Advances in liquid flow metering and control

Life science and biotech requirements for accurate measurement and control of tiny liquid flows of the order of nanolitres through to millilitres per minute are satisfied by the latest developments.

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 a lot of applications in the life science, analysis (eg HPLC), biotech, synthesis (for example of pharmaceuticals) 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 for instance Peek or fused silica as wetted material for the flow sensor tube as alternative to stainless steel.

Furthermore the instruments should have a modular setup, 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 the material 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, none of today's commercially available flow sensors were equipped with the above mentioned features.

In this article, a new generation of liquid flow sensors is presented that is 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.

These flow sensor structures and operating principles have the following innovative features and advantages.

The flow sensor comprises a short straight flow tube, with an internal diameter varying between 20 and 200 microns, thus having an extremely small internal volume, varying between 15 nanolitres and 1.5 microlitre.

The smallest measurable flow range is 25-500 nanolitre/min, the largest measurable flow range is 100-2000 microlitre/min; the response time t98% is of the order of 1s.

The measurable flow range can easily be adjusted, by varying the internal diameter, material and wall thickness of the flow tube and the measurement principle, so a wide flow range can be covered with the same type of instrument.

The material of the flow tube can be either stainless steel, Peek or fused silica; other materials may also prove to be feasible.

The length of the flow sensor tube is the same for all flow ranges: this enables a modular setup and exchangeability of the instruments.

The active elements are placed outside the flow tube, so all wetted parts are either stainless steel, Peek or fused silica.

Both the constant power and the constant temperature measurement principle need their own specific electronic circuitry, which is based on a Wheatstone bridge configuration.

The electronic circuitry provides an output voltage that shows a linear relation with the mass flow.

Flow control is achieved by integrating a control valve onto the body of the liquid flow meter.

This control valve has a purge connection on top of the sleeve that enables easy elimination of air or gas when starting up the system.

The electronic control function forms part of the standard circuitry in the liquid flow meter, so the need for an external controller is eliminated.

To illustrate the performance of the sensors, several liquid flow sensors were used, with flow tubes made out of stainless steel, Peek and fused silica, and with internal diameters varying between 20 and 200 microns.

Furthermore a liquid flow controller was used, comprising one of the sensors working according to the constant temperature measurement principle.

The output signal of the flow sensors was measured for flow ranges varying between 25-500 nanolitres/min and 100-2000 microlitres/min water.

The following three stepwise variations in setpoint were performed: 0 to 100 to 0%; 20 to 80 to 20%; 20 to 40 to 60 to 80 to 100%.

The measured output signals as a function of the mass flow illustrate that the measured response times are all within the value of t(98%) of 2s.

(Full graphical results can be provided on request).

These new generation liquid flow sensors are capable of meeting the requirements imposed by the life science, analysis, biotech and other markets.

To summarise: the actual flow sensor consists of a straight flow tube with two active elements around it.

The internal diameter of the flow tube may vary between 20 and 200 microns.

The corresponding internal volume of the sensor tube is 15 nanolitres and 1.5 microlitres, respectively, which is extremely small.

Instruments with a flow tube made of stainless steel, fused silica or Peek were tested successfully.

The feasibility of other materials suitable for the above mentioned markets will be further investigated.

The flow sensors have been driven with two different measurement principles, namely the constant power (CP) and the constant temperature (CT) method.

The CP measurement principle proved to be useful for flow ranges below circa 100 microlitres/min, the CT measurement principle was suited for flow ranges above circa 100 microlitres/min.

The tested instruments were capable of measuring flow ranges between 25 - 500 nanolitres/min (smallest flow range) and 100 - 2000 microlitres/min (largest flow range) water, with operating pressures up to 100 bar (up to 400 bar for flow meters with flow ranges below 100 microlitres/min).

The measured response time of the flow sensors is of the order of t(98%) of 2 seconds.

Products based on this new technology were introduced to the market in 2003 and were found to be a reliable solution for many low liquid flow applications, for example in HPLC systems.

This article was provided by Dr Joost Lotters, R and D Manager for Bronkhorst High-Tech in Ruurlo, The Netherlands.

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