diff --git a/cara/models.py b/cara/models.py
index 7ea4668e..c876e3af 100644
--- a/cara/models.py
+++ b/cara/models.py
@@ -111,7 +111,7 @@ class PiecewiseconstantFunction:
     def __post_init__(self):
         if len(self.transition_times) != len(self.values)+1:
             raise ValueError("transition_times should contain one more element than values")
-        if sorted(list(set(self.transition_times))) != self.transition_times:
+        if tuple(sorted(set(self.transition_times))) != self.transition_times:
             raise ValueError("transition_times should not contain duplicated elements and should be sorted")
 
     def value(self,time) -> float:
@@ -134,8 +134,8 @@ class PiecewiseconstantFunction:
 
 # Geneva hourly temperatures as piecewise constant function (in Kelvin)
 GenevaTemperatures = {
-    month: PiecewiseconstantFunction(list(range(24)),
-                                     (273.15+np.array(temperatures)).tolist())
+    month: PiecewiseconstantFunction(tuple(range(24)),
+                                     tuple(273.15+np.array(temperatures)))
     for month,temperatures in Geneva_hourly_temperatures_celsius_per_hour.items()
 }
 
diff --git a/cara/tests/test_known_quantities.py b/cara/tests/test_known_quantities.py
index dae53343..350efa68 100644
--- a/cara/tests/test_known_quantities.py
+++ b/cara/tests/test_known_quantities.py
@@ -152,33 +152,33 @@ def test_expiration_aerosols():
 
 def test_piecewiseconstantfunction_wrongarguments():
     # number of values should be 1+number of transition times
-    pytest.raises(ValueError,models.PiecewiseconstantFunction,[0,1],[0,0])
-    pytest.raises(ValueError,models.PiecewiseconstantFunction,[0],[0,0])
+    pytest.raises(ValueError,models.PiecewiseconstantFunction,(0,1),(0,0))
+    pytest.raises(ValueError,models.PiecewiseconstantFunction,(0,),(0,0))
     # two transition times cannot be equal
-    pytest.raises(ValueError,models.PiecewiseconstantFunction,[0,2,2],[0,0])
+    pytest.raises(ValueError,models.PiecewiseconstantFunction,(0,2,2),(0,0))
     # unsorted transition times are not allowed
-    pytest.raises(ValueError,models.PiecewiseconstantFunction,[2,0],[0,0])
+    pytest.raises(ValueError,models.PiecewiseconstantFunction,(2,0),(0,0))
 
 
 def test_piecewiseconstantfunction():
-    transition_times = [0,8,16,24]
-    values = [2,5,8]
+    transition_times = (0,8,16,24)
+    values = (2,5,8)
     fun = models.PiecewiseconstantFunction(transition_times,values)
     assert (fun.value(10) == 5) and (fun.value(20.5) == 8) and \
             (fun.value(8) == 2) and (fun.value(0) == 2) and (fun.value(24) == 8)
 
 
 def test_constantfunction():
-    transition_times = [0,24]
-    values = [20]
+    transition_times = (0,24)
+    values = (20,)
     fun = models.PiecewiseconstantFunction(transition_times,values)
     for t in [0,1,8,10,16,20.1,24]:
         assert (fun.value(t) == 20)
 
 
 def test_piecewiseconstantfunction_vs_interval():
-    transition_times = [0,8,16,24]
-    values = [0,1,0]
+    transition_times = (0,8,16,24)
+    values = (0,1,0)
     fun = models.PiecewiseconstantFunction(transition_times,values)
     interval = models.SpecificInterval(present_times=[(8,16)])
     assert interval.transition_times() == fun.interval().transition_times()
@@ -187,8 +187,8 @@ def test_piecewiseconstantfunction_vs_interval():
 
 
 def test_windowopening():
-    tempOutside = models.PiecewiseconstantFunction([0,10,24],[273.15,283.15])
-    tempInside = models.PiecewiseconstantFunction([0,24],[293.15])
+    tempOutside = models.PiecewiseconstantFunction((0,10,24),(273.15,283.15))
+    tempInside = models.PiecewiseconstantFunction((0,24),(293.15,))
     w = models.WindowOpening(active=models.SpecificInterval([(0,24)]),
                 inside_temp=tempInside,outside_temp=tempOutside,
                 window_height=1.,opening_length=0.6)