The internal structure of a thermal by-pass flow meter is based around creating a predictable and repeatable split in the flow between the laminar flow element (LFE) and the by-pass sensor. The better the laminar flow element works, the more predictable the flow of gas is and the more accurate the split of the flow and therefore the performance of the flowmeter. With the split of a thermal by-pass instrument being based on mechanical dimensions the absolute temperature and pressure virtually do not influence the split. With pressure based instruments the viscosity in the LFE is directly influencing the reading, viscosity strongly depends on temperature and pressure, and this may lead to the instrument being susceptible to subtle variations in the flow.
Accuracy
Overall accuracy in a thermal by-pass instrument is dependent on just one sensor (measuring direct thermal mass flow), on the other end of the scale pressure based instruments need to calculate mass flow from the measured volume flow, temperature and pressure. This could mean using up to 4 sensors. When measuring with this many sensors the individual errors will add up. Pressure based instruments have to measure temperature at two positions; one in the pressure sensor to compensate for temperature errors in the pressure sensor and one in the LFE to correct the pressure drop at actual gas temperature.
Conclusions
Thermal by-pass instruments cover a much larger range of flows and pressures; they are also less complex requiring fewer sensors to generate the same data. It is also easier to define the working parameters of the thermal by-pass instruments, fewer sensors means fewer combined limitations and inaccuracies. The internal design and basic principle of the differential pressure devices appears to require a more complex system of measuring to get to the same end point. As always we are happy to discuss and talk about the differences between all of the flow meters available, in the coming weeks we will be looking closely at other flow meter technology including; Coriolis, CTA (Constant Temperature Anemometry) and MEMS.
Read more about these technologies in the related blog posts: