# Limitations of Using DAQ Cards for Absolute Vibration Measurement in Hydraulic Machines
## 2 Vibration Measurement, Specificity of Vibration Measurement in Hydraulic Machines, Standards
#### ISO 10816-5 and ISO 20816-5 Standards  
The ISO 10816-5 [36] and ISO 20816-5 standards [37] contain recommendations on measuring and assessing absolute vibrations on non-rotating components of hydrounits (the ISO 20816-5 standard also covers the measurement and assessment of shaft vibrations).  
Both standards are based on broadband vibration measurement; in both cases, two vibration assessment criteria are considered--based on qualification to a specific classification zone (criterion I) and based on the change in vibration level. However, the boundaries of classification zones (criterion I) are defined differently; there are also different recommendations for the measured quantity and different requirements for the measurement bandwidth.  
The standard [36] distinguishes four classification zones:  
* Zone A--the vibration of newly commissioned machines;
* Zone B--machines with vibration within this zone are considered acceptable for unrestricted long-term operation;
* Zone C--machines with vibration within this zone are considered unsatisfactory for long-term continuous operation; machines can be operated for a limited period until needed action will be carried out;
* Zone D--machines at risk of damage.  
For machines with a nominal rotational speed of 60 to 300 rpm, the standard recommends measuring the peak-to-peak values of absolute vibration displacements, while for machines with a rotational speed in the range of 300-1800 rpm, the standard recommends measuring the rms value of vibration velocity.  
In the case of measuring peak-to-peak values of vibration displacements, the bandwidth of the measuring system should be in the range from 0.25-f\({}_{\text{n}}\) to 3-f\({}_{\text{n}}\)-n (f\({}_{\text{n}}\)--rotational frequency, n--number of blades). In the case of measuring the rms value of vibration velocity, the bandwidth should be in the range of 2-1000 Hz (for diagnostic measurements, it may be necessary to extend the measurement bandwidth).  
As an example, Table 1 shows the boundaries of classification zones according to [36] for the group 3 machine set.  
\begin{table}
\begin{tabular}{c c c} \hline
**Zone Boundary** & **v\({}_{\text{rms}}\) (mm/s)** & **S\({}_{\text{p-p}}\) (\(\upmu\)m)** \\ \hline A/B & 1.6 & 30 \\ B/C & 2.5 & 50 \\ C/D & 4 & 80 \\ \hline \end{tabular}
\end{table}
Table 1: ISO 10816-5, zone boundaries for machines of group 3.  
In the case of criterion II, the standard indicates that a significant change in the vibration level can be assumed to be an increase or decrease in vibration by 25% of the upper limit of zone B.  
ISO 20816 also provides four classification zones defined in a similar way as in [12]. However, in the fifth part [12], relating to hydraulic machines, zones A and B are combined and described as zones A-B. The boundaries of zones A-B/C are defined as Action limit 1, while the boundary of zones C/D as Action limit 2, and the standard additionally provides recommended actions when these limits are exceeded.  
According to [37], the evaluation of absolute vibrations of non-rotating elements is made on the basis of the measurement of the rms value of vibration velocity.  
The measuring system should provide bandwidth from 0.1f\({}_{\text{n}}\) to 3-f\({}_{\text{n}}\)-n\({}_{\text{r}}\) (n\({}_{\text{r}}\)--number of runner blades). For machines with a rotational frequency below 2 Hz, the lower limit of the bandwidth can be raised to 0.25-f\({}_{\text{n}}\). In the case of slow-speed Kaplan and tubular turbines, the standard indicates a possible need to raise the upper frequency to 3-f\({}_{\text{n}}\)-n\({}_{\text{g}}\) (n\({}_{\text{g}}\)--number of guide vanes).  
As an example, Table 2 shows the boundaries of classification zones according to [37] for selected group 3 machines.  
In the case of criterion II, the standard indicates that a significant change in the vibration level can be considered as an increase or decrease in vibration by 25% or a phase change by more than 30\({}^{\circ}\).