diff --git a/src/pspm_cfg/pspm_cfg_selector_channel_action.m b/src/pspm_cfg/pspm_cfg_selector_channel_action.m
index 1e3e71bb..e82e3ab2 100644
--- a/src/pspm_cfg/pspm_cfg_selector_channel_action.m
+++ b/src/pspm_cfg/pspm_cfg_selector_channel_action.m
@@ -6,4 +6,4 @@
 channel_action.values = {'add', 'replace'};
 channel_action.labels = {'Add', 'Replace'};
 channel_action.val = {'add'};
-channel_action.help = {'Choose whether to add a new channel, or to replace the last existing channel of the same type and with the same units (if any).'};
+channel_action.help = {'Choose whether to add a new channel, or to replace the processed channel.'};
diff --git a/src/pspm_convert_au2unit.m b/src/pspm_convert_au2unit.m
index 55b6667a..207c4404 100644
--- a/src/pspm_convert_au2unit.m
+++ b/src/pspm_convert_au2unit.m
@@ -11,7 +11,7 @@
 %   input data if the recording method is area. This is performed to always
 %   return linear units.
 %   Using the given variables, the following calculations are performed:
-%   0. Take square root of data if recording is 'area'.
+%   0. Take 2*sqrt(data/pi) of data if recording is 'area'.
 %   1. Let from unit to reference_unit converted recording distance be Dconv.
 %   2. x ← A*(Dconv/Dref)*x
 %   3. Convert x from ref_unit to unit.
@@ -163,25 +163,32 @@
         [sts, channeldata, infos, pos_of_channel(i)] = pspm_load_channel(alldata, channel{i}, 'pupil');
             if sts < 1, return; end
             % recursive call to avoid the formula being stated twice in the same function
-            [sts, convert_data.data{i}] = pspm_convert_au2unit(channeldata.data, unit, distance, record_method, ...
+            [sts, convert_data{i}.data] = pspm_convert_au2unit(channeldata.data, unit, distance, record_method, ...
                 multiplicator, reference_distance, reference_unit, options);
             if sts < 1, return; end
             convert_data{i}.header = channeldata.header;
             convert_data{i}.header.units = unit;
         end
-        [f_sts, f_info] = pspm_write_channel(fn, convert_data, options.channel_action, struct('channel', pos_of_channel));
-        if f_sts < 1, return; end
+        [sts, f_info] = pspm_write_channel(fn, convert_data, options.channel_action, struct('channel', pos_of_channel));
+        if sts < 1, return; end
         outchannel = f_info.channel;
      % convert data
     case 'data'
         convert_data = data;
         if strcmpi(record_method, 'area')
-            convert_data = sqrt(convert_data);
+            [f_sts, convert_data] = pspm_convert_area2diameter(convert_data);
+            if f_sts < 1, return; end
         end
-        [~, distance] = pspm_convert_unit(distance, unit, reference_unit);
+
+        [f_sts, distance] = pspm_convert_unit(distance, unit, reference_unit);
+        if f_sts < 1, return; end
         convert_data = multiplicator * (distance / reference_distance) * convert_data;
+
         %% convert data from reference_unit to unit
-        [~, convert_data] = pspm_convert_unit(convert_data, reference_unit, unit);
+        [f_sts, convert_data] = pspm_convert_unit(convert_data, reference_unit, unit);
+        if f_sts < 1, return; end
         outchannel = convert_data;
+        sts = 1;
 end
-sts = 1;
+ 
+end
\ No newline at end of file
diff --git a/src/pspm_write_channel.m b/src/pspm_write_channel.m
index 2f3dbd7d..b716b896 100644
--- a/src/pspm_write_channel.m
+++ b/src/pspm_write_channel.m
@@ -111,7 +111,7 @@
         for iChannel = 1:numel(newdata)
             warning off
             [sts, ~, pos_of_channels] = pspm_select_channels(data, ...
-                newdata{iChannel}.header.chantype, newdata{iChannel}.header.units);
+                newdata{iChannel}.header.chantype);
             warning on
             if sts < 1
                 channeli(iChannel) = 0;
@@ -132,8 +132,7 @@
                 warning off
                 sts = pspm_select_channels( ...
                     data(options.channel(iChannel)), ...
-                    newdata{iChannel}.header.chantype, ...
