diff --git a/src/pymorize/units.py b/src/pymorize/units.py
index e34bffba..f231901c 100644
--- a/src/pymorize/units.py
+++ b/src/pymorize/units.py
@@ -29,6 +29,8 @@
 
 ureg = pint_xarray.unit_registry
 
+ureg.define("practical_salinity_unit = g/kg = psu = PSU")
+
 
 def handle_chemicals(
     s: Union[str, None] = None, pattern: Pattern = re.compile(r"mol(?P<symbol>\w+)")
@@ -114,9 +116,13 @@ def handle_unit_conversion(da: xr.DataArray, rule: Rule) -> xr.DataArray:
     else:
         _to_unit = to_unit
     if _to_unit == to_unit:
-        logger.info(f"Converting units: ({da.name} -> {cmor_variable}) {from_unit} -> {to_unit}")
+        logger.info(
+            f"Converting units: ({da.name} -> {cmor_variable}) {from_unit} -> {to_unit}"
+        )
     else:
-        logger.info(f"Converting units: ({da.name} -> {cmor_variable}) {from_unit} -> {_to_unit} ({to_unit})")
+        logger.info(
+            f"Converting units: ({da.name} -> {cmor_variable}) {from_unit} -> {_to_unit} ({to_unit})"
+        )
     new_da = new_da.pint.to(_to_unit).pint.dequantify()
     if new_da.attrs.get("units") != to_unit:
         logger.debug(
diff --git a/tests/unit/test_dimensionless_units.py b/tests/unit/test_dimensionless_units.py
index 18bc26c6..07516232 100644
--- a/tests/unit/test_dimensionless_units.py
+++ b/tests/unit/test_dimensionless_units.py
@@ -28,6 +28,6 @@ def test_units_with_psu():
     new_da = handle_unit_conversion(da, rule)
     assert new_da.attrs.get("units") == "0.001"
     # Check the magnitude of the data
-    # 1 g/kg is approximately 1 psu, so the values should be 10 * 1000
+    # 1 g/kg is approximately 1 psu, so the values should be 10
     # after conversion:
-    assert np.allclose(new_da.values, 10000)
+    assert np.allclose(new_da.values, 10)