16 and two drain valves 17 sampling valve 16 and both drain valves 17 like control valves 6 7 are made as electrically controllable valves and can be controlled for example via an electronic control circuit not shown here the measured values of flow meter sensor 14 and temperature measurement sensor 15 in the exemplary embodiment shown here in fig1 can also be output in electronic form and can be processed for example by the electronic control circuit moreover equalizing tank 9 is provided with a ventilation valve 10 which can also be controlled electrically as for example by the electronic control circuit the pump output of electric pump 11 and the heat output of heater 12 can also be regulated by the electronic control circuit an exemplary embodiment for a measuring process is evident from the flowchart 18 shown in fig2 first of all test system 1 is initialized 19 provided this has not already taken place to this end rinsing cycle 2 of test system 1 more precisely rinsing cycle 2 with bypass 5 with bypassing of main circuit 4 or heat exchanger 3 to be tested is filled with the test fluid in the present case the test fluid is a 10 aqueous formic acid solution here with a volume of 12 l to prepare the 10 formic acid solution 10 8 l of deionized water.