-                    newdata{iChannel}.header.units);
+                    newdata{iChannel}.header.chantype);
                 warning on
                 if sts < 1
                     channeli(iChannel) = 0;
diff --git a/test/pspm_convert_au2unit_test.m b/test/pspm_convert_au2unit_test.m
index b13056d9..2fb4c055 100644
--- a/test/pspm_convert_au2unit_test.m
+++ b/test/pspm_convert_au2unit_test.m
@@ -1,6 +1,230 @@
 classdef pspm_convert_au2unit_test < pspm_testcase
-  % ● Description
-  % unittest class for the pspm_convert_au2unit function
-  % ● Authorship
-  % (C) 2019 Eshref Yozdemir (University of Zurich)
+% ● Description
+% unittest class for the pspm_convert_au2unit function
+% ● Authorship
+% (C) 2019 Eshref Yozdemir (University of Zurich)
+% Updated in 2026 by Bernhard von Raußendorf
+
+methods (Test)
+
+%% data mode
+%   [sts, converted_data] = pspm_convert_au2unit(data, unit, distance, record_method,
+%                           multiplicator, reference_distance, reference_unit, options)
+function testDiameterSameUnits(testCase)
+    [sts, out] = pspm_convert_au2unit(20, 'mm', 60, 'diameter', 0.1, 50, 'mm');
+    testCase.verifyEqual(sts, 1);
+    testCase.verifyEqual(out, 2.4);
 end
+function testDiameterUnitConversion(testCase)
+    [sts, out] = pspm_convert_au2unit(30, 'cm', 6, 'diameter', 0.2, 50, 'mm');
+    testCase.verifyEqual(sts, 1);
+    testCase.verifyEqual(out, 0.72);
+end
+function testAreaSameUnits(testCase)
+    data = 100;
+    [sts, out] = pspm_convert_au2unit(data, 'mm', 60, 'area', 0.1, 50, 'mm');
+    testCase.verifyEqual(sts, 1);
+    
+    % expected with formula diameter = 2.*sqrt(area./pi);
+    expected_from_formula = 0.1 * (60 / 50) * 2*sqrt(data./pi);
+    testCase.verifyEqual(out, expected_from_formula);
+
+    % expected with [sts, diameter] = pspm_convert_area2diameter(area)
+    [sts, diam] = pspm_convert_area2diameter(data);
+    expected_from_pspm = 0.1 * (60 / 50) * diam;
+    testCase.verifyEqual(out, expected_from_pspm);
+
+end
+function testAreaUnitConversion(testCase)
+    data = 400;
+    [sts, out] = pspm_convert_au2unit(data, 'mm', 100, 'area', 0.1, 100, 'm'); % reference_unit in m
+    testCase.verifyEqual(sts, 1);
+
+    
+    % expected with formula diameter = 2.*sqrt(area./pi);
+    % ref. units in m: Dconv 100mm -> 0.1 m and A(Dconv/Dref) m -> mm (*1000)
+    expected_from_formula = 0.1 * (0.1 / 100) * 2 * sqrt(data ./ pi) * 1000;
+    testCase.verifyEqual(out, expected_from_formula);
+
+    % expected with [sts, diameter] = pspm_convert_area2diameter(area)
+    [sts, diam] = pspm_convert_area2diameter(data);
+    expected_from_pspm = 0.1 * (0.1 / 100) * diam * 1000;
+    testCase.verifyEqual(out, expected_from_pspm);
+
+
+    
+end
+function testVectorAreaSameUnits(testCase)
+    data = [25 36 49]
+    [sts, out] = pspm_convert_au2unit(data, 'mm', 80, 'area', 0.05, 40, 'mm');
+    testCase.verifyEqual(sts, 1);
+
+
+    % expected with formula diameter = 2.*sqrt(area./pi);
+    expected_from_formula = 0.05 * (80 / 40) * 2 * sqrt(data ./ pi) ;
+    testCase.verifyEqual(out, expected_from_formula);
+
+    % expected with [sts, diameter] = pspm_convert_area2diameter(area)
+    [sts, diam] = pspm_convert_area2diameter(data);
+    expected_from_pspm = 0.05 * (80 / 40) * diam ;
+    testCase.verifyEqual(out, expected_from_pspm);
+
+end
+function testVectorAreaUnitConversion(testCase)
+    data = [100 400 900];
