# Limitations of Using DAQ Cards for Absolute Vibration Measurement in Hydraulic Machines
## 5 Measurement of Absolute Vibrations of Hydraulics Machines Using DAQ Cards, Measurement Setup
### NI 6218  
Features of the NI 6218 DAQ card from National Instruments (National Instruments Inc., Austin, TX, USA):  
* resolution: 16 bits;
* number of analog inputs (multiplexed): 32/16 (single-ended/differential);
* maximum sampling rate (sum of sampling rates of used channels): 250 kHz;
* measurement ranges: \(\pm\)10, \(\pm\)5, \(\pm\)1, \(\pm\)0.2 V.  
Figure 3 shows exemplary graphs of the recorded voltage with the measuring card input shorted for the card input range of \(\pm\)10 V. The recorded quantization noise is within \(\pm\)1 mV.  
The noise of the A/D converter (DAQ card) visible in Figure 3 has an impact on the results of vibration measurements performed using the accelerometer and measuring cards. Figure 4 shows the signal corresponding to the vibration velocity and vibration displacement obtained by integration and double integration of the signal shown in Figure 2, respectively, using high-pass filtration with a cut-off frequency of 0.5 Hz. The noise shown in Figure 2, with a value of \(\pm\)1 mV, causes the vibration velocity determined on its basis to exceed the range of \(\pm\)0.5 mm/s and the vibration displacements to exceed the range of \(\pm\)50 \(\upmu\)m.  
Figure 2: Schematic diagram of the measurement setup.  
Figure 5 shows the amplitude spectrum, in the range of up to 2 Hz, corresponding to the vibration velocity determined based on the signal from Figure 3. Figure 6 shows the amplitude spectrum of vibration displacements. In both figures, the scales were selected so as to illustrate the change in amplitude as a function of frequency, as far as possible, as a result of which the maximum amplitudes of the components with the lowest frequencies are not visible in both figures.  
The information contained in Figures 2-5 is intended only to illustrate the problems related to the use of measurement cards for absolute vibration measurements in water machines and will, therefore, not be repeated for the remaining DAQ modules.  
Table 3 presents the results of the test described above, in which the input of the measuring card was shorted, and the recorded signal was processed in a manner corresponding to the processing of the signal from an accelerometer with a sensitivity of 100 mV/g to vibration velocity and displacement, using high-pass filters with different cut-off frequencies. The results from Table 3 are illustrated in Figure 7.  
Figure 4: Signal corresponding to vibration velocity and vibration displacement resulting from measurement card noise (A/D converter) for 1 s (**a**) and 60 s (**b**).  
Figure 5: The amplitude spectrum of vibration velocity.  
Figure 3: Exemplary signals recorded with closed input of measuring card for 1 s (**a**) and 60 s (**b**).  
The recorded signal corresponds to vibration velocity from 0.01 to 0.2 mm/s depending on the card's measurement range and the high-pass filter's cut-off frequency. Vibration displacements are in the range from 0.71 to 298.82 \(\upmu\)m. Both the velocity and vibration displacements increase with the reduction of the filter cut-off frequency. It should be noted that for a frequency of 0.1 Hz, vibration displacements exceed the limit of zone A of the ISO 10816-5 standard for group 3 machines (Table 1) regardless of the measurement range of the card, and for ranges above \(\pm\)1 V, they qualify machines to zone D only on the basis of measurement noise. The displacement value of no more than 1 \(\upmu\)m is achieved only for the range of 0.2 V and the filter frequency of 2 Hz.