Keywords: solar water heater, NTC, water level detection

Abstract: In order to improve the reliability and stability of water level detection of domestic solar water heaters, a solution of water level detection using NTC is proposed in view of the problems existing in the current water level detection technology. The working principle and technical points of NTC water level detection are described.

This paper discusses the water level measurement of the domestic solar water heater tank (hereinafter referred to as the solar water heater) which is widely used at present. Its water tank is of non-pressure type and is integrated with the glass tube collector.

First, the role of solar water heater water level detection

As the solar water heater and water tank are installed outdoors, with the injection and discharge of water, the amount of water in the water tank changes accordingly, so the water level detection has its special role.

First, users need to know the amount of water in the tank when using hot water. Prevent dehydration during use, especially during bathing. Solar energy water heater and water storage pressure type water heater is different, under normal circumstances should not be in the process of water to the water tank water, otherwise will make water temperature drops, the impact of normal use. Of course, most of the current solar water heater has electric auxiliary heating function, but in addition to special circumstances, users should try to play the role of solar heating, less electricity heating.

Secondly, automatic water injection control. If there is no water level detection, it is impossible to realize the automation of water injection to the water tank. Whether the water tank should be filled with water and stop the control of water injection after the water is full, and the water overflow of the water tank can not be separated from the water level detection in the water tank.

Two, the current situation of water level detection of solar water heater

Restricted by the structure and shape of the solar water heater tank, the water level test shall not change or destroy the size of the tank and the insulation parts. The water level detection component can only be extended into the box from the upper or bottom of a reserved Ф round hole of about 20mm.

At present, most solar water heaters use electrodes to detect water level (referred to as electrode type), the principle is to use the electrical conductivity of water. The electrode pieces are distributed on an insulating rod according to certain size requirements to form a measuring rod and inserted from the top of the water tank. As shown in Figure 1, where A, B, C and D, including the common end, are electrode plates. When the A, B, C, and D electrodes are immersed in water, A current is generated with the common end. The actual detection is based on whether there is current on the electrode piece, to judge whether it is immersed in water. Electrode control and detection circuit is relatively simple, low cost, so it has been widely used, is the mainstream technology at present. However, when the electrode scales (the attachment of sodium and magnesium ions), the problem leading to measurement error has not been fundamentally solved. Because the electrode after scaling, or scale thickness reached a certain extent, the water level can not be measured normally. The situation is more serious in places with poor water quality. In order to ensure the accuracy of the measurement, the measuring rod must be replaced regularly.

A small number of solar water heaters use a water pressure sensor to measure the water level. The pressure sensor installed at the bottom of the water tank determines the water level (quantity) by measuring the water pressure, referred to as the pressure type. This is shown in Figure 2. The pressure type avoids scale measurement problems to some extent, however, the sensor is not able to withstand temperatures above 60 ° C (the actual water temperature often exceeds 60 ° C). Although some insulation measures can be taken, it is not possible to prevent damage to the sensor caused by excessive temperature. In addition, scale can also affect the measurement results after plugging the measurement holes.

There are also the use of dry reed tube water level detection, referred to as dry reed tube type. Principle is within a measuring bar in accordance with the requirements for a certain size (like the electrode type) distributed embedded dry reed pipe, each dry reed pipe section set a ring float with permanent magnets, dry reed pipe in each section of the top of the limit and float to move up and down in the segment, only when the buoyancy of water is raised float to the top of the block permanent magnets attract dry reed pipe. The measuring rod is inserted from the top of the tank, as shown in Figure 3. It can be seen that when all floats are located at the top of the section, all the dry reed pipes are in action, so the water level is the highest. On the contrary, when all floats are located at the bottom of the section, all the dry reed pipes do not operate, then the water level is the lowest. The dry-reed type has lower reliability due to mechanical motion (up and down movement of the float).

