A digital alternative to traditional VA-meters/purge meters

May 8, 2018 Frank Doornbos

The first Variable Area (VA) meter with rotating float was invented by Karl Kueppers in Aachen in 1908. The device was patented in Germany that same year. Felix Meyer was among the first to recognize the significance of Kueppers’ work and implemented the process for offering the meter for sale. In 1909, the firm of "Deutsche Rotawerke GmbH" was created in Aachen (Germany). They improved this invention with new shapes of the float and of the glass tube. It didn’t take long for the new device to capture attention in Europe, the United Kingdom, and other areas.

VA flow meters (or purge meters)

Over time, different types of VA flow meters (also called purge meter) have been developed, usually in response to some specific need. Nowadays a purge meter usually consists of a tapered tube, typically made of glass or plastic. Inside this tapered tube there is the ‘float’ which is made either of anodized aluminum or a ceramic. The ‘float’ is actually a shaped weight that is pushed up by the drag force of the flow and pulled down by gravity. The drag force for a given fluid and float cross section is a function of flow speed squared only.

While the meters are still relatively simplistic in design, relatively low cost, low maintenance and ease of installation they are widely used in many kinds of application. Despite these facts, the traditional VA meter has a number of drawbacks. For instance, graduations on a given purge meter will only be accurate for a given substance at a given temperature and pressure. Either way, due to the direct flow indication, the resolution is relatively poor anyway. Especially when they are built into a machine, reading might be hard. Moreover, the float must be read through the flowing medium, so you can imagine that some fluids may obscure the reading.

MASS-VIEW flow meter in application
MASS-VIEW flow meter in application

9 reasons why to use a thermal mass flow meter instead of a traditional purge meter

As for the current century, Bronkhorst developed the MASS-VIEW flow meter series which is the digital high-tech alternative to these traditional VA meters. Thanks to today’s digital possibilities, many other advantages arises for many industrial processes and chemical plants.

  1. The MASS-VIEW flow meter series operate on the principle of direct thermal mass flow measurement (no by-pass); rather than measuring the volume flow it measures the actual mass flow, without the need of temperature and pressure correction.
  2. The digital OLED display provides an easy direct or relative reading of the actual flow. Herewith parallax errors are excluded.
  3. With this digital mass flow meter it is easily possible to obtain the accumulated flow. This availability of data gives insight in costs, leading to data driven decision making power.
  4. In contrast to the traditional VA meter which need to be mounted in a vertical position, this digital alternative can be mounted in any position.
  5. The flow path is made of sustainable aluminum rather than plastic or glass which is fragile.
  6. The instruments are standard equipped with 0-5V, RS-232 and Modbus-RTU output signals. Note that the traditional VA meters usually do not have any output signal available at all.
  7. As a standard feature, there are 2 built-in relays which indicate an alarm situation. Herewith, external devices can be controlled.
  8. Multi Gas; as opposed to traditional VA meters, which are produced for one particular fluid only, the digital alternative has up to 10 pre-installed gases available as a standard feature.
  9. Multi Range; traditional VA meters usually have a rangeability of 1:10 and one single full scale range only, the digital alternative has a rangeability of 1:100 as well as up to 4 pre-installed flow ranges.

Manual control valve
Manual control valve

Achieve a stable flow

A VA meter, whether it is a conventional or a digital one, can be equipped with a built-in needle valve. This needle valve enables the user to regulate the flow rate by means of a restriction inside the flow channel. As long as the inlet pressure is stable, the subsequent flow will be stable too. On the other hand, once pressure conditions are susceptible to change, the flow rate will become equally unstable. If this is not desirable, you’ll have to compensate these pressure fluctuations.

This effect can be eliminated by using a manual control valve like the FLOW-CONTROL series which keeps the pressure drop across the needle valve (delta-P) constant. This is accomplished by a second (normally open) valve, though it is operated by a membrane this time. The operating principle is based on a balance that forms between the pre-pressure, back-pressure and the spring force on the mebrane. A change in the pressure conditions leads to a change of the equilibrium and thus a change in the valve position as well (as shown in the picture below).

Although Bronkhorsts’ pressure compensation technology is suitable for either gases and liquids, the nice thing about this is that both technologies, the digital VA meters and pressure compensation, lend themselves well to being built together. However, in that case it is applicable for gases only.

Working principle of a pressure compensated control valve
Working principle of a pressure compensated control valve

Bronkhorst High-Tech B.V.

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NL-7261 AK Ruurlo (NL)
Tel. +31 573 45 88 00

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