GB2252823A

GB2252823A - Measuring dissolved gas concentration in liquids

Info

Publication number: GB2252823A
Application number: GB9202454A
Authority: GB
Inventors: Philip Nelson
Original assignee: INDEX INSTR Ltd
Current assignee: INDEX INSTR Ltd
Priority date: 1991-02-12
Filing date: 1992-02-05
Publication date: 1992-08-19
Also published as: GB9102963D0; GB9202454D0

Abstract

Measurement includes the step of holding the sample under pressure during testing to maintain the dissolved gas in solution. Suitable apparatus comprises a measuring chamber (6) having an inlet and outlet (7, 8), and a pump (4) connected to pump liquid (2) into the chamber through the inlet, characterised by means for closing the outlet, in that the chamber is a pressure chamber and that the pump is adapted to pump liquid into the chamber under pressure. Process has application for determining dissolved CO2 in canned or bottled soft drinks. Apparatus may incorporate a refractometer. <IMAGE>

Description

TITLE: LIOUID MEASURING APPARATUS AND METHOD DESCRIPTION The invention relates to a method and apparatus for measuring liquids, and more particularly but not exclusively to the measurement of the refractive index or a related parameter of liquids.

It is difficult to make precise measurements of dissolved solids in solutions containing large amounts of dissolved gas, e.g., when testing bottled and canned drinks, e.g., beer and lager, colas and most other soft drinks containing CO2. Whilst the fluid is under pressure in the container, the gas remains in solution, but when the pressure is released by opening the container, the gas starts to come out of solution in the form of foam and bubbles, which form around the walls of the container and throughout the solution.

Refractometry, among other methods, is a useful and simple way of measuring the solids content in a wide range of food substances including drinks; and it lends itself to precise quality control of the product. However, gas in solution has always created a problem and makes precision measurement virtually impossible. Traditionally the gas has been removed prior to measurement. De-gassing methods range from pouring the solution through a filter paper, which is normally the preferred method for beers, to ultrasonic bombardment or irradiating with microwaves. One thing which these de-gassing methods have in common is that time is wasted carrying out the de-gassing step. The time involved can be anything from one to fifteen minutes depending on the sample and the amount of dissolved gas.

The problem has become even more difficult in recent years with the introduction of low calorie, so-called super saturated, soft drinks.

It is an object of this invention to eliminate the need for the de-gassing step prior to measurement of the liquid sample.

According to the invention there is provided a method of testing a liquid sample containing dissolved gas, comprising the step of holding the sample under pressure during testing to maintain the dissolved gas in solution.

Preferably the method comprises the steps of pumping the liquid sample into a measuring chamber until the chamber is filled, closing the exhaust while continuing to pump liquid into the chamber to pressurise the liquid, and testing the liquid in the chamber while under pressure.

Thus the invention provides a method of measuring the refractive index or a related parameter of a liquid sample containing dissolved gas, comprising the steps of pumping the liquid to be tested into a measuring chamber until the chamber is filled, continuing to pump liquid into the chamber after it is filled while preventing escape of liquid from the chamber to pressurise the liquid-in the chamber, and measuring the liquid in the chamber while so pressurised.

The method may comprise the step of filling the chamber with liquid to the extent that surplug liquid is exhausted from the chamber before the exhaust is closed.

From another aspect the invention is apparatus for testing a liquid sample containing dissolved gas, comprising a measuring chamber having an inlet and outlet, and a pump connected to pump liquid into the chamber through the inlet, characterised by means for closing the outlet, in that the chamber is a pressure chamber and in that the pump is adapted to pump liquid into the chamber under pressure.

Preferably the pump is a peristaltic pump, which pumps the liquid through an elastically deformable tube. The apparatus may comprise a refractometer for measuring the liquid sample in the chamber.

The invention is diagrammatically illustrated, by way of example, in the accompanying drawing.

In the drawing there is shown a measuring system intended to form part of a precision refractometer, although the system might be applicable to other equipment where entrained gas is a problem. The system consists of a peristaltic pump 4 which, through suitable conduits e.g.

in the form of tubing 3,5,10,11, ingests a partly gas saturated liquid sample 2 from a container 1 and pumps it through a measuring cell, 6 which is in the form of a pressure chamber, having an inlet 7 and an outlet 8 and then via a solenoid exhaust valve 9 and tubing 10 to waste.

The system is preferably controlled by a microprocessor (not shown).

In operation, fluid is ingested by the pump 4, the running time of which is presettable by the user, and is fed to the cell 6 to fill the cell. A typical time for the pump to run might be 10 seconds and during this time any previous sample in the system is washed to waste via the open exhaust valve 9. Usually the system fills with foam at this time. Approximately 1 to 3 seconds (also settable by the user) before the pump 4 stops the solenoid valve 9 closes, preventing the fluid from continuing to run to waste, which causes a pressure rise in the connecting tubing 5,11 and the measuring cell 6. This action forces much of the gas to re-dissolve in the solution, thus making an accurate measurement possible.

The invention is thus a method of measuring solutions containing dissolved gases such as CO2 thus eliminating the need for time-consuming special sample preparation. The system in addition to being simple, is sufficient to enable measurement direct from the can or bottle.

Claims

1. A method of testing a liquid sample containing dissolved gas, comprising the step of holding the sample under pressure during testing to maintain the dissolved gas in solution.

2. A method according to claim 1, comprising the steps of pumping the sample into a measuring chamber, allowing excess liquid to exhaust from the chamber, closing the exhaust while continuing to pump into the chamber to pressurise the liquid, and testing the liquid while under pressure.

3. A method of measuring the refractive index or a related parameter of a liquid sample containing dissolved gas, comprising the steps of pumping the liquid to be tested into a measuring chamber until the chamber is filled, continuing to pump liquid into the chamber after it is filled while preventing escape of liquid from the chamber to pressurise the liquid in the chamber, and measuring the liquid while so pressurised.

4. A method of testing a liquid sample substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

5. Apparatus for testing a liquid sample containing dissolved gas, comprising a measuring chamber having an inlet and outlet, and a pump connected to pump liquid into the chamber through the inlet, characterised by means for closing the outlet, in that the chamber is a pressure chamber and in that the pump is adapted to pump liquid into the chamber under pressure.

6. Apparatus according to claim 4, wherein the pump is a peristaltic pump.

7. Apparatus according to claim 4 or claim 5, comprising a refractometer for measuring the liquid sample in the chamber.

8. Apparatus for testing a liquid sample containing dissolved gas substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

9. A refractometer comprising apparatus for testing a liquid sample containing dissolved gas substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.