Having provided the means of measuring or sensing various aspects of a system, it is now necessary to record and analyse the data obtained in order to ensure the system is operating to specification or to predict likely failures.
Data logging options
Data loggers record data in one of two main ways. In the first, data is recorded continuously over time, much like an old-fashioned tape recorder: start, record, stop.
This is ideal to get a feel for what is happening in a hydraulic system, or where all the data is required for development purposes and it can be analysed afterwards. The only downside to this approach is the large amount of data that may be obtained, which may take a significant amount of time to analyse (Table 9.2). It is usually possible to alter the sampling rate (which is quoted as either the number of readings recorded per second or the time between readings) from milliseconds to minutes or hours, depending on the nature of the property being recorded.
table 9.2 Typical file size (kB), based on sample rate and duration of test, for an eight-channel data logger
Test duration
Sample rate
0.1 ms 1 ms 10 ms 100 ms 1 s 10 s 60 s
1 s 1,800 200 20 0 0 0 0
2 s 3,600 400 40 0 0 0 0
5 s 8,900 900 80 0 0 0 0
1 min 107,200 10,700 1,100 100 20 0 0
5 min 536,100 53,600 5,400 500 60 0 0
1 h 6,433,600 643,400 64,300 6,400 600 60 20
24 h 154,406,300 15,440,600 1,544,100 154,400 15,400 1,500 300 The data can typically be analysed on a data-logging device (Fig. 9.16) by using PC software supplied by the manufacturer, or by exporting it to an analysis package such as Microsoft Excel. A data logger is typically a stand-alone electronic device (normally battery powered) that either interfaces with or incorporates the relevant sensor and stores the data obtained in its memory.
The second method of recording data uses a trigger. A snapshot of data is recorded every time a button is pressed, so a key press could be the trigger. This is ideal when carrying out a pump test, for example, to record the flow and pressure at various points in order to build a power curve. Alternatively, a data logger can be used to record a set of data over a pre-set period every time a measured value hits a certain target value. For example, if the system pressure is normally 210 bar (3000 psi) the data logger can be set to trigger when the pressure exceeds 220 bar (3300 psi) and record all channels for 2 seconds. In this way, much less but more relevant data is captured. This approach is an ideal means of checking for occasional pressure spikes in the system.
Data analysis
Armed with measurements of what is really happening within the hydraulic system the user will typically want to produce a graph, a table of results or a combination of the two. Using the software supplied, these results can easily be turned into a report format by adding titles, comments, arrows and highlights. This report, or the complete dataset, can then be shared with other engineers at the user’s company or at the customer’s company to confirm the situation or seek help and direction.
How to select a data logger
While not exhaustive, the following checklist covers the key areas to consider.
• What is your application? For example: fault-finding, research and development, condition monitoring, pre-dispatch inspection.
• Is it fixed or mobile? Will the data logger be moved from job to job or mounted permanently in a system or test stand?
• What type and duration of tests need to be run? See above for examples.
• What is the distance from the sensors to the readout? It is better to use digital (CAN) rather than analogue sensors for distances over 5–10 m (15–30 ft).
• What environment will the data logger be used in? How much electrical noise, dust and water (rain) will it be exposed to? Check the signal types and ingress protection (IP) rating of the device.
• What needs to be measured? For example: flow, pressure, temperature, contamination, any custom sensors.
• How many channels/sensors are required simultaneously? Fewer channels generally mean lower cost. Some sensors can transmit two values, such as flow and temperature.
Fig. 9.16 A typical data logger (Webtec)
DEFINITION The ingress protection (ip) rating of a component provides an indication of how resistant the component is to environmental dust and moisture. The higher the number the better the protection.
• What is the fastest sampling rate required? Most data loggers will sample at 1 ms, some sample at 0.1 ms. The sampling rate may have an impact on the amount of memory required if long tests are carried out at high speed.
• What is the skill level of the operator? Consider ease of use from the operator’s perspective. A simpler data logger may get used much more than a more complex and expensive model.
• What is the available budget? There are lots of solutions available at all price levels. If it is not possible to find a device within budget, consider which of the points above may be flexible. Check that the data logger is not being over-specified for the job in hand.
FURTHER READING For further information and white papers on how to select flow or pressure test equipment, go to
www.webtec.com/education