## 2 Measurement Methodology and Used Devices  
The Mehrer TZW 60/S6 (Mehrer Compression GmbH, Balingen, Germany) piston compressor is used for the compression of the mixture of 75% H\({}_{2}\) and 25% N\({}_{2}\) gases, from a relative pressure (overpressure) of 3 kPa to the max. relative pressure of 900 kPa. The two-stage compressor with an intercooler and an aftercooler of the gas mixture uses a central cooling system (Figure 1). The water is also used to cool the compressor cylinders. Thermostatic regulation of the cooling water temperature ensures that the maximum temperature of the water at the outlet from the cylinders is maintained at the maximum value of 50 \({}^{\circ}\)C. The cooling of the intercooler must be adjusted manually, using a regulating valve, so that the gas temperature at the inlet to Stage 2 is \(\leq\)45 \({}^{\circ}\)C. The cooling water inlet to the aftercooler must be adjusted, using a regulating valve, so that the output gas temperature is as much as 5-20 \({}^{\circ}\)C higher than the cooling water temperature. Specification of the used compressor of the TZW 60/S6 type, including the description of its borderline parameters, is contained in Table 1.  
Figure 1: Compressor connection diagram including the description of the measuring and regulation technology (1—Stage 1 of the compressor, 2—Stage 2 of the compressor, 3—intercooler, 4—aftercooler, 5—thermostatic regulating valve controlled by the TC1 sensor, 6—inlet filter, 7, 8—blow-down device, 9, 10—valves, 11—cooling water inlet valve, TC1—temperature sensor for the thermostatic regulation, PI1 to PI3—manometer, TR1 to TR8—temperature sensors with the recording of the SMT160-30 (Smartec BV, Breda, Netherlands), FR1—UTXDR ultrasound flow meter (GE, Waltham, MA, USA)). During the measurement, the rate of the cooling water flow through the intercooler and the aftercooler was set so that the gas temperature at the inlet to Stage 2 of the compressor did not exceed 32 \({}^{\circ}\)C, and so that the maximum difference between the gas temperatures at the outlet from the aftercooler and the temperature of the cooling water was 7.2 \({}^{\circ}\)C. Date of measurement: 19 July 2017. Measurements of the thermal and energy parameters were based on the calorimetric principle, as no phase transition of the liquid medium occurs while it is being transported through the individual compressor parts. The temperatures were measured using SMT160-30 smart temperature sensors (with the measurement accuracy of \(\pm\)0.4%) connected to the B-Pi microcomputer, with a pre-set measurement interval of 10 s at the following device points:  
* TR1--cooling water temperature at the inlet pipe behind the filter;
* TR2--cooling water temperature at the outlet pipe (total cooling water temperature at the discharged from the compressor);
* TR3--cooling water temperature at the outlet pipe from the cylinders;
* TR4--cooling water temperature at the outlet pipe from the intercooler;
* TR5--gas temperature at the inlet to Stage 1 of the compressor;
* TR6--gas temperature at the outlet from Stage 1 of the compressor;
* TR7--gas temperature at the inlet to Stage 2 of the compressor; and,
* TR8--gas temperature at the outlet from Stage 2 of the compressor. During the measurement, the compressor was running in a regular operation (Figure 2), and the cooling water flow rate measurement was carried out using the ultrasonic flow meter (without any intervention in the cooling circuit). The type of the used flow meter: GE Transport PT878, UTXDR measuring probe (2 MHz) (GE, Waltham, MA, USA), measurement accuracy of \(\pm\)2%; pipe thickness measuring probe--Panametrics D868 (5 MHz) (GE, Waltham, MA, USA), and measurement accuracy of \(\pm\)5%. The connection of the UTXDR measuring probe of the flow meter to the cooling water inlet pipe is depicted in Figure 3.