diff --git a/satpy/tests/reader_tests/test_gerb_l2_hr_h5.py b/satpy/tests/reader_tests/test_gerb_l2_hr_h5.py
index 3eb115759f..65c8583b19 100644
--- a/satpy/tests/reader_tests/test_gerb_l2_hr_h5.py
+++ b/satpy/tests/reader_tests/test_gerb_l2_hr_h5.py
@@ -25,6 +25,23 @@
 FNAME = "G4_SEV4_L20_HR_SOL_TH_20190606_130000_V000.hdf"
 
 
+def make_h5_null_string(length):
+    """Make a HDF5 type for a NULL terminated string of fixed length."""
+    dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
+    dt.set_size(7)
+    dt.set_strpad(h5py.h5t.STR_NULLTERM)
+    return dt
+
+
+def write_h5_null_string_att(loc_id, name, s):
+    """Write a NULL terminated string attribute at loc_id."""
+    dt = make_h5_null_string(length=7)
+    name = bytes(name.encode('ascii'))
+    s = bytes(s.encode('ascii'))
+    at = h5py.h5a.create(loc_id, name, dt, h5py.h5s.create(h5py.h5s.SCALAR))
+    at.write(np.array(s, dtype=f'|S{len(s)+1}'))
+
+
 @pytest.fixture(scope="session")
 def gerb_l2_hr_h5_dummy_file(tmp_path_factory):
     """Create a dummy HDF5 file for the GERB L2 HR product."""
@@ -34,77 +51,30 @@ def gerb_l2_hr_h5_dummy_file(tmp_path_factory):
         fid.create_group('/Angles')
         fid['/Angles/Relative Azimuth'] = np.ones(shape=(1237, 1237), dtype=np.dtype('>i2'))
         fid['/Angles/Relative Azimuth'].attrs['Quantisation Factor'] = np.array(0.1, dtype='float64')
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(7)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/Angles/Relative Azimuth'].id, b'Unit', dt, h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'Degree', dtype='|S7'))
         fid['/Angles/Solar Zenith'] = np.ones(shape=(1237, 1237), dtype=np.dtype('>i2'))
         fid['/Angles/Solar Zenith'].attrs['Quantisation Factor'] = np.array(0.1, dtype='float64')
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(7)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/Angles/Solar Zenith'].id, b'Unit', dt, h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'Degree', dtype='|S7'))
+        write_h5_null_string_att(fid['/Angles/Relative Azimuth'].id, 'Unit', 'Degree')
         fid['/Angles/Viewing Azimuth'] = np.ones(shape=(1237, 1237), dtype=np.dtype('>i2'))
         fid['/Angles/Viewing Azimuth'].attrs['Quantisation Factor'] = np.array(0.1, dtype='float64')
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(7)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/Angles/Viewing Azimuth'].id, b'Unit', dt, h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'Degree', dtype='|S7'))
+        write_h5_null_string_att(fid['/Angles/Viewing Azimuth'].id, 'Unit', 'Degree')
         fid['/Angles/Viewing Zenith'] = np.ones(shape=(1237, 1237), dtype=np.dtype('>i2'))
         fid['/Angles/Viewing Zenith'].attrs['Quantisation Factor'] = np.array(0.1, dtype='float64')
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(7)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/Angles/Viewing Zenith'].id, b'Unit', dt, h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'Degree', dtype='|S7'))
+        write_h5_null_string_att(fid['/Angles/Viewing Zenith'].id, 'Unit', 'Degree')
         fid.create_group('/GERB')
         dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
         dt.set_size(3)
         dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/GERB'].id, b'Instrument Identifier', dt, h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'G4', dtype='|S3'))
+        write_h5_null_string_att(fid['/GERB'].id, 'Instrument Identifier', 'G4')
         fid.create_group('/GGSPS')
         fid['/GGSPS'].attrs['L1.5 NANRG Product Version'] = np.array(-1, dtype='int32')
         fid.create_group('/Geolocation')
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(44)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/Geolocation'].id, b'Geolocation File Name', dt, h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'G4_SEV4_L20_HR_GEO_20180111_181500_V010.hdf', dtype='|S44'))
-        fid['/Geolocation'].attrs['Line of Sight North-South Speed'] = np.array(0.0, dtype='float64')
+        write_h5_null_string_att(fid['/Geolocation'].id, 'Geolocation File Name',
+                                 'G4_SEV4_L20_HR_GEO_20180111_181500_V010.hdf')
         fid['/Geolocation'].attrs['Nominal Satellite Longitude (degrees)'] = np.array(0.0, dtype='float64')
-        fid.create_group('/Geolocation/Rectified Grid')
