1. Field of Invention
The present invention relates to a method and apparatus for sensing a fluid level in a device; and more particularly to a method and apparatus for sensing a fluid level in a steam boiler during operation in order to control the water level therein.
2. Description of Related Art
Conductance style probes are installed in boilers in order to control the water level in the boiler and to prevent the burner from firing if the water level is too low. When water is below the probe, the probe has low conductance and the probe control unit signals the water feeder or pump to turn on and feed water into the boiler. In addition, the burner is prevented from firing because the water level is too low. If the probe is in water, the conductance is high and the probe control unit signals the water feeder or pump to turn off. In addition, the burner is now allowed to fire, if required.
In particular, FIG. 1 shows a steam boiler 30 having a container 32 with a probe control 34 arranged on a side wall 32a thereof. The probe control 34 has a conductance probe (See FIGS. 2(a) to (c)) arranged therein and inside the side wall 32a. The conductance probe monitors the water level in the container 32 to make sure a minimum safe water level is maintained therein.
FIGS. 2(a), 2(b) and 2(c) show a conductance probe 18 that may be used in the steam boiler 30 in FIG. 1, which includes a base 50, an assembly rod 52, a ceramic insulator 54, a spring washer 56, screws 56a, 56b and nuts 58. The assembly rod 52 is a conducting element partially wrapped with Teflon 53 for providing an electric insulator between the base 50 and the rod assembly 52, leaving a conducting tip 55 as shown. The operation of the conductance probe 18 in relation to the device 30 (Figure is known in the art. By way of example, when installed in a boiler or other suitable device, a wire 57a from a printed circuit board (PCB) 57 in the probe control 18 is coupled between the nuts 58 and the base 50, and a pair of wires such as 57b are coupled from the screws 56a, 56b to ground. When the probe 18 comes into contact with the fluid, a conductance signal is provided to the PCB 57 via the wire arrangement 57a, 57b. The conductance probe may also take the form of a capacitive based probe which is known in the art having a layer of Teflon or other suitable insulator or dielectric material covering the whole tip.
One known technique for monitoring the water level uses a threshold value to determine where the water line is. The problem with this approach is that foam on top of the water line can appear as water to a conductance sensor, if the foam is conductive. This technology overcomes this problem by detecting changes in the stability of the conductance, allowing it to determine when the sensor is near the liquid/foam interface.
Other known techniques for monitoring the water level uses a timing circuit to shut down the process in order to let the foam settle. For example, U.S. Pat. Nos. 5,739,504 and 6,390,027, both issued to Lyon et al., both require periodically turning off a heater for a predetermined time period independent of any detected foam condition, whereas after detecting a foam condition and turning off the device, you wait for a predetermined number of consecutive in-water readings. However, these approaches cause inefficiencies in the process, since the process cannot run 100% of the time. Moreover, while the '504 and '027 patents disclose shutting off the device periodically to minimize the effects of foam, they do not disclose techniques for detecting the presence of foam in the container.
There are many other known devices related to detecting and using fluid levels to control boilers and other devices, such as humidifiers. Of these patents, U.S. Pat. No. 6,078,729, issued to Kopel, relates to using foam detection to provide fluid level control. In the '729 patent, when a capacitive sensor detects normal water levels and a conductive foam detector begins conducting, then a controller concludes that foam is present, thereby triggering corrective action. See the '729 patent, column 3, lines 62-67. However, there is no disclosure in the '729 patent to monitor the volatility/variability of two or more conductive resistance readings. Other known patents do not relate to controlling the operation of the device based on the detection of foam, but instead relate to controlling the operation of a device based on various techniques for monitoring the water level in boilers and other devices having containers of fluid-like humidifiers.
In view of the aforementioned, there is a need in the art for a method and apparatus for sensing a fluid level in a device and controlling the device based on detecting the presence an undesirable foam or unstable fluid line, especially for use in steam boilers and other devices for heating fluids.