Variable Area Flowmeters for Gases with attractive Design

A VA-flowmeter consists of a tapered tube, typically made of glass with a ‘float’, actually a shaped weight, inside that is pushed up by the drag force of the flow and pulled down by gravity. Drag force for a given fluid and float cross section is a function of flow speed squared only.

A higher volumetric flow rate through a given area increases flow speed and drag force, so the float will be pushed upwards. However, as the inside of the VA-flowmeter is cone shaped (widens), the area around the float through which the medium flows increases, the flow speed and drag force decrease until there is mechanical equilibrium with the float’s weight.

Floats are made in many different shapes, with spheres and ellipsoids being the most common.

The variable area flow meter for gases is highly dependent on the pressure in the measuring tube. The position of the control valve must be considered thereby (see also valve at the inlet or outlet).

Gases can be compressed. Consequently, pressure and temperature (and the kind of gas) have a direct impact on the display of the flow meter. We therefore calibrate the unit for the specific customer conditions that have been provided at the time of the order.

The flow values indicated on the flow table or on the measuring tube always refer to the depressurised state at 0°C and 1013.25 mbar abs. A different reference temperature or reference pressure is not possible.

If the temperature and pressure varies a lot we recommend you us a digital red-y compact flowmeter unit which automatically compensates for changes in pressure and temperature.

The following principles apply:

It is significant for the design that conditions, which are as constant as possible, are to be strived for in the measuring tube. The application determines the point of installation of the regulating valve. Most of the processes are close to atmospheric at the outlet side. In this case, the flow control valve is placed on the inlet.

There are processes, in which the pressure is not stable downstream from the flow meter. In this case, the regulating valve needs to be placed at the outlet. Warning: In this configuration, it is very important for the adjusted inlet pressure to be stable and to coincide with the designed pressure in the measuring tube. This arrangement is considerably more stable, because the depressurizing takes place downstream from the regulating valve (see also stable flow in response to changing conditions).

A flow can be stabilized with a simple trick in the event of an unstable back pressure. When using regulating valves, the following principle applies in the case of stable system pressure: The flow remains constant, provided that the downstream pressure does not exceed 50% of the inlet pressure.
This principle can be applied to all regulating valves from our assortment of goods.

We require the following information in order to provide you with an optimal solution: gas type, desired measuring range, temperature, inlet pressure and outlet pressure. It is wise to submit the range under which these variable change as well.

The center of the sphere, thus where the sphere has the largest horizontal diameter, serves as read-out point.