+    [sts,out]=pspm_convert_au2unit(data,'mm',100,'area',0.1,50,'m'); 
+    testCase.verifyEqual(sts,1); 
+
+    % expected with formula diameter = 2.*sqrt(area./pi);
+    expected_from_formula = 0.1 * (0.1 / 50) * 2 * sqrt(data ./ pi) * 1000 ;
+    testCase.verifyEqual(out, expected_from_formula);
+
+    % expected with [sts, diameter] = pspm_convert_area2diameter(area)
+    [sts, diam] = pspm_convert_area2diameter(data);
+    expected_from_pspm = 0.1 * (0.1 / 50) * diam * 1000;
+    testCase.verifyEqual(out, expected_from_pspm);
+end
+function testVectorAreaUnitConversionTEST(testCase)
+    data = [100 400 900];
+    [sts,out]=pspm_convert_au2unit(data,'mm',100,'area',0.1,50,'mm'); 
+    testCase.verifyEqual(sts,1); 
+    % 
+    % % expected with formula diameter = 2.*sqrt(area./pi);
+    % expected_from_formula = 0.1 * (0.1 / 50) * 2 * sqrt(data ./ pi) * 1000 ;
+    % testCase.verifyEqual(out, expected_from_formula);
+    % 
+    % % expected with [sts, diameter] = pspm_convert_area2diameter(area)
+    % [sts, diam] = pspm_convert_area2diameter(data);
+    % expected_from_pspm = 0.1 * (0.1 / 50) * diam * 1000;
+    % testCase.verifyEqual(out, expected_from_pspm);
+
+    data2 = unit2au(out,'mm',100,'area',0.1,50,'mm');
+    testCase.verifyEqual(data   , data2, 'AbsTol', 1e-12);
+
+    
+
+end
+
+%% Error handeling
+function testErrorHandling(testCase)
+    % invalid inputs
+    [sts,out] = pspm_convert_au2unit();
+    testCase.verifyEqual(sts,-1);
+    testCase.verifyEmpty(out);
+    
+    [sts,out] = pspm_convert_au2unit(20);
+    testCase.verifyEqual(sts,-1);
+    testCase.verifyEmpty(out);
+    
+    [sts,out] = pspm_convert_au2unit(20,'mm');
+    testCase.verifyEqual(sts,-1);
+    testCase.verifyEmpty(out);
+    
+    [sts,out] = pspm_convert_au2unit(20,'mm',60);
+    testCase.verifyEqual(sts,-1);
+    testCase.verifyEmpty(out);
+    % invalid unit inputs
+    [sts,out] = pspm_convert_au2unit(20,'mm',60,'wrong',0.1,50,'mm');
+    testCase.verifyEqual(sts,-1);
+    testCase.verifyEmpty(out);
+    
+    [sts,out] = pspm_convert_au2unit(20,'mm','far','diameter',0.1,50,'mm');
+    testCase.verifyEqual(sts,-1);
+    testCase.verifyEmpty(out);
+    
+    [sts,out] = pspm_convert_au2unit(20,'mm',60,'diameter','bad',50,'mm');
+    testCase.verifyEqual(sts,-1);
+    testCase.verifyEmpty(out);
+    
+    [sts,out] = pspm_convert_au2unit(20,'mm',60,'diameter',0.1,'bad','mm');
+    testCase.verifyEqual(sts,-1);
+    testCase.verifyEmpty(out);
+end
+
+%% Pspm files
+function testFileRoundTripConvertAu2Unit(testCase)
+    fn = '/home/bernd/git/PsPM/ImportTestData/eyelink/pspm_u_sc4b31.mat';
+    fn_roundtrip = '/home/bernd/git/PsPM/ImportTestData/eyelink/pspm_u_sc4b31_au.mat';
+
+    % copy file
+    S = load(fn);
+    save(fn_roundtrip, '-struct', 'S');
+
+    unit = 'mm';
+    distance = 600;
+    record_method = 'diameter';
+    multiplicator = 0.04;
+    reference_distance = 500;
+    reference_unit = 'mm';
+
+    options = struct();
+    options.channel = 'pupil';
+    options.channel_action = 'replace';
+
+    %% 1) load original file
+    [sts, infos, data] = pspm_load_data(fn);
+    testCase.verifyEqual(sts, 1);
+
+    %% 2) load pupil channel
+    [sts, original_channel, ~, pos] = pspm_load_channel(fn, options.channel, 'pupil');
+    testCase.verifyEqual(sts, 1);
+
+    %% 3) change units to au
+    au_data = unit2au(original_channel.data, unit, distance, record_method, ...
