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Frequently asked questions

What are the conversion factors for other gases?

You can use the VPFlowMate and VPFlowScope with various gases, as long as they are non-corrosive and non-explosive.

Examples are:

  • Helium
  • Argon
  • Carbon Dioxide
  • Corgon (C02-Argon)
  • Nitrogen
  • Breathing air mixtures

For oxygen, special caution must be taken and additional regulations/ legislation may be applicable.  The grease in the VPFlowMate and VPFlowScope may not be approved for use with oxygen. Please consult your company on internal safety regulations and make sure that the wetted materials (anodized aluminium, stainless steel 316, glass and epoxy) are approved.

Where can I find the UL listing of VPInstruments products?

A copy of the listing is found below:


Mass flow meters, Series VPFlowMate, Cat. Nos. VPF-Rxxx-Mxxx-Dx-Sxxx-Exxx, may be followed by PN or PN 20, VPP-Rxxx-Pxxx-Dx-Sxxx-Exxx, may be followed by PN PN 20 or PN 40 where x can be any letter/number.

Series VPFlowScope, Cat. Nos. VPS-Rxxx-Pxxx may be followed by PN, PN20 or PN40, where x can be any letter/number.

The actual status can be checked here.  Search for Van Putten Instruments or E231429

Can I use a lage T-piece as an insertion point?

Insertion flow meters are great tools for compressed air measurements. But you have to be careful when installing these devices. The insertion hole has to be as small as possible, otherwise it will create a turbulence around the probe tip. This turbulence can cause misreadings. : in this case, you want to narrow the hole as close as possible to 0,5 inch, plus some tolerance for drilling. If this T-piece is still to be mounted, you can insert a small ring in between the two pipe ends to fill the space.


What air quality does your flow meter need?

The ISO 8573.1 air quality standard classes is the most suitable compressed air. It ensures that your factory will get the compressed air quality you specified. It can also be used to define the required air quality for sensors used in the compressed air network.


The best air quality for solids is up to class 5. The after cooler and pre-filter of the compressor have taken out most harmful particles. An additional source of particles can be rust in the pipework. Using a general purpose 1 micron filter will take out any harmful parcicle


The best air quality for water is up to class 6 (non condensing). We have not seen any significant effect on our flow meters. (for more information on water, read on below).


For short audits, oil does not give any problems, however, a thin film may occur over time. We do recommend a regular check in permanent installations after six months. Once you determine that it is still clean, you can postpone a following check till after one year.

Humidity Effects

Thanks to its high operating temperature, the flow sensor will keep the surrounding air very dry. This minimizes the effect of normal humidity variations.

Variations in Dew Point

In our in-house test facility, we have researched the effects of dew point variations between -20 and +15° C (Class 3 … Class 6 according tot DIN 8573-1).  Our thermabrigde sensor does not show any significant change in sensitivity over this range, which covers most industrial compressed air applications where at least a refrigerant drier is installed.

Condensation effects (droplets)

Once condensation occurs, the gas temperature is close to the dew point temperature.
Example: The air coming from an air compressor has a dew point of 35° C. The outlet temperature is around 50° C. As soon as the temperature drops below 35° C, the water vapor will condensate and form small droplets. Note that the thermal conductivity of water is about 25 times higher than air. Every time a water droplet hits the sensor, the amount of heat conducted to the medium will dramatically increase. Depending on the size of the droplet, the water will evaporate or it will temporarily cover the sensor. In most cases, large spikes can be seen in the output signal. In general, high readings up to 50% have been observed with thermal mass flow sensors.

Solutions :

1 Place the flow meter downstream of a refrigerant/ desiccant drier, or at least at a point where most of the water is separated from the air, for instance behind a receiver tank.
2 When wet air is the permanent condition, a VPFlowScope DP is the best choice.  Please make sure that the installation conditions meet the specific requirements of the DP.

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