# Limitations of Using DAQ Cards for Absolute Vibration Measurement in Hydraulic Machines
## 6 Conclusions  
The study considers the possibility of using measurement cards to measure the absolute vibrations of non-rotating hydraulic machine elements. For the purposes of the  
Figure 11: Vibration measurements using the NI 9232 (National Instruments Inc., Austin, TX, USA) card depend on the high-pass filter’s cut-off frequency: vibration velocity (**a**) and vibration displacement (**b**). \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**2 Hz**} & \multicolumn{3}{c}{**1.5 Hz**} & \multicolumn{3}{c}{**1 Hz**} & \multicolumn{3}{c}{**0.5 Hz**} & \multicolumn{3}{c}{**0.25 Hz**} & \multicolumn{3}{c}{**0.1 Hz**} \\ \hline
**Range** & \(\mathbf{v_{rms}}\) & \(\mathbf{s_{p-p}}\) & \(\mathbf{v_{rms}}\) & \(\mathbf{s_{p-p}}\) & \(\mathbf{v_{rms}}\) & \(\mathbf{s_{p-p}}\) & \(\mathbf{v_{rms}}\) & \(\mathbf{s_{p-p}}\) & \(\mathbf{v_{rms}}\) & \(\mathbf{s_{p-p}}\) & \(\mathbf{v_{rms}}\) & \(\mathbf{s_{p-p}}\) \\
**(V)** & **(mm/s)** & **(\(\mathbf{\mu}\)m)** & **(mm/s)** & **(\(\mathbf{\mu}\)m)** & **(mm/s)** & **(\(\mathbf{\mu}\)m)** & **(mm/s)** & **(\(\mathbf{\mu}\)m)** & **(mm/s)** & **(\(\mathbf{\mu}\)m)** & **(mm/s)** & **(\(\mathbf{\mu}\)m)** \\ \hline \(\pm 10\) & 0.006 & 1.24 & 0.008 & 2.06 & 0.012 & 4.08 & 0.023 & 15.22 & 0.032 & 40.95 & 0.043 & 120.67 \\ \(\pm 1\) & 0.001 & 0.16 & 0.001 & 0.26 & 0.002 & 0.69 & 0.004 & 2.40 & 0.006 & 6.26 & 0.010 & 37.68 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Vibration measurement results using the NI 4492 card for different cut-off frequencies of high-pass filters. Figure 12: Vibration measurements using the NI 4492 (National Instruments Inc., Austin, TX, USA) card depend on the high-pass filter’s cut-off frequency: vibration velocity (**a**) and vibration displacement (**b**). analysis, it was assumed that vibrations are measured using an accelerometer with a sensitivity of 100 mV/g. The properties of the vibration sensor were not considered, and only the properties of the measurement cards were focused on. In order to determine the suitability of measurement cards for measuring absolute vibrations of hydraulic machines (containing slowly changing signal components), tests were carried out consisting of the analysis of the signal recorded with the input of the measurement card closed (a constant signal simulating the absence of vibrations). The signal measured in this way was processed appropriately for the acceleration signal so as to determine the velocity and displacement of vibrations using high-pass filtration. In this way, the values of the velocity and displacement of vibrations were determined for various cut-off frequencies, and then they were referred to vibration standards appropriate for hydraulic machines. Based on the tests that were conducted, it can be stated that the use of DAQ cards for measuring absolute vibration velocity does not generally pose any major problems. The rms values of vibration velocity determined in the described manner are in the range from 0.001 mm/s (NI 4492, range \(\pm\)1 V, low-pass filter cut-off frequency 2 Hz) to 0.225 mm/s (DaqBoard 2000, range \(\pm\)10 V, cut-off frequency 0.1 Hz). Much more attention is required to measure absolute vibration displacements. In this case, the smallest peak-to-peak vibration displacement value determined is 0.16 \(\upmu\)m (NI 4492, range \(\pm\)1 V, high-pass filter cut-off frequency 2 Hz), and the largest is 334 \(\upmu\)m (DaqBoard 2000, range \(\pm\)10 V, cut-off frequency 0.1 Hz). The studies carried out indicate that in this case, an incorrect selection of the measurement card or range can cause measurement errors exceeding the permissible vibration values. The minimization of the influence of the DAQ card on the measurement result is facilitated by using devices with a higher resolution of the A/D converter and by choosing the smallest possible measurement range.