1: directional flow measurement
A flow meter needs to measure several things in order to show the complete picture: flow, temperature, pressure and the direction of the flow. Bi-directional flow measurement plays an important role in getting the right information, especially in ring networks. This is shown in the following example from one of our clients, a large electronic component manufacturer. Standard unidirectional air flow meters provided wrong readings because of back-flow towards the receiver tanks. The total flow was even higher than the compressors could possibly generate. The issue was solved by using a bi-directional sensing VPFlowScope flow meter. It now indicates reverse flow as negative values in the graphs, allowing a correction of the data. So, in conclusion: be sure to use a bi-directional flow meter for an accurate result.
2: reference conditions are vital
Have you ever compared two flow meters and found the result to be strangely different? Or did a flow meter reading not match the compressor output? Be sure to check that you use the same reference conditions. These conditions define the mass of a gas, at a fixed temperature and pressure. Without taking this information into account, results can be off up to 10%! This is especially true for mass flow, but applies to volumetric flow as well. Read more in this article about reference conditions and be prepared for all your future measurements.
3: surprises in your factory
Unexpectedly high numbers on your air flow meter display can be puzzling. Do you really consume that much compressed air in your factory? Chances are that some of that compressed air is wasted, for example because of leakages in the system. Another common reason is a bypass that is unaccounted for. This may cause strange flow meter readings at a certain point in the production process. Compressed air systems can be somewhat of a maze, especially when the factory is old. It is therefore worthwhile to do a thorough scan of your facility and look for leakages, malfunctioning air drains or unaccounted for bypass pipes. Fixing these issues not only sorts your compressed air numbers, but also reduces your energy costs!
4: calibration matters
When purchasing a new flow meter, it is only natural to compare it with the previous one. If the numbers are very different, you might think that the fault lies with the new flow meter. However, it is more likely that the old flow meter did not function properly. A common reason for that is a lack of calibration. Any measuring device is no longer trustworthy if you do not calibrate it. Deviation happens in various ways: sensors and electronics may drift, get polluted or mechanically wear out. These issues need to be checked and then corrected using a reliable reference flow meter in a lab. An annual calibration for any device is recommended, although this also depends on process parameters, air quality and intensity of use.
5: manage your expectations
A flow meter is calibrated under ideal conditions. Reality, however, is not as perfect. A field accuracy of 5% is quite typical for most mass flow meters, and the output still provides valuable insight into your compressed air system! Be aware of the direct impact of installation conditions, especially in older compressed air systems it can be complex to meet installation requirements. Obtaining accurate measurement data can be tricky with disturbances such as double elbows, abrupt diameter changes and large T-pieces. These disturbances can have major impact where we have seen even deviations up to 50%! Under such circumstances make sure all parties involved are aware. Measurement can still be valuable, especially in case of trend monitoring over time.
6: the right flow meter for the right situation
No flow meter is the same and each one has its own applications. Pending the application, the process conditions, the required field accuracy, and purpose of the measurement, it is most important to select the right type of flow meter. Using a flow meter incorrectly hampers the result. Three examples illustrate this:
- Thermal flow meters require a certain distance for an accurate measurement. This distance is often not maintained (because of a T-piece, for example) with turbulence and inaccurate measurements as a result.
- Special circumstances (such as wet or extremely hot air) call for specific technology. The VPFlowScope DP is specifically designed for wet and hot compressed air flow measurement. Any other flow meters will give an incorrect result.
- The flow meter’s sensor has to be situated wholly in the pipe. Incorrect installation of the insertion point, or ball valve may tamper with this. This results in incomplete measurement readings, such as a display of pressure and temperature only (without the required total flow).