The water level Controller is used to indicate and control the height of water level in overhead water tanks. An LED Bar graph is used to display the water level.Copper probes are used to sense the water level. The probes are inserted into the water tank which is to be monitored.
The 8 bit Microprocessor 8085 is used in the circuitry. Water level is displayed continuously and it turns on the indicator when water level goes beyond a certain level.
The water level is also displayed over the LED display. All functions are done through the Programmable Peripheral Interface IC 8255.
Present Liquid Level measuring devices that are used rely on the change of resistivity of the probe with temperature.. A significant decrease in the resistivity of the probe over its value when it was measured in air indicates that the probe is submerged in liquid. Devices that rely on change in resistivity are usually only capable of determining whether or not a certain level, such as the location of the probe, is reached. Liquid levels can only be determined at discrete locations of the probe only. Intermediate levels between two probes is the only thingthat can not be determined. Resistive probes are temprature sensitive and are accurate only at the temperatures in which they are calibrated.at other operating temperatures they need to be caliberateed. Furthermore, resistive probes usually require such components, resulting in a that is susceptible to electromagnetic interference or electrical noise from other electrical/electronic equipment which lie within its vicinity.
This circuitry precludes the shortcomings inherent in liquid level devices which employ resistive probes, because actual temperature are used and processed, and the temperatures measured are dependent on heat transfer mechanisms rather than change in resistivity of the probe material. Furthermore, the circuit is capable of not only determining liquid level at discrete points where the temperature sensors are located, but can also measure liquid levels at intermediate points between two temperature sensor locations, which the resistive type device is incapable of doing.
The circuit described here is a means of measuring the level of a liquid in a liquid container such as a fuel tank by means of a probe to which heat is applied and the temperature along the length of the probe is measured. The circuit makes use of the difference in cooling efficiency between liquid and gas such as air, or between any two different liquids, such as water and oil. When heat is applied to the probe, the temperature of the portion of the probe submerged in liquid is lower than the temperature of that portion of the probe outside of the liquid and is typically exposed to air. This is because the liquid removes heat at faster rate than air, so that the temperature difference between the surface of the probe is much lower in liquid than it is in air. Water removes heat more efficiently, and oil does not do so effectively .This principle is used to operate it. Temperature sensors, such as thermocouples or thermistors that are attached to various points on the probe measure the temperatures at various locations on the probe. This invention is not only capable of determining where the liquid level is at discrete points where the temperature sensors are attached, it can also determine where the liquid level is between two discrete points to with great accuracy, when precision temperature measurement devices are used in conjunction with suitable microprocessor, which process the signals received from the temperature sensors.
The purpose of this invention is to provide a device that can measure liquid levels, such as that of fuel in an automobile fuel tank or lubrication oil level in an automobile engine compartment fairly accurately and with minimal effort, such as simply pushing a button on an instrument panel. The advantages of this invention are (A) It can measure liquid levels accurately;
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(B) It requires less power to operate;
(C) It is compact and light weight;
(D) It is very reliable;
(E) it does not generate any significant amount of electromagnetic energy;
(6) with certain modifications to the device, it can be used to measure other important liquid parameters such as viscosity and density. This device can also be adopted for the detection of ice formation on the external surface of an aircraft, such as the external surface of an aircraft wing or fuel tank.
DETAILED DESCRIPTION OF THE CIRCUIT
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An application of the liquid level measuring apparatus is measuring the liquid level of fuel in an automobile fuel tank or lubrication oil level . The liquid level sensor probe 20 is installed inside the oil pan 10. A plurality of electrical wires 21 connect the probe 20 to a microprocessor 22 which may be located behind the automobile instrument panel 30. The microprocessor is in turn connected by one or more wires 23 to a display 31 located on the automobile control panel 30. A power suppl 24 made of one or two batteries is located either in the engine compartment, behind the automobile control panel or close to the probe and electrically connected to the microprocessor 22, the probe 20 and an activation button or switch 33 located on the automobile control panel, provides electrical energy to the liquid level sensing system. Sensing of the lubrication oil level is accomplished by activating the button or switch 33, sending electrical energy to the probe 20 and the microprocessor 22. The lubrication oil level is displayed on the display 31.
The circuit makes use of the cooling efficiency between liquid and gas, such as air, or between two different liquids. The embodiment is comprised of a probe 40 made from 0.002 inch diameter Nichrome wire three inches in length but maybe of any suitable lengths, a microprocessor 50, a display 51, an electrical power source 52, a switch 53, electrical wires 54 and, connecting wires 56. Six thermocouples beads 41, 42, 43, 44, 45 and 46 from 0.008 inch diameter or Copper-Constantan pairs of wires are attached to the probe 40 by wrapping the probe Nichrome wire around the thermocouples beads 41 through 46. The number of thermocouples beads may be varied depending on the length of the probe and the accuracy desired. The thermocouples 41 through 46 are electrically connected to the microprocessor by a Copper-Constantan wires 47 of suitable size and length. The probe 40 is coated with an insulating material to electrically isolate it from the thermocouples beads 41 through 46.
