Gas Flow Meter
All BMP sets counts with a Gasflowmeter, where the gas generation is measured using a liquid displacement gas flow meter.
The gas flow meter is a single Perspex block with 15- 0.2l cells and Perspex tumbling buckets of around 7ml gas volume. A spare cell (cell 16) is used for automatic monitoring of temperature. A barometer in the Arduino logger continuously monitors atmospheric pressure for STP correction. The liquid in all cells is interconnected to keep them at exactly the same head, for equal head pressure in all reactors (liquid specifications: Table 2). Each cell has a tumbling bucket with active volume of around 7ml (easily calibrated by user- See section 3.1). The smaller measuring volume allows for better recording of the kinetics of the test, especially after the initial phase of high gas production. However, the smaller the measurement volume, the larger the potential error from uncounted gas bubbles. To minimise potential error during gas measurement a minimum counting volume of 5ml has been designed and attention has been paid to the size of gas bubbles entering the measuring vessel. The diameter of the nozzle orifice where gas enters the measurement cell is machined to 1.5mm, reducing the size of gas bubbles and minimising the potential impact of uncounted odd bubbles by-passing the tumbling bucket.
Biological tests using heterogeneous inoculum and samples present small differences in gas production. However, the minimising potential physical differences, such as temperature and mixing helps improve consistency between sample sets. For a same substrate, assuming full homogenisation, and assuming homogenised inoculum in all reactors, the main parameters to control to have lower kinetic differences are temperature and mixing. Good quality water baths assure minimum differences in temperature, even if un-stirred. Anaero Technology minimises differences in mixture intensity by mixing all the reactors in a bath through a gearbox that uses only one motor. This also reduces replacement costs by having only one readily available motor to service