DETECTION OF THE SURFACE OF A LIQUID OR FOAM

Abstract: In an apparatus for detecting the surface of a liquid or foam, for example in a fermentation vessel a detector electrode (10) is surrounded at its lower end by a protective shroud (12) so as to protect a confined vapour space (14) surrounding the detector electrode. The vapour in the space (14) is maintained at a relatively low humidity either by heating the detector electrode or some other part of the apparatus or by purging the space (14) with a relatively dry air or gas. It is thereby ensured that no condensation onto the O-ring seal (20) and surrounding parts takes place. The shroud (12) also protects the parts from splashing.

Description: The present invention relates to an apparatus for and method of detecting the surface of a liquid or of a foam. The present invention is particularly useful in connection with fermentation processes. In carrying out fermentation processes in a vessel or vat, it is customary to detect the surface of the liquid or foam by detecting a change in the electrical conductivity or capacitance between a detector electrode and a counter-electrode. The detector electrode is arranged at the top of the fermentation vessel and projects downwardly for the purpose of detecting when the liquid or foam rises to the tip of the electrode. The counter-electrode is submerged in the liquid or foam to establish and maintain good electrical contact therewith. The detector electrode must be insulated. However it remains in contact with the vapour within the vessel above the surface to be detected. When the temperature of the liquid is higher than that of the vapour, the humidity of the vapour is increased to the extent that moisture condenses on the insulating parts by which the detector electrode is insulated in relation to other parts, including the counter-electrode. Condensation on to the detector electrode itself also takes place. This phenomenon increases the electrical conductivity and capacitance. As a result, erroneous detection signals can be generated. In the event of vigorous agitation or foaming of the content of the fermentation vessel ocurring during aerobic fermentation or aeration, the liquid is splashed and becomes deposited on the detector electrode and on the insulating parts so that erroneous detection signals may be produced as described above. It is known to introduce anti-foaming agents during fermentation in order to break up foam once foam has been detected. In many cases, an excessive amount of anti-foaming agent is introduced, particularly prior to the expected time at which foam will likely occur, because of the poor reliability of known detecting devices. Anti-foaming agents are expensive and also may become an inhibiting factor in connection with fermentation or may act as an inhibitor with respect to separation and refining of fermentation products. Accordingly it is desirable to add a minimal quantity of anti-foaming agent in order to optimise fermentation. It is therefore an object of the present invention to provide a foam detector of good reliability. More specifically it is an object of the invention to provide an apparatus for detecting the surface of a liquid or of a foam wherein the insulating parts for the detector electrode can be kept dry and preferably also the detector electrode itself. DE-A-3446060 discloses an apparatus for detecting the surface of a liquid or of a foam comprising a detector electrode arranged to be touched by the liquid or foam, a counter-electrode for electrical contact with the liquid or foam being provided, a change in electrical conductivity or capacitance between the electrodes being indicative as to whether the detector electrode is touching the surface of the liquid or foam, a shroud surrounding the detector electrode, and an insulator insulating the detector electrode from the counter-electrode. Air, which may be heated, is blown downwardly through the space between the detector electrode and the shroud to remove any moisture which may have splashed on to the electrode. An apparatus for detecting the surface of a liquid or of a foam, according to the invention is characterised by a heater for heating the detector electrode for maintaining vapour surrounding the detector electrode and confined by the shroud in an unsaturated state. EP-A-0115106 discloses a low-water cut-off for a boiler in which a shroud surrounding a detector electrode is electrically conductive and serves as the counter-electrode. US-A- 4476028 discloses a fuel-water separator provided with a sensor for sensing the water level. The sensor is electrically insulated from a conductive reservoir portion of the separator housing in which the water separated from the fuel is collected. When the water level reaches the sensor, an electrical circuit between the sensor and the reservoir portion is completed. The sensor contains a heater for preventing the fuel from becoming too viscous in cold weather. Looking at the invention in another way, means are provided for drying the interior of the preferably cylindrical shroud. The invention includes a method of detecting the surface of a liquid or foam in which a change in the electrical resistance or capacitance between a detector electrode, which is surrounded by a shroud and which is arranged to be touched by the liquid or foam, and a counter electrode, which is electrically in contact with the liquid or foam and from which the detector electrode is insulated by insulating parts surrounding the detector electrode, is sensed, characterised in that the detector electrode is heated to maintain the vapour surrounding the detector electrode and confined by the shroud in an unsaturated state to inhibit the formation or deposition of moisture on the insulating parts surrounding the detector electrode. The actual detection of the surface of the liquid or foam is conducted in a conventional manner, the invention being characterised by drying or maintaining dry the detector electrode. The invention is further described by way of example with reference to the accompanying drawings in which:- Fig. 1 is a longitudinal sectional view of one embodiment of detecting apparatus according to the invention, Fig. 2 is a diagrammatic view of the detecting apparatus in use in a fermentation vat, and Fig. 3 is a diagrammatic view of a second embodiment of detecting apparatus. An apparatus for detecting the surface of a liquid or foam as shown in Fig. 1 comprises a central detector electrode 10 which is surrounded at its lower end portion by a cylindrical plastic shroud or envelope 12 open at its lower end. A confined space 14 for vapour is thereby substantially enclosed by the shroud 12 and surrounds the central detector electrode 10. The plastic shroud 12 is secured to a main body 16 by means of a stainless steel screw threaded bush 18. An O-ring 20 is confined between the bush 18 and internal shoulder 22 inside the shroud 12 and is urged against the detector electrode 10 to establish and maintain a fluid seal and also to insulate the detector electrode. Two further O-rings 22 and 24 also help to insulatively support the centre electrode 10 and are confined between the bush 18 and an internal shoulder 26 in the body 16 with the interposition of a stainless steel spacer 28. A connector 30 is screwed into the rear or upper end of the body 16 and is sealed thereto by an O-ring 32. The connector 30 has a terminal connected via a lead wire 34 to the detector electrode 10 by means of solder 36. Two further terminals are connected by means of lead wires 38 and 40 to a heater (not shown) disposed within the electrode 10 which is made hollow for this purpose. As shown in Fig. 2, the detecting apparatus 42 of Fig. 1 depends downwardly into a fermentation vat and is supported by means of a stainless steel head plate 44 with the interposition of a rubber plug 46. A counter electrode 48 is attached to the head plate 44 electrically conductively and extends downwardly below the surface 50 of the liquid or foam to be detected so as to remain immersed in and in permanent electrically conductive relationship with such liquid or foam. In use, the three terminals of the detecting apparatus 42 and the counter-electrode 48 are connected to suitable electrical circuitry whereby the heater is heated and whereby the electrical resistance and/or capacitance between the detector electrode 10 and the counter-electrode 48 is monitored. Preferably, the heater heats the detector electrode to a temperature of 10°C to 50°C above that of the vapour in the confined space 14. When the surface 50 rises so as to touch the tip of the detector electrode 10 a change in the electrical resistance or capacitance is detected, thereby indicating that the liquid or foam has reached the indicated level. If too high a temperature is used, there is a risk of components of the culture solution in the vat being burnt and adhered to the detector electrode. This would reduce the sensitivity of the detector electrode. The effect of the shroud 12 is to confine within the space 14 the vapour surrounding the detector electrode 10. Thus, when this electrode is heated the vapour is also heated whereby its relative humidity is reduced. It is therefore ensured that there is substantially no condensation of liquid onto the detector electrode 10, onto the O-ring 20 or onto the interior of the shroud 12 which would affect the electrical resistance or capacitance. Also in the event of splashing or vigorous foaming in the vat it is ensured that liquid being splashed about does not reach the O-ring 20. Any liquid which does become deposited on the shroud 12 or on the detector electrode 10 is only deposited on the lower ends thereof and quickly runs away. To further promote running away of liquid that may become deposited on the shroud 12 the bottom end of the shroud 12 is chamfered as shown. Instead of making the shroud 12 of plastics material, it could be made of an electrically conductive material, such as stainless steel. In that event, the shroud could also serve as the counter-electrode, whereby the level of the surface of the liquid or foam is detected when both electrodes are contacted by the liquid or foam. A further possibility is to use two of the detector apparata of the kind shown in Fig. 1, wherein the centre electrode of one such apparatus serves as the counter-electrode. The embodiment of Fig. 3 differs from that of Fig. 1 in that the heater 52 is not disposed within the centre electrode 10 but instead surrounds the upper end of the apparatus above the rubber plug 46 by which the apparatus is supported on the stainless steel head plate 44. Heat from the heater 52 is conducted via the body 16 and the centre detector electrode 10 to the shroud 12 and to the O-ring 20 such that this embodiment operates in the same way as that of Fig. 1.

