Working with moist gases

Let's have a quick, comparative look on purely calorimetric volume flow measuring devices: the mere presence of liquid droplets in the streaming air changes their "measured" results to a random number, no kidding. But not so with the VSM-02! In principle, neither the measuring precision nor the reliability are affected at all.

As in the case of compressed air, e.g., it is immediately clear that the air at the suction side, which is taken from the environment, has a natural content of water vapour. This moisture condenses during the generation of the compressed air and the resulting water must be strictly kept away (with considerable effort) from the tubes and consumers. Mind you that corrosion and damage would be just inevitable, if the water wasn't kept away from the compressed air. However, the separation will not always reach 100 percent efficiency. So occasional droplets do occur. As with most calorimetric measuring devices, the sensor areas are so small that a single droplet can easily cover up most of the sensing area. The very high cooling effect of the liquid now mimics an enormous (!) volume flow. If this occurs repeatedly, the overall value of the "measured" consumption will be void. Not more and not less: just void. So, if somebody had to actually pay for his measured consumption, why then any bothering with calorimetric measurings at all, if sometimes a simple, archaic estimate would be a much better solution?

As said already, the VSM-02 uses a very different, more reliable physical method. The pressure difference due to the Bernoulli effect occurs over the whole sectional area, thereby integrating over the said area. It is therefore neither a spot measurement that could be voided by any small local turbulence (which is, by the way, just another nuisance of many competing calorimetric devices), nor is it affected by the occasional water or oil droplet travelling by. The measuring method does not depend on cooling properties, so one can forget about the related hassles. When working with moist air or gas mixtures there are some easy guidelines to keep in mind, though:

To the right: Wrong nozzle placement Wrong nozzle placement

Mistake: Obeying the fundamental laws of gravity, water and oil droplets tend to flow downwards, of course. It is obviously no good idea to place the pressure hoses of the VSM-02 just in their way. The liquids would fill the hoses after some time and might give the unsuspecting operator even a sudden shower on opening the couplings.

To the right: Correct nozzle placement Correct nozzle placement

Right: Some droplets will always, by chance, find their way into the lower sections of the pressure hoses. But this time, again thanks to gravity, the droplets find their way out just on their own. No external help needed. The streaming air finally carries the droplets away. As a result, the pressure hoses of the VSM-02 stay permanently open and dry. With a length of 2 m per hose and because of their flexibility, the hoses can also be led from the nozzle to the measuring device in the shape of an arc. The only important thing is to lead the hoses upwards away from the nozzle.

The same applies to vertically mounted tubes. The hoses should form an arc leading to the VSM-02. In other words: the mounting site of the device should be somewhere above the nozzle.

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