When the switch 53 is in the open position and no power is applied to the probe 40, the temperature of the thermocouples 41 through 46 will measure the same temperature as the media which surrounds the probe. When the switch 53 is in the closed position, current flows through the circuit including the probe and heat is generated at the probe 40 in the form of I2 R POWER losses. The heat generated at the probe 40 is dissipated to the surrounding medium. In order for heat to be dissipated to the surrounding medium the temperature of the probe has to be higher than that of the surrounding medium. At steady-state condition, that is when the temperatures have stabilized some time after the. switch 53 is closed, usually several seconds, the characteristics temperature difference between the medium and the probe 40 is established. For example, if 6.0 milliwatts of power is applied to the probe and the entire probe is in air which is maintained at a constant temperature of 20 degrees C., the temperature at the thermocouple location 41 through 46 are approximately 35 degrees C., approximately 15 degrees C. higher than the temperature of the surrounding air when Steady-state condition is reached. If the entire probe is immersed in water, also maintained at 20 degrees C., the temperature of the probe at the thermocouples 41-46 locations will only be slightly above 20 degrees C. The actual temperatures at the thermocouples locations are found in Table 1. This is because water can remove heat from the probe at much faster rate than air.
TABLE 1 ______________________________________ TEMPERATURES AT 6 THERMOCOUPLE LOCATIONS LIQUID LEVEL DEVICE TEMPERATURES (DEG C.) THERMO- TC 4, 5 & COUPLE NO. ALL IN AIR ALL IN WATER 6 IN WATER ______________________________________ 1 34.493 20.145 33.207 2 34.493 20.145 31.914 3 34.493 21.145 28.035 4 34.493 20.145 20.52 5 34.493 20.145 20.149 6 34.493 20.145 20.145 ______________________________________ NOTE: BOTH AIR AND WATER TEMPERATURES = 20 DEG C.
So the water requires only a small temperature difference (less than 1 degree C.) to remove the same heating rate as the air has to remove. The temperature profile of the probe is:
(A) where the entire probe with 6.0 milliwatts power is in air whose temperature is 20 deg (55),
(B) where the probe is completely immersed in water whose temperature is 20 deg (56), and
(C) where the probe is immersed in water from thermocouples location 43 to 46, with both air and water maintained at 20 degrees C.
In this example heat is transferred from the surface of the probe to the surrounding medium by free convection. The basic convection heat transfer equation (applicable to both free and forced convection) is
Where qis the heat transfer rate
h is the convection (free convection in this case) heat transfer coefficient.
A is the area of the probe exposed to the medium
Tp is the temperature of the probe surface exposed to the medium
Tm is the temperature of the medium (air or water in this example)
The temperature difference between the probe surface and the medium is expressed as DT or
In this example the values of Q and A in equations 1 and 2 are held constant. Only h, which is a measure of the heat transfer or heat removal efficiency, is varied. The higher h is the lower DT is. Water, which is a good heat transfer liquid,is orders of magnitude better than air in removing heat from the probe.
When only liquid levels at discrete locations are desired, such as where the six thermocouples 41-46 are located, the processing of the temperature data becomes too simple. The points (thermocouples locations) that are completely immersed in water will indicate a much smaller DT. For example, if thermocouples 44, 45 and 46 are completely immersed in water and thermocouples 41, 42 and 43 are in air, the temperature of the six thermocouples 41-46 cannot be constant. The DT's of the thermocouples immersed in water will be much lower. The temperature distribution along the probe when the thermocouples 41, 42 and 43 are in air and when thermocouples 44, 45 and 46 are immersed in water are shown as 57 in FIG. 3. From comparison of the difference in temperatures of the six thermocouples 41-46 to each other, it can be determined which thermocouples or discrete points are immersed in water.
The device can also be used to determine the liquid levels at any intermediate points between the thermocouples locations.For e.g. the liquid level is somewhere between thermocouple 43 and thermocouple 44 and it is desired to determine the location of the liquid level within 1.3 millimeter. The space between thermocouple 43 and thermocouple 44 of the probe 40 is 12.7 millimeters. If the space is divided into ten equal spaces, the distance between each intermediate mark is 1.27 millimeters, within the 1.3 millimeter accuracy desired. As the level of the water is varied from thermocouple 43 (o distance from thermocouple 43) one intermediate mark at a time to thermocouple 44 (12.7 millimeter distance from thermocouple 43), the actual temperature of thermocouple 43 and thermocouple 44 and the difference between the two temperatures will vary, as shown in Table 2 and FIG. 5. These data can be processed by the microprocessor to where the actual liquid level is. The thermocouples or equivalent temperature sensors used to measure the temperatures at the various locations will have to be able to provide much more accurate readings than when only discrete temperature levels are being measured. This can be accomplished by using the entire spatial profile of differential rather than absolute thermocouple readings.
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