diff --git a/swash/processing/post_reflex.py b/swash/processing/post_reflex.py
index 582032e..7833f00 100644
--- a/swash/processing/post_reflex.py
+++ b/swash/processing/post_reflex.py
@@ -44,10 +44,24 @@ f = 1 / dt
 idx = [arg_x0 - 5, arg_x0, arg_x0 + 7]
 wl = watl[arg_t0:, idx]
 
+# Reflex3S
 res = reflex3S(*wl.T, *x[idx], botl[idx].mean(), f, 0.02, 0.2)
 f_, ai_, ar_ = reflex3s(wl.T, x[idx], botl[idx].mean(), f)
 ai, ar, _, _, _, _, fre = res
 
+# Custom PUV
+eta = watl[t > t0, arg_x0]
+u = vel[t > t0, 0, arg_x0]
+
+phi_eta = sgl.welch(eta, f)
+phi_u = sgl.welch(u, f)
+phi_eta_u = sgl.csd(eta, u, f)
+
+R = np.sqrt(
+    (np.abs(phi_eta[1]) + np.abs(phi_u[1]) - 2 * phi_eta_u[1].real)
+    / (np.abs(phi_eta[1]) + np.abs(phi_u[1]) + 2 * phi_eta_u[1].real)
+)
+
 out = pathlib.Path(config.get("post", "out")).joinpath(f"t{t0}x{x0}")
 log.info(f"Saving plots in '{out}'")
 out.mkdir(parents=True, exist_ok=True)
@@ -58,6 +72,7 @@ plt.ylim((0, 1))
 plt.savefig(out.joinpath("reflex3s_pa.pdf"))
 plt.clf()
 plt.plot(f_, ar_ / ai_)
+plt.plot(phi_eta[0], R)
 plt.xlim((0, 0.3))
 plt.ylim((0, 1))
 plt.savefig(out.joinpath("reflex3s.pdf"))