+        multiplicator, reference_distance, reference_unit);
+
+    newdata = original_channel;
+    newdata.data = au_data;
+    newdata.header.units = 'au';
+
+    [sts, info_write] = pspm_write_channel(fn_roundtrip, newdata, 'replace', struct('channel', pos));
+    testCase.verifyEqual(sts, 1);
+
+    %% 4) convert back to units
+    [sts, outchannel] = pspm_convert_au2unit( ...
+        fn_roundtrip, unit, distance, record_method, ...
+        multiplicator, reference_distance, reference_unit, ...
+        struct('channel', pos, 'channel_action', 'replace'));
+    testCase.verifyEqual(sts, 1);
+
+    %% 5) load converted channel
+    [sts, reconverted_channel] = pspm_load_channel(fn_roundtrip, outchannel, 'pupil');
+    testCase.verifyEqual(sts, 1);
+
+    %% 8) Test
+    testCase.verifyEqual(reconverted_channel.data, original_channel.data, 'AbsTol', 1e-12);
+    testCase.verifyEqual(reconverted_channel.header.units, unit);
+
+    %% cleanup
+    if exist(fn_roundtrip, 'file')
+        delete(fn_roundtrip);
+    end
+end
+
+end
+end
+
+function data = unit2au(outchannel, unit, distance, record_method, ...
+    multiplicator, reference_distance, reference_unit)
+
+    % [~, outchannel_ref] = pspm_convert_unit(outchannel, unit, reference_unit);
+    % [~, distance_conv] = pspm_convert_unit(distance, unit, reference_unit);
+
+    switch lower(record_method)
+        case 'diameter'
+            data = outchannel ./ ...
+                (multiplicator * (distance / reference_distance ));
+        case 'area'
+
+            data = (outchannel ./ ...
+                (multiplicator * ( distance/ reference_distance)));
+            data = ((data./2).^2 ).*pi;
+            
+        otherwise
+            error('Invalid record_method');
+    end
+end
+
+
+
+
diff --git a/test/pspm_convert_gaze_test.m b/test/pspm_convert_gaze_test.m
index c839b127..bb934957 100644
--- a/test/pspm_convert_gaze_test.m
+++ b/test/pspm_convert_gaze_test.m
@@ -98,7 +98,7 @@ function conversion(this, target, from, channel_action)
             this.verifyTrue(~isempty(out_channel));
             if strcmpi(target, 'sps')
                 extra = 1;                
-            elseif strcmpi(channel_action, 'add') || ~strcmpi(target, from) 
+            elseif strcmpi(channel_action, 'add') %|| ~strcmpi(target, from) 
                 extra = 2;
             else
                 extra = 0;
diff --git a/test/pspm_write_channel_test.m b/test/pspm_write_channel_test.m
index dcafb909..1931e3c7 100644
--- a/test/pspm_write_channel_test.m
+++ b/test/pspm_write_channel_test.m
@@ -197,11 +197,11 @@ function test_replace_units(this)
       gen_data.data{1}.header.units = 'degree';
       [~, ~] = this.verifyWarningFree(@() pspm_write_channel(this.testdatafile, gen_data.data{1}, 'replace'));
       [~, post_unit_change.infos, post_unit_change.data] = pspm_load_data(this.testdatafile);
-      % should be one more channel as degrees did not exist
-      this.verifyEqual(length(post_unit_change.data), length(new.data) + 1);
+      % should be the same nr. of channels
+      this.verifyEqual(length(post_unit_change.data), length(new.data));
       % assert one mm gaze channel and one degree gaze channel
-      this.verifyEqual(length(find(cellfun(@(c) strcmp(c.header.units, 'mm') && ...
-        strcmp(c.header.chantype, 'gaze_x_l'), post_unit_change.data))), 1);
+      % this.verifyEqual(length(find(cellfun(@(c) strcmp(c.header.units, 'mm') && ...
+      %   strcmp(c.header.chantype, 'gaze_x_l'), post_unit_change.data))), 1);
       this.verifyEqual(length(find(cellfun(@(c) strcmp(c.header.units, 'degree') && ...
         strcmp(c.header.chantype, 'gaze_x_l'), post_unit_change.data))), 1);
     end