Fix converting to offset units of higher dimension e.g. gauge pressure

CHANGES

@@ -10,6 +10,8 @@ Pint Changelog
   (PR #1926, fixes Issue #1841)
 - Fix LaTeX siuntix formatting when using non_int_type=decimal.Decimal.
 
+- Fix converting to offset units of higher dimension e.g. gauge pressure (#1949).
+-
 
 0.23 (2023-12-08)
 -----------------

pint/facets/nonmultiplicative/registry.py

@@ -192,7 +192,7 @@ def _add_ref_of_log_or_offset_unit(
         self, offset_unit: str, all_units: UnitsContainer
     ) -> UnitsContainer:
         slct_unit = self._units[offset_unit]
-        if slct_unit.is_logarithmic or (not slct_unit.is_multiplicative):
+        if slct_unit.is_logarithmic:
             # Extract reference unit
             slct_ref = slct_unit.reference
 
@@ -204,6 +204,11 @@ def _add_ref_of_log_or_offset_unit(
                 (u, e) = [(u, e) for u, e in slct_ref.items()].pop()
                 # Add it back to the unit list
                 return all_units.add(u, e)
+
+        if not slct_unit.is_multiplicative:  # is offset unit
+            # Extract reference unit
+            return slct_unit.reference
+
         # Otherwise, return the units unmodified
         return all_units
 

pint/testsuite/test_issues.py

@@ -1147,7 +1147,7 @@ def test_issue1725(registry_empty):
     assert registry_empty.get_compatible_units("dollar") == set()
 
 
-def test_issues_1505():
+def test_issue1505():
     ur = UnitRegistry(non_int_type=decimal.Decimal)
 
     assert isinstance(ur.Quantity("1m/s").magnitude, decimal.Decimal)
@@ -1203,6 +1203,16 @@ def test_issues_1841_xfail():
     # print(q)
 
 
+def test_issue1949(registry_empty):
+    ureg = UnitRegistry()
+    ureg.define(
+        "in_Hg_gauge = 3386389 * gram / metre / second ** 2; offset:101325000 = inHg_g = in_Hg_g = inHg_gauge"
+    )
+    q = ureg.Quantity("1 atm").to("inHg_gauge")
+    assert q.units == ureg.in_Hg_gauge
+    assert_equal(q.magnitude, 0.0)
+
+
 @pytest.mark.parametrize(
     "given,expected",
     [