*Ny*Nz);
+	if (! pBC && phi_s > 0.0){
+		FlipID(id,Nx*Ny*Nz);
+		phi_s = -phi_s;
+	}
 	
 #endif
 
diff --git a/tests/lb2_Color_wia_mpi.cpp b/tests/lb2_Color_wia_mpi.cpp
index 67f7cd32..e09d52c4 100644
--- a/tests/lb2_Color_wia_mpi.cpp
+++ b/tests/lb2_Color_wia_mpi.cpp
@@ -516,7 +516,10 @@ int main(int argc, char **argv)
 		FlipID(id,Nx*Ny*Nz);
 	}
 */
-	if (! pBC )	FlipID(id,Nx*Ny*Nz);
+	if (! pBC && phi_s > 0.0){
+		FlipID(id,Nx*Ny*Nz);
+		phi_s = -phi_s;
+	}
 	
 #endif