BS 5857-2.1:1980

Compares the dilution method with the method based on transit time measurement. Also covers choice of tracers and measuring/mixing lengths.

This International Standard deals with the measurement of gas flow in conduits by using tracer methods.

In a steady flow of compressible fluid, the only conservative parameter is the mass rate of flow q_m. Therefore the whole of this International Standard will refer to mass rate of flow q_m.

However, for those fluids for which the composition (and therefore, the mass density) may not be known accurately, the volume rate of flow q_v could be measured, it being understood that this volume rate of flow is only valid for the conditions of temperature and pressure at which it has been measured.

This International Standard applies to flow measurement in conduits into which a tracer can be injected in such a way that effective mixing in single phase with the gas flowing in the pipe can be achieved.

The fluid in the conduit can be a mixture of several gases provided the thermodynamic state and conditions of flow of this mixture are well defined.

Two fundamental test procedures are used:

• — The first, known as the constant rate injection method, is based on the dilution principle: a tracer solution is injected into the conduit and the dilution (ratio) of this tracer in the gas flowing in the conduit is determined, this dilution being proportional to the rate of flow.
• — The second is a method of measurement of the transit time (formerly called Allen velocity method): the tracer is injected into the conduit and the time taken by the tracer to travel a specified length between two cross-sections in each of which it is detected, is measured.

The advantages and disadvantages of these two methods are considered in clause 4. The distance between injection and measuring sections shall be sufficient to achieve mixing of the tracer with the gas flowing in the conduit according to the methods; the adequate mixing distance is considered in clause 6.

A large number of different tracers may be used, such as radioactive or non-radioactive, mineral or organic materials. The choice of tracer depends on the circumstances of the measurement (see clause 5). The uncertainty of the measurements may be less than 1 % under good conditions (see clause 7).