III. Advantages of NTC water level detection

NTC is a widely used thermistor whose main characteristic is that the resistance value changes significantly with the temperature, and decreases with the temperature rising. The full name for NTC is negative temperature coefficient thermistor. NTC has the characteristics of low price, simple circuit composition, stable performance and so on, and is widely used in temperature measurement. In fact, NTC can also be used for liquid level measurement. In the author's opinion, NTC used in the water level measurement of solar water heaters has already had the technical conditions to replace the electrode type, it has some other advantages besides the electrode type small size, simple structure and so on.

When NTC is used in water level detection, it not only retains some advantages of NTC in temperature measurement, but also shows some advantages that are not available in current water level detection methods. First of all, NTC belongs to semiconductor devices, solid devices are characterized by no mechanical movement and long life; Secondly, in addition to the simple circuit structure, it can also take into account the function of measuring water temperature; The most important thing is that it completely gets rid of the trouble caused by scale.

Objectively speaking, electrode type water level detection technology, if there is no trouble with scale should be a more realistic water level detection solutions (still need to solve the problem of asymmetric electroplating), however, the scale on the electrode due to the failure of the electric control system, solar water heater is run stable and reliable for a long time has not been able to overcome obstacles. In fact, it is not difficult to eliminate the problem of scale, the surface of the surface of Teflon (PTFE) coating (for example, similar to the surface of non-stick pan coating) can be very effective in preventing scale, the key is that these coatings are insulated. Electrode-type electrodes happen not to be coated with this kind of coating, because it needs to conduct electricity to work. The NTC works in a different way when measuring water levels, so it can be coated. This prevents the formation of scale and does not affect the detection of water level, thus effectively avoiding the problem of scale. On the other hand, from the working principle of NTC water level detection, it is not sensitive to scale in the detection process, that is to say, a small amount of scale does not affect its detection results. In fact, assuming that the surface of the NTC has been fouled, the temperature variation of the NTC during the inspection process will facilitate the scaling off.

Four, the working principle of NTC water level detection

Take advantage of the temperature difference. In general, the temperature of the air and water in the water tank of the solar water heater is inconsistent. It is imagined that the temperature in the water tank can be measured through NTC and the temperature difference between the air and water can be used to detect the water level. In-depth analysis, there are several insurmountable problems: (1) it is not easy to distinguish the state full of water and no water; (2) The temperature of water at different depths is often different. The upper temperature of water in the process of heat collection is higher than the lower temperature. ③ When the ambient temperature is low, the temperature near the measuring hole changes greatly. Although rigorous logic judgment circuits and software can solve these problems, physical judgment is more direct, more reliable, simpler and cheaper. At present, there is no market application case of using temperature difference to detect water level, but this idea can be used for reference.

Since the heat. The use of NTC always through a certain amount of current, the current to NTC itself to generate heat, referred to as self-heating. Because NTC is a heat sensitive element, self-heating will lead to a decline in the resistance value. When NTC is used for temperature measurement, self-heating should be avoided as far as possible, or the temperature rise caused by self-heating should be controlled to a negligible degree, otherwise the measured temperature will be inaccurate. If the heat generated in the NTC artificially, and set the thermal constant power and constant power, and the surrounding temperature for a certain value, at this point, its temperature rise depends on the surrounding medium, heat capacity (heat) big medium small medium to low temperature specific heat capacity, because of the heat capacity big medium specific heat capacity of small medium absorbs more heat NTC. For example, the heat capacity of water is larger than that of air, which is 2.5 times that of air. The temperature of self-heated NTC in water is lower than that in air at the same temperature. That is to say, when the temperature of NTC is lower, it indicates that it is in water. On the other hand, when the NTC temperature is high, it is in the air. This is the basic principle of measuring water level. NTC has the property of distinguishing medium with different heat capacity under the same temperature condition in the self-heating state, the premise is to know the temperature of the medium. In the actual detection, the temperature of water and air is often inconsistent, so it is necessary to add some conditions and data to detect the water level, otherwise it will lead to errors. For example, when the air temperature is too low and the water temperature is too high, it is impossible to determine whether NTC is in the water or in the air only according to the size of the NTC temperature rise under the condition that the NTC self-heating power remains unchanged.