Claims: An apparatus for detecting the surface of a liquid or of a foam comprising a detector electrode (10) arranged to be touched by the liquid or foam, a counter-electrode (48) for electrical contact with the liquid or foam being provided, a change in electrical conductivity or capacitance between the electrodes being indicative as to whether the detector electrode (10) is touching the surface of the liquid or foam, a shroud (12) surrounding the detector electrode (10), and an insulator (20) insulating the detector electrode (10) from the counter-electrode (48); characterised by a heater for heating the detector electrode (10) for maintaining vapour surrounding the detector electrode (10) and confined by the shroud (7) in an unsaturated state. Apparatus according to claim 1, in which the shroud (12) is electrically non-conductive. Apparatus according to claim 1, in which the shroud is electrically conductive and serves as the counter-electrode. Apparatus according to claim 1, 2 or 3, in which the lower end of the shroud (12) is chamfered. Apparatus according to any of claims 1 to 4, in which the detector electrode (10) is supported relative to the shroud (12) by means of at least one insulating O-ring (20 or 66). Apparatus according to any of claims 1 to 5, in which the heater is disposed inside the detector electrode. Apparatus according to any of claims 1 to 5, in which the heater surrounds the rear end of the detector electrode. Method of detecting the surface of a liquid or foam in which a change in the electrical resistance or capacitance between a detector electrode, which is surrounded by a shroud and which is arranged to be touched by the liquid or foam, and a counter-electrode, which is electrically in contact with the liquid or foam and from which the detector electrode is insulated by insulating parts surrounding the detector electrode, is sensed, characterised in that the detector electrode is heated to maintain the vapour surrounding the detector electrode and confined by the shroud in an unsaturated state to inhibit the formation or deposition of moisture on the insulating parts surrounding the detector electrode.

Inventors: ISHIKAWA, YOICHI
Applicants: ABLE CORPORATION
Application year: 1988
Main classification: G01F23
IPC codes: G01F23