Precise electronic gas flow control

More about precise mass flow measurement and control

Gas flow control is an important factor in the reproducibility of microbial and cell cultures.

Even well-known producers of laboratory fermenters supply only simple rotameters (floating ball capillaries) with their systems. The reading of rotameters in not precise and varies with pressure and temperature. Pressure variations are frequent during fermentations and cell cultures because the output filter will progressively be blocked by droplets of medium.

Furthermore, the reading of rotameters can neither be recorded nor can they be electronically controlled. Therefore, the reproducibility of such cultures cannot be assured (which would not be conforming to GLP, GMP and other quality systems). Thus, rotameters must be considered rather a toy than a real measuring device in biotechnology.

The only reason for providing rotameters is to lower the initial price of a fermenter or bioreactor system.

LAMBDA does not reduce costs at the expense of quality and therefore does not deliver systems with rotameters at all! All LAMBDA fermentors-bioreactors (even in the lowest cost start-up version) are equipped with high precision thermal mass flow meters electronically coupled with a proprietary proportional needle valve.

The thermal mass flow measurement is based on the electronic measurement of heat transport (see the picture below) which is equivalent to a precise amount of gas molecules transported through the detector. The mass flow signal is independent of variation of temperature, pressure and other factors and delivers a high quality signal which can be recorded. However, such a high quality gas control costs us fifteen times more than a simple rotameter!

The measuring principle of thermal mass flow measurement is particularly well suited for the measurement of gas flows. One of the key advantages is that this measurement method is largely independent of pressure and temperature. Thus, in contrast to volumetric systems (e.g. rotameter), the pressure and temperature do not have to be measured in addition.

The measuring system (mass flow sensor) consists of a heating element (H) and two temperature measurement points (temperature gauges T1 and T2). The gas flowing through the sensor draws off the heat from the heating element.

With mass flow measuring and mass flow control instruments, a constant heating capacity ensures a flow-dependent temperature difference. When the gas flow is zero, the heating element (H) distributes the heat evenly so that the temperature difference ΔT=T1-T2 is zero. The presence of a gas flow is accompanied by two effects which generate a temperature difference ΔT: 1) the temperature sensor (T1) located at the entrance of the channel measures a lower temperature. This is due to the cooling of the gas as it enters the chamber. 2) the gas flowing over the heating element transports heat to the temperature sensor (T2) located after the heating element, which results in an increased temperature T2. The hereby generated temperature difference ΔT is a direct measure of the mass flow of the corresponding gas.

The high grade gas flow control allows also a high quality regulation of dissolved oxygen (DO) by air flow rate regulation a not just by variation of stirrer speed as is generally proposed by other providers. We think that DO should be controlled at any stirrer speed. Or should one tolerate bad agitation at low DO values? The bad mixing could lead to accumulation of acid during pH control or generate other problems.

Gas flow modules are proposed in separate instruments MASSFLOW 500 and MASSFLOW 5000, which can be used for the measurement and regulation of other gases (oxygen, nitrogen, carbon dioxide and others). They allow setting up any gas station according to the specific needs of the cultures (see Autonomous precise gas flow control modules).