Basic circuit. This refers specifically to self-heating circuits. In order to make the actual circuit of water level detection simple and reliable, the constant power mode or even the constant current source mode is generally not adopted. Just use the metal film resistance voltage divider circuit structure, like the temperature measurement circuit, is very simple, as shown in Figure 4.

Basic data. Two sets of "voltage-temperature" data for air and water can be obtained by measuring and collecting voltage values at different temperatures and under conditions of two media (water and air), respectively, according to the basic circuit. According to these two sets of data, we get two voltage values below each temperature value. Obviously, the larger value is obtained from water. The smaller value is the number obtained in the air.

The basic method. For each NTC water level detection process can be carried out in accordance with the following steps: ① temperature measurement, to determine the location of the NTC temperature value; (2) voltage, the measurement of self-heating voltage value; (3) search, find two data in the basic data set under the corresponding temperature conditions; (4) judging the results, the voltage value is equal to or close to the larger (smaller) value, it indicates that in water (air); ⑤ Temperature measurement, return to the first step of circular detection.

External structure. The household solar water heater industry is accustomed to display the water level of the water tank in four grades. NTC is installed on a measuring rod, shape and electrode type is basically the same, NTC position and the position of the electrode sheet consistent. This is shown in Figure 5.

Technical points of NTC water level detection of solar water heater

As a result of solar water heater installation in the outdoor, sun and rain, water level detection of the control cable is longer, environmental conditions than other household appliances more severe, has its own technical requirements.

Temperature rise requirement. NTC components and leads in the tank shall withstand high temperature. Considering that there will be no water in the water tank of the solar water heater, the glass vacuum collector tube is still collecting heat normally at this time, and the maximum temperature in the water tank is 260℃. While the odds of this happening are small, it must be taken into account. The temperature of NTC and lead shall be 300℃.

Waterproof requirements. NTC must be able to withstand the test of years of immersion in water, its parameters must be stable, electrical performance must remain safe and reliable.

Cut cables. The control cable of water level detection has a relatively long distance from the outdoor water tank to the indoor controller, so it has practical significance to reduce the number of wires as far as possible, both from the perspective of reducing cost and improving reliability. In fact, the NTC of measuring temperature and the NTC of self-heating can be shared, so that the NTC can be saved by half, and the number of cables can be reduced by half. Measuring temperature and generating self-heating can be achieved by switching the NTC's divider resistor (or applying a different voltage value) with a control circuit. It takes a certain time for NTC to change from temperature-measuring state to autoheating state. Similarly, NTC from thermal state transition to cool the temperature measuring state needs certain time, the state transition time caused the fall of water level detection speed, in spite of this, control process can still meet the detection requirements, because of rising water tank (water injection process), and fell on the water (process) reaction speed slower than the speed of the circuit.

First power on. The circuit design shall measure the temperature first. When the temperature is higher than 100℃, it indicates that the water tank is out of water (water shall not be injected at this time, otherwise the glass tube collector will burst and damage). At the same time, all NTC need not enter the self-heating state.

State of the NTC. When the water level is detected, only one NTC above and below the water surface is in the self-heating state, and the other NTC is in the temperature measuring state. For example, if the water level is between Cd, the two NTCs C and D are in the self-heating state, while A and B are in the temperatures-measuring state. When water is full, only D NTC is in the self-heating state, and the others are in the temperature-measuring state; When there is no water, when the temperature is below 60℃ (in the state of preparing water injection), only A NTC is in the state of self-heating before water injection control, and the others are in the state of temperature measurement. This is shown in Figure 5.

Meet the EMC. In order to reduce the cost, NTC needs to use unshielded cable, which is easy to form the interference source to the electronic control system, which is the anti-interference focus of the system.

The above content is the academic discussion of the utility model patent "A kind of solar water heater and water tank level detection device" (ZL 2005 2 0120442.6), the author is the designer of this patent.