GB2333843A

GB2333843A - Fluid level control system

Info

Publication number: GB2333843A
Application number: GB9802043A
Authority: GB
Inventors: Anthony Keith Cummins; Jose Losada; Peter John Cummins; Ian John Benson
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-01-31
Filing date: 1998-01-31
Publication date: 1999-08-04
Also published as: GB9802043D0

Abstract

A fluid level control system includes level detection probes P1 which may have shaped tips to increase the surface area of the probe which comes into contact with the fluid F. The probe tip may also be provided with a bimetallic or other thermally sensitive disc such that temperature fluctuations cause the tip to flex and dislodge any solid deposits from the probe. The probe may be supplied by a photo-voltaic isolator G, dc to dc converter or constant current source and current flow through the probe may be detected using an opto-coupler O.

Description

Fluid Level Control System This invention relates to a fluid level control system The Problem There are often occasions when the level of fluids in a vessel needs to be detected and controlled accurately and reliably. Various forms of proximity detector are available but are costly to produce and suffer from lack of accuracy. Other methods using floatation switches are susceptible to the effects of heat and deposition of contaminants such as calcium and magnesium salts in water which cause unreliable operation of moving mechanical parts. Methods depending on electrical conductivity require high currents to flow, which can cause electrolytic deposition or erosion of the sensing probes. Low currents are difficult to detect and can not usually be used to distinguish between liquid and froth or foam. The level of the fluid may have to rise an unacceptably variable distance up the probe before the probe to fluid electrical resistance is reduced to an effectively detectable value.

The Invention The benefit of this invention is that the fluid level in a container, vessel, tank, engine or boiler can be controlled very accurately due to the large "tip" connected to the probe.

The increased surface area in contact with the fluid will allow the probe to transmit the signal to the electronics at a much lower voltage and current than in existing level control systems.

In conditions where fluids create froth or bubbles, the lower voltage will not allow sufficient current to pass through the froth or bubbles. However, when the liquid which has a lower electrical resistance, reaches the increased surface area of the probe, the rapidly reduced resistance allows a current to flow which can be detected.

A further benefit of the invention is that by being able to use lower voltages and currents, electrolytic effects are decreased resulting in reduced mineral accumulation around the probes and less probe erosion. By adding a bi-metallic or similar thermal disc, spring or mechanical thermal device, the tip will be caused to flex by temperature fluctuations and this movement will dislodge solids that normally build-up on probes The electronics employs a novel technique for generating the low driving voltage and detecting the small resultant probe current. An electrical safety isolation barrier may readily be formed to prevent possibly hazardous voltages at the probes reaching the control circuits and the user.

A specific embodiment of the invention will now be described with reference to the following drawings in which: Figure 1 shows the Electronic circuit diagram; Figure 2 shows various probe tip shapes Figures 3a and 3b illustrates varying contact resistance between fluid levels and probe tip position.

Figure 1 shows the circuit arrangement of the level control system. Vessel V contains the fluid F which enters via solenoid control valve S. The valve S is operated by the valve control circuit VC which is driven in response to the condition of the output terminal of detector circuit D.

Generating device G produces a potential which is applied between level detecting probe P 1 and either the vessel V if it is electrically conducting or to fluid contact probe P2 via connection C. When fluid F makes contact with probe Pi, current passes through the current detecting device 0 which is typically an opto-coupler.

Devices G, 0 and S provide an electrical isolation barrier indicated by the dotted line I such that dangerous electrical potentials arising from devices such as an electrical heater H, can not reach the detector D or power source V. This is often a requirement to meet electrical safety standards.

The generating device G may be a photo-voltaic isolator although other device types such as a DC to DC converter may be used. Power source V, which may also supply voltage Vcc for the detector D, supplies driving current to generator G. Resistor RI limits this current to a safe value. The generator G acts as a constant current generator over its range of output voltages (typically 0-5 volts) such that current in the order of 25 to 50 micro-amps flows when the probe Pi is in contact with the fluid F. Using a constant current device overcomes problems of high resistance with probes in poor condition or poorly conducting fluids and avoids excessive currents encountered with low resistance. Device O is typically a photo-Darlington opto-coupler. Current causes the LED in the opto-coupler 0 to emit light and cause the photo-transistor in the device to conduct. In this condition current passes from source Vcc via the phototransistor through resistor R2 which is of a high value, typically 1 - 2 megohms. The resulting change in potential across R2 causes the FET to conduct via R3 which produces a large change in voltage of the output. This voltage may be monitored by a control circuit typically containing a micro-processor, or may be applied directly to the valve control circuit VC.

When the fluid level F is below the level of probe P1, no current flows through device O and the output of detector D causes solenoid valve S to open. When the fluid reaches probe P1, the resulting current through 0 causes a change in the state of the output of detector D and the valve is caused to shut.

This circuit arrangement is able to work with a variety of probe designs. Extremely accurate level control can be achieved if the initial contact area between probe and fluid is maximised.

Level Control Probe The level of the fluid can be controlled very accurately due to the surface area created at the tip of the probe. By increasing the horizontal surface area the contact resistance is decreased. This allows much lower voltages to be applied and so avoids false detection caused by froth and bubbles.

Figure 2 shows examples of probe tips that allow extended surface area contact.

These examples are not intended to limit the scope of this patent to those shown.

Figure 3 shows the fluid just reaching the probe. As the fluid rises to touch the probe tip, surface tension causes a sudden change to the whole of area A being in contact.

This results in a sudden reduction in contact resistance. If the fluid continues to rise as in Figure 3b, then a second step change occurs as the upper surface suddenly becomes flooded. This produces a further reduction in contact resistance due to the much larger area surface B.

Claims

Claims 1. A level control system incorporating an electronic circuit which includes a generating and current-detecting means in which the generating device is a photo-voltaic isolator or DC to DC converter.
2. A level control system incorporating an electronic circuit which includes a generating and current-detecting means in which the generating device is a constant current source.
3. A level control system incorporating an electronic circuit which includes a generating and current-detecting means in which the current-detector is an opto-coupler.
4. A level control system as in claim 1 or 2 in which the current-detector is an opto-coupler.
5. A fluid-level control system incorporating an electronic circuit which includes both a generating and current-detecting means including but not limited to the types identified in claims 1, 2, 3 and 4 and having a probe which causes a large surface area of probe to come into contact with the fluid.
6. A level control system as in any of the above claims in which the generating and detection means form an isolation barrier of the form required to inhibit the transmission of dangerous potentials.
7. A fluid-level control probe having a shaped tip which causes a large surface area to come into initial contact with the fluid.
8. A fluid-level control probe having a conductive plate at the tip which causes a large surface area to come into initial contact with the fluid.
9. A fluid-level control probe having a conductive thermally sensitive plate at the tip which is caused to deflect by temperature change as it comes into contact with fluids at higher or lower temperatures.
10. A fluid-level control probe having a shaped tip which causes a large surface area to rapidly come into contact with the fluid as the fluid rises up the probe.