-        fid['/Geolocation/Rectified Grid'].attrs['Grid Orientation'] = np.array(0.0, dtype='float64')
-        fid['/Geolocation/Rectified Grid'].attrs['Lap'] = np.array(0.0, dtype='float64')
-        fid['/Geolocation/Rectified Grid'].attrs['Lop'] = np.array(0.0, dtype='float64')
-        fid['/Geolocation/Rectified Grid'].attrs['Nr'] = np.array(6.610674630916804, dtype='float64')
-        fid['/Geolocation/Rectified Grid'].attrs['Nx'] = np.array(1237, dtype='int32')
-        fid['/Geolocation/Rectified Grid'].attrs['Ny'] = np.array(1237, dtype='int32')
-        fid['/Geolocation/Rectified Grid'].attrs['Xp'] = np.array(618.3333333333334, dtype='float64')
-        fid['/Geolocation/Rectified Grid'].attrs['Yp'] = np.array(617.6666666666666, dtype='float64')
-        fid['/Geolocation/Rectified Grid'].attrs['dx'] = np.array(1207.4379446281002, dtype='float64')
-        fid['/Geolocation/Rectified Grid'].attrs['dy'] = np.array(1203.3201568249945, dtype='float64')
-        fid.create_group('/Geolocation/Rectified Grid/Resolution Flags')
-        fid['/Geolocation/Rectified Grid/Resolution Flags'].attrs['East West'] = np.array(0.014411607, dtype='float64')
-        fid['/Geolocation/Rectified Grid/Resolution Flags'].attrs['North South'] = \
-            np.array(0.014411607, dtype='float64')
         fid.create_group('/Imager')
         fid['/Imager'].attrs['Instrument Identifier'] = np.array(4, dtype='int32')
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(7)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/Imager'].id, b'Type', dt, h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'SEVIRI', dtype='|S7'))
+        write_h5_null_string_att(fid['/Imager'].id, 'Type', 'SEVIRI')
         fid.create_group('/RMIB')
-        fid['/RMIB'].attrs['Product Version'] = np.array(10, dtype='int32')
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(16)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/RMIB'].id, b'Software Identifier', dt, h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'20220812_151631', dtype='|S16'))
         fid.create_group('/Radiometry')
         fid['/Radiometry'].attrs['SEVIRI Radiance Definition Flag'] = np.array(2, dtype='int32')
         fid['/Radiometry/A Values (per GERB detector cell)'] = np.ones(shape=(256,), dtype=np.dtype('>f8'))
@@ -117,78 +87,32 @@ def gerb_l2_hr_h5_dummy_file(tmp_path_factory):
         fid['/Radiometry/Shortwave Correction'].attrs['Quantisation Factor'] = np.array(0.005, dtype='float64')
         fid['/Radiometry/Solar Flux'] = np.ones(shape=(1237, 1237), dtype=np.dtype('>i2'))
         fid['/Radiometry/Solar Flux'].attrs['Quantisation Factor'] = np.array(0.25, dtype='float64')
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(22)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/Radiometry/Solar Flux'].id, b'Unit', dt, h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'Watt per square meter', dtype='|S22'))
+        write_h5_null_string_att(fid['/Radiometry/Solar Flux'].id, 'Unit', 'Watt per square meter')
         fid['/Radiometry/Solar Radiance'] = np.ones(shape=(1237, 1237), dtype=np.dtype('>i2'))
         fid['/Radiometry/Solar Radiance'].attrs['Quantisation Factor'] = np.array(0.05, dtype='float64')
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(36)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/Radiometry/Solar Radiance'].id, b'Unit', dt, h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'Watt per square meter per steradian', dtype='|S36'))
+        write_h5_null_string_att(fid['/Radiometry/Solar Radiance'].id, 'Unit', 'Watt per square meter per steradian')
         fid['/Radiometry/Thermal Flux'] = np.ones(shape=(1237, 1237), dtype=np.dtype('>i2'))
         fid['/Radiometry/Thermal Flux'].attrs['Quantisation Factor'] = np.array(0.25, dtype='float64')
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(22)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/Radiometry/Thermal Flux'].id, b'Unit', dt, h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'Watt per square meter', dtype='|S22'))
+        write_h5_null_string_att(fid['/Radiometry/Thermal Flux'].id, 'Unit', 'Watt per square meter')
         fid['/Radiometry/Thermal Radiance'] = np.ones(shape=(1237, 1237), dtype=np.dtype('>i2'))
         fid['/Radiometry/Thermal Radiance'].attrs['Quantisation Factor'] = np.array(0.05, dtype='float64')
