diff --git a/bfps/cpp/full_code/test_interpolation.cpp b/bfps/cpp/full_code/test_interpolation.cpp
index 9c4cf62d11f9bece1aea8f06d033e0c0f6de16b7..b7efdebdf8bb54321ec76d4907d1eb77b00e4720 100644
--- a/bfps/cpp/full_code/test_interpolation.cpp
+++ b/bfps/cpp/full_code/test_interpolation.cpp
@@ -116,6 +116,23 @@ int test_interpolation<rnumber>::do_work()
     this->velocity->ift();
     this->nabla_u->ift();
 
+    *this->vorticity = 1.0;
+    *this->velocity = 2.0;
+    *this->nabla_u = 3.0;
+    DEBUG_MSG("changed values\n");
+    DEBUG_MSG("some velocity values: %g %g %g\n",
+            this->velocity->rval(20, 1),
+            this->velocity->rval(200, 2),
+            this->velocity->rval(741, 0));
+    DEBUG_MSG("some vorticity values: %g %g %g\n",
+            this->vorticity->rval(20, 1),
+            this->vorticity->rval(200, 2),
+            this->vorticity->rval(741, 0));
+    DEBUG_MSG("some velocity gradient values: %g %g %g\n",
+            this->nabla_u->rval(20, 1, 0),
+            this->nabla_u->rval(200, 2, 1),
+            this->nabla_u->rval(741, 0, 1));
+
     // allocate interpolation arrays
     std::unique_ptr<double[]> p3data(new double[3*this->ps->getLocalNbParticles()]);
     std::unique_ptr<double[]> p9data(new double[9*this->ps->getLocalNbParticles()]);
@@ -135,6 +152,7 @@ int test_interpolation<rnumber>::do_work()
 
     /// sample velocity at particles' position
     this->ps->sample_compute_field(*this->velocity, p3data.get());
+    DEBUG_MSG("first vel value is %g\n", p3data.get()[0]);
     this->particles_sample_writer_mpi->template save_dataset<3>(
             "tracers0",
             "velocity",
@@ -145,6 +163,7 @@ int test_interpolation<rnumber>::do_work()
             this->ps->get_step_idx()-1);
     /// sample vorticity at particles' position
     this->ps->sample_compute_field(*this->vorticity, p3data.get());
+    DEBUG_MSG("first vort value is %g\n", p3data.get()[0]);
     this->particles_sample_writer_mpi->template save_dataset<3>(
             "tracers0",
             "vorticity",
@@ -155,6 +174,7 @@ int test_interpolation<rnumber>::do_work()
             this->ps->get_step_idx()-1);
     /// sample velocity gradient at particles' position
     this->ps->sample_compute_field(*this->nabla_u, p9data.get());
+    DEBUG_MSG("first vel gradient value is %g\n", p9data.get()[0]);
     this->particles_sample_writer_mpi->template save_dataset<9>(
             "tracers0",
             "velocity_gradient",
diff --git a/bfps/test/test_interpolation.py b/bfps/test/test_interpolation.py
index 6a8cf0fad71af7411f36a1d5a80b00a1b07fa685..a3991a1c37b375d59f6cbb94603aa1292cdbd240 100644
--- a/bfps/test/test_interpolation.py
+++ b/bfps/test/test_interpolation.py
@@ -16,7 +16,7 @@ except ImportError:
 
 
 def main():
-    nparticles = 10
+    nparticles = 100
     c = TEST()
     c.launch(
             ['test_interpolation',
@@ -32,8 +32,13 @@ def main():
     ofile = h5py.File(
             'test_output.h5',
             'r')
+    pos0 = ifile['tracers0/state/0'].value
+    pos1 = ofile['tracers0/position/0'].value
+    print('maximum position error is ', np.max(np.abs(pos0-pos1) / np.abs(pos0)))
     vort0 = ofile['tracers0/vorticity/0'].value
+    print(vort0)
     vel_gradient = ofile['tracers0/velocity_gradient/0'].value
+    print(vel_gradient)
     vort1 = vort0.copy()
     vort1[:, 0] = vel_gradient[:, 7] - vel_gradient[:, 5]
     vort1[:, 1] = vel_gradient[:, 2] - vel_gradient[:, 6]