Life science and biotech requirements for accurate measurement and control of tiny liquid flows of the order of nanolitres through to millilitres/minute are said to have been satisfied
Accurate measurement and control of tiny liquid flows of the order of nanolitres through to millilitres per minute is becoming more and more important for of applications in the life science, analysis (eg HPLC), biotech, synthesis, and nanotechnology markets.
Accompanying demands for flow sensors suited to this low flow range are requirements for an extremely small internal volume and the use of Peek or fused silica as wetted material for the flow sensor tube, as an alternative to stainless steel.
Furthermore the instruments should have a modular set-up, so they can be easily exchanged and adapted to a new need.
For example, the separation column at the detector side of analytical equipment is sometimes made of fused silica with an internal diameter typically of the order of 100 microns.
In some cases, it is necessary to measure the - very small - flow at the detector side to improve the accuracy of the analysis.
In this application, the internal diameter and wetted material of the flow sensor tube should preferably be the same as those of the separation column, to avoid disturbances in the flow and to minimise the internal volume.
Until recently, no commercially available flow sensors were equipped with these features.
A new generation of liquid flow sensors is said to be capable of meeting the requirements as imposed by the life science, analysis, biotech and other markets.
The actual flow sensor consists of a straight flow tube with two active elements around it.
The wetted material of the flow tube is stainless steel, or as an option, fused silica or Peek.
The internal diameter of the flow tube may vary between 20 and 200 microns, depending on flow range.
The corresponding internal volume of the mass flow meter is 1.5 to 20 microlitres.
Two measurement principles can be distinguished, namely, the constant power (CP) and the constant temperature (CT) method.
The CP measurement principle is used for the flow ranges below circa 100 microlitres/min.
In this case, the two elements are used both as heater and as temperature sensor.
Both elements are provided with an equal amount of constant power, the temperature difference between them is a measure for the flow.
The CT measurement principle is used for the flow ranges above circa 100 microlitres/min.
In this case, the first element acts as temperature sensor, and the second element acts as a heater.
The heater is heated to a certain constant temperature difference over the monitored temperature of the medium.
The heater power necessary to maintain this difference is a measure of the flow.
