AU2002100092A4 - Optical level sensor - Google Patents

Description

Optical Level Sensor: Description: The difficulty of deriving highly precise measurement of small changes in fluid level in various industrial applications, chemical processes and in biomedical research, among many other endeavours, has been the subject of many instrument designs. The formidable problem of accommodating various disruptive elements in the measuring process, especially with reference to negating surface tension artifacts, has produced some quite effective designs. These become increasingly deficient as the range of measurement decreases, particularly when attempting to measure level changes over the range of a few millimetres while trying to resolve to a few microns.

This invention utilises a unique application of well-established electromagnetic and acoustic phenomena and, in this embodiment using the equivalent optical phenomenon, derives an extraordinarily sensitive signal from the interface of two fluids. This signal is capable of representing fluid level.

This technique, along with additional stratagems for the use of the derived signals, produces unambiguous information on the displacement of the optical interface from the effective optical axis of the transmitter -receiver assembly.

The principle of operation is derived from the fact that wave fronts arriving at an angle perpendicular to the interface of essentially-transparent media are generally transmitted through the interface while only a small percentage is reflected. In contradistinction, wave fronts arriving at the interface at a low, grazing angle, somewhat lower than a critical value, are generally reflected.

Only a small and generally negligible percentage is transmitted.

This phenomenon applies equally to each side of the interface and an important aspect of this invention, in the application of this phenomenon, is that the measuring of the fluid level takes place from the underside rather than from above. This approach is taken in order to reduce the effect of extraneous light and physical disruption.

This invention applies this phenomenon in the following manner: A 'transmitting' optical fibre is introduced into a fluid chamber containing the fluid whose depth is to be monitored. The submerged optical fibre is positioned such that its cone of emission of light has an axis orientated slightly upwards and toward the underside of the interface at an angle of, typically, 5 degrees to the underside of that interface.

A second, submerged 'receiving' optical fibre is introduced into the same fluid chamber and disposed symmetrically to the vertical centreline and at an identical angle to the interface and having the optical axis of its cone of receptivity aligned with and intersecting the vertical plane of the transmitting fibre.

The point of intersection of these axes is effectively the datum from which measurements can be made.

The fluid chamber is arranged in such a way that the fluid level is normally never low enough come in contact with or to expose the polished ends of the optical fibre to direct physical contact with the interface. This avoids contending with the optically distorting effects of such a contact.

Furthermore, the geometry of the fluid chamber is so arranged as to ensure that the meniscus of the fluid being measured is always away from the receptive field of the optical fibres and that the surface of the interface is undistorted. This means that level changes are accurately and unambiguously represented and the optical pathway is undistorted.

It is possible to render the polished ends of the optical fibres slightly convex in order to enlarge the cone angle of the emerging light beam. This technique enlarges the range of operation of the detector.

The light injected into the transmitting fibre emerges through the convex fibre/fluid interface and progresses through the fluid in a cone-shaped beam where it eventually encounters the fluid interface to be monitored.

As described previously, the light is largely reflected at a low and equal angle back down into the fluid.

This reflected light is, in part, collected by the receiving fibre through its convex or planar fluid/fibre interface and the light is carried via the fibre to a suitable photosensitive device. In the photosensitive device it is received and transduced, typically by a phototransistor, to a voltage which represents the intensity of the received light.

The topology of this disposition of optical fibres is such that a response curve is generated which commences from a low, fixed value determined, in part, by the minimum level of the fluid chamber. Other factors, which determine this base level, are various electronic and optical parameters, which are arbitrary but stable.

As the fluid level increases there is more light able to be collected by the receiving acceptance angle of the receiving fibre as the effective ellipse of reflected light originating from the transmitting fibre moves upwards through the receiving fibre's receptive field. The resulting received flux variation has a response curve, which typically increases, to a maximum level which represents the maximum flux of emitted light entering the receptive field.

The shape of the response curve can readily be used to represent a nominal working range and, by nominating the 'set point' level in the centre of the longest and most linear slope of the response curve, variations in level can be derived.

In practice this system produces generous signal levels, good linearity over a useful range of, typically, plus or minus 3 mm, is stable in value and has no discernable drift when appropriate optical, mechanical an electronic stability is provided.

It is easily possible to detect movements in fluid level in the order of a few microns and to detect surface motion in the microphonic range.

If there were no 'ceiling' associated with the fluid chamber the fluid level would be able to continue to increase and, detrimentally, produce an erroneous representation of level due to the transmitted light no longer being in the receptive field. It is important to ensure that there is no 'out of range', negative-going output when the fluid level is rising This potential anomaly is overcome by having a highly reflective 'ceiling' placed in such a position in the fluid chamber that as soon as the fluid level rises to the maximum permitted level the surface of the fluid would 'wet' the ceiling and dramatically increase the signal level received by the receiving optical fibre. This would signal an 'over range' state to the receiving electronics.

Alternatively it is possible to employ a similar pair of optical fibres as a duplicate detector, placed above the fluid interface, to derive a complementary signal which could be used to detect an 'over range' state to the electronics controlling the detector.

One currently developing application of many possible applications of this detector is in the monitoring of fluid level in apparatus used for perfusing biological specimens. The derived signal could, in closed loop systems, control pumps used to supply and scavenge the vessels containing such material.

The high sensitivity and the stability, simplicity and robust nature of this invention will enable many further applications to be applied in the future.

Claims (4)

1. This invention constitutes a new method of deriving highly sensitive indications of level variations of fluid interfaces over a range of, in this embodiment, several millimeters while attaining a resolution of a few microns.

2. This invention employs a non-distorting, submerged optics technique to obviate the difficulties encountered with contact-type detectors. This enables readings to be taken which do not have the inaccuracies introduced by effects of the meniscus adhering to the wall of the vessel or of inaccuracies and distortions inherent in using intrusive elements such as floats, contacts or reflectors.

3. This invention utilises no moving parts and requires no maintenance in normal use. It is immune to corrosion and flow irregularities and remains mechanically and chemically stable indefinitely.

4. The electronics required for the operation of this invention consists of simple circuitry, principally for thermal stabilisation of the optical source and receiver, and for the basic signal processing and display.