## 4 Discussion  
Authors [11; 21; 22; 23; 24; 25] dealt with the issue of testing hydrostatic transducers and hydraulic fluids in their work, and created a set of measuring laboratory equipment for testing hydrostatic elements and hydraulic fluids.  
Authors [11] solved the design of a test equipment for parametric tests of hydraulic pumps used in an open hydraulic circuit, where a gearbox was included between the motor and the hydraulic pump, on the output shaft of which was located the torque sensor. The flow in the pressure part of the hydraulic circuit was measured by a high-pressure flow meter connected to the outlet of the hydraulic pump.  
Authors [12] used an electro-hydraulic proportional pressure valve in the test case for dynamic loading of the hydraulic pump, by which the hydraulic motor and the hydraulic pump were loaded at the same time, while the hydraulic motor was firmly connected via a coupling connected to the hydraulic pump. The difference between the technical solution of the proposed hydraulic test equipment is that it allows the loading of hydrostatic transducers with operating pressure in the primary and secondary circuit simultaneously, but also separately. It is also possible to use an electro-hydraulic proportional valve to simulate the load only in the primary circuit, in case the circuits are mechanically disconnected after disconnecting the torque sensor.  
Authors [26] describe a test device for the implementation of life tests of gear hydraulic pumps, where the cyclic compressive stress is provided by an electro-hydraulically controlled switchboard connected to the outlet of the hydraulic pump. The change in the position of the distributor resulted in a change in the flow of the fluid, which at the same time changed the pressure loading conditions at the outlet of the hydraulic pump.  
As reported by [27], the authors used a pressure valve to change the pressure in the hydraulic circuit and to load the gear hydraulic pump, which derived the load by a gradual and uniform pressure rise (5 MPa each), and the result was a calculation of the flow efficiency of the hydraulic pump.  
Authors [28] in their research verified the flow efficiencies of the hydraulic pump depending on the speed rotation, to increase the temperature of the operating fluid also used a throttle valve with stabilization, and the temperature was regulated via a heat exchanger (cooling device) and flow efficiencies were monitored at different fluid temperatures.  
Authors [29] state in their work that a change in the flow efficiency of a hydraulic pump indicates such changes in the parameters of the hydraulic pump, it is also possible to evaluate the technical condition of the monitored hydraulic pump. Said monitoring of the technical condition of the hydraulic pump is also enabled by the proposed test labora  
Figure 10: Flow characteristics of the transducers in the secondary circuit.  
tory equipment for monitoring the service life tests of the hydraulic pump, the technical condition of which will be assessed by changing the flow efficiency of the hydraulic pump.  
It will be possible to monitor the service life of hydrostatic transducers under cyclic loading and dynamic loading with operating pressure on the proposed laboratory test equipment. Moreover, it is possible to monitor the influence of used hydraulic fluids on individual elements of the hydraulic system by monitoring changes in physico-chemical properties of hydraulic fluids. The proposed device is designed to allow a smooth change in the temperature of the working fluid by using a throttle valve with stabilization and regulation of air flow through the radiator, comparing the change in viscosity of the working fluid, as the temperature in the primary and secondary hydraulic circuit can be changed according to oil manufacturers.