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(36)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/Radiometry/Thermal Radiance'].id, b'Unit', dt, h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'Watt per square meter per steradian', dtype='|S36'))
+        write_h5_null_string_att(fid['/Radiometry/Thermal Radiance'].id, 'Unit', 'Watt per square meter per steradian')
         fid.create_group('/Scene Identification')
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(13)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/Scene Identification'].id, b'Solar Angular Dependency Models Set Version', dt,
-                             h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'CERES_TRMM.1', dtype='|S13'))
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(7)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/Scene Identification'].id, b'Thermal Angular Dependency Models Set Version', dt,
-                             h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'RMIB.3', dtype='|S7'))
-        fid['/Scene Identification/Aerosol Optical Depth IR 1.6'] = np.ones(shape=(1237, 1237), dtype=np.dtype('>i2'))
-        fid['/Scene Identification/Aerosol Optical Depth IR 1.6'].attrs['Quantisation Factor'] = \
-            np.array(0.001, dtype='float64')
-        fid['/Scene Identification/Aerosol Optical Depth VIS 0.6'] = np.ones(shape=(1237, 1237), dtype=np.dtype('>i2'))
-        fid['/Scene Identification/Aerosol Optical Depth VIS 0.6'].attrs['Quantisation Factor'] = \
-            np.array(0.001, dtype='float64')
-        fid['/Scene Identification/Aerosol Optical Depth VIS 0.8'] = np.ones(shape=(1237, 1237), dtype=np.dtype('>i2'))
-        fid['/Scene Identification/Aerosol Optical Depth VIS 0.8'].attrs['Quantisation Factor'] = \
-            np.array(0.001, dtype='float64')
+        write_h5_null_string_att(fid['/Scene Identification'].id,
+                                 'Solar Angular Dependency Models Set Version', 'CERES_TRMM.1')
+        write_h5_null_string_att(fid['/Scene Identification'].id,
+                                 'Thermal Angular Dependency Models Set Version', 'RMIB.3')
         fid['/Scene Identification/Cloud Cover'] = np.ones(shape=(1237, 1237), dtype=np.dtype('uint8'))
         fid['/Scene Identification/Cloud Cover'].attrs['Quantisation Factor'] = np.array(0.01, dtype='float64')
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(8)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/Scene Identification/Cloud Cover'].id, b'Unit', dt, h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'Percent', dtype='|S8'))
+        write_h5_null_string_att(fid['/Scene Identification/Cloud Cover'].id, 'Unit', 'Percent')
         fid['/Scene Identification/Cloud Optical Depth (logarithm)'] = \
             np.ones(shape=(1237, 1237), dtype=np.dtype('>i2'))
         fid['/Scene Identification/Cloud Optical Depth (logarithm)'].attrs['Quantisation Factor'] = \
             np.array(0.00025, dtype='float64')
         fid['/Scene Identification/Cloud Phase'] = np.ones(shape=(1237, 1237), dtype=np.dtype('uint8'))
         fid['/Scene Identification/Cloud Phase'].attrs['Quantisation Factor'] = np.array(0.01, dtype='float64')
-        dt = h5py.h5t.TypeID.copy(h5py.h5t.C_S1)
-        dt.set_size(34)
-        dt.set_strpad(h5py.h5t.STR_NULLTERM)
-        at = h5py.h5a.create(fid['/Scene Identification/Cloud Phase'].id, b'Unit', dt, h5py.h5s.create(h5py.h5s.SCALAR))
-        at.write(np.array(b'Percent (Water=0%,Mixed,Ice=100%)', dtype='|S34'))
-        fid['/Scene Identification/Dust Detection'] = np.ones(shape=(1237, 1237), dtype=np.dtype('uint8'))
-        fid['/Scene Identification/Dust Detection'].attrs['Quantisation Factor'] = np.array(0.01, dtype='float64')
-        fid['/Scene Identification/Solar Angular Dependency Model'] = np.ones(shape=(1237, 1237), dtype=np.dtype('>i2'))
-        fid['/Scene Identification/Surface Type'] = np.ones(shape=(1237, 1237), dtype=np.dtype('uint8'))
-        fid['/Scene Identification/Thermal Angular Dependency Model'] = \
-            np.ones(shape=(1237, 1237), dtype=np.dtype('uint8'))
+        write_h5_null_string_att(fid['/Scene Identification/Cloud Phase'].id, 'Unit',
+                                 'Percent (Water=0%,Mixed,Ice=100%)')
         fid.create_group('/Times')
         fid['/Times/Time (per row)'] = np.ones(shape=(1237,), dtype=np.dtype('|S22'))