Abstract:
This paper is intended to provide an economical solution which is based on PP float and magnet, whose water level could be detected and controlled by reed switches. This solution can be used widely in household appliances field, such as air-conditioning fan and water fountain.

Introduction
In the field of household appliances, liquid medium is widely used, among which water accounts for the main part, such as making coffee and soya-bean milk, washing clothes, etc. The measurement and control of water level can improve food taste and save water for energy savings.

Considering that it’s related to food processing field and public’s high requirement of sanitation and safety for food contact materials as well as the impact on food taste of water precise control, household appliances such as coffee machine and soya-bean milk machine etc. are widely equipped with externally no water-contact level controller of high precision capacitors, inductance and infrared ray, but these electronic water level detection devices need being installed distributedly at the limited level, the cost is relatively high and have a shorter service life, compared with mechanical water level controllers.


Figure 1 Capacitance water-level detection in coffee and soya-bean milk machines

The reason why these water level control plans mentioned above cost higher is that water level detection devices need to be installed in distributed type at the limited level (or more level detection points) yet electronic device’s service life will be affected because of its bad weather ability (heated by fluid with high temperature) but the good point is that external water level detector can be used regardless of sanitation and safety for food contact materials, waterproof problem and size.

Thinking about cost and service life, more customers are hoping to find a more economical water-level detect plan with quality assurance. This paper is introducing one magneto electric type of water level sensor in air-conditioning fan field. This water level sensor in the plan uses PP, eco-friendly material, in accord with requirement of sanitation and safety for food contact materials, has reed switch with 106 service life to control limit level mechanically and it owns compact leak-proof structure. Its size is controlled by process and the small and simple structure can be waterproof.


Structure and application of designing plan

The soft and natural wind from air-conditioning fan can improve different body's response. And compared with high power cooling of air conditioner, the air-conditioning fan is with lower power consumption of only 50W-80W, catered to the new market needs. According to statistics, market capacity of air-conditioned fan in 2011 reached to 2 million sets with output value of over 1 billion yuan, accounting for 10% of the fan market [1]. 


Figure 2: Application of water level control driven by PP float and magnet in field of air-conditioning fan and water fountain

Magneto electric water level controller consists of magnet, PP float, reed switch, PP shell and wire. PP float and magnet are integrally molded by foaming, magnet design is with thin wall (less than 2mm) and the magnetic flux density is no less than 80mT for driving reed switch off after foaming mold. The overall density of PP float and magnet is set to 0.7g / m3 and the material is Sinopec S700 or IRPC 1100nk.


a) Structure of water level controller driven by reed switch PP float
b) PP floater with magnet embedded
Figure 3: Structure of water level controller driven by PP float and magnet
Given the defined structure qualification (space) of PP shell, so the welding structure of reed switch and wire and encapsulation process have a major impact on the waterproof performance of products, which are the key parameter for product reliability.

Test verification

After testing the selected water level controller with magnet of t=1.9 by Lakeshore480 Fluxmeter according to IEC 60404-14 standards, results meet the design requirements and realize the magnetic drive for reed switch. And this slim magnet which can drive reed switch can effectively control the overall density of float and magnet.



Foam sample of PP float is tested according to GB 9688-1988 Hygienic Standard for Polypropylene Products Used as Food Packing and FDA 21 CFR 177.1520. It should be noted that, foam density for FDA is 0.85-1.0g/cm³, which is not possible to reach based on the industrial foaming conditions and special adjustments need to be done for mold parameter of tested sample for FDA. Likewise, as required in FDA for foam density, the reliability couldn't be guaranteed, taking into account the impact of water absorption on the product after a long-term immersion process.

Test result of extracting 230g sample according to standard of GB 9688


FDA 21 CFR Part 177.1520 Test Result


Testing and application of the water level controller depends on the way how water limit level is controlled. The typical one is with air-conditioning fan’s normal open controller structure and the water level reaches to the ON (maximum water level) submersible pumps starts to work.

Test is done to the normal open water level controller in accordance with corporate standards. Namely, contact resistor and water limit level are used to test power on and off with resistance instrument, test action cycle with life-span test machine, test electric strength with withstand voltage tester, insulation resistance meter and soaking tank and to do thermal impact test with high temperature box and low temperature box.

Test of on/off Distance and Value of Contact Resistor for Normal Open Water Level Controller of CPS-8500



Normal Open Water Level Controller of CPS-8500 Electric Strength Test


Normal Open Type Water Level Controller of CPS-8500 Thermal impact test


Epilogue

This paper is intended to provide an economical solution which is driven by PP float and magnet and whose small type magnetic-sensing water level controller could be controlled by reed switches. In the application of air-conditioning fan, it can realize the measurement and control of water level limit by connecting submersible pumps to pump water at the max. level and adding more water at the minimum level. Products meet the requirements of sanitation and safety for food contact materials of GB 9688 and FDA 21 CFR 177.1520. It meets electrical safety requirements under the condition of 150℃ temperature difference between the hot and cold shock and soaking for 12h, gets the CQC product certification and has been widely used in field of air-conditioning fan and water fountain.

It is worth mentioning that another type, MR reluctive transducer can reach to the accuracy of 0.03mm after repeated positioning, its response time is12.5μs, magnetic drive is less than 1.5mT. Therefore, it’s thinkable to displace the reed switch as magneto sensor so as to meet the requirements of water level measurement and control in the field of high precision. But also it needs to be pointed out that the weather ability of MR reluctive transducer still needs to be improved.


Cited Reference

[1]   Air-conditioning fan: Niche industry is unworthy or popular. [Z]. Huicong Air-Conditioning Cooling Network, May 4, 2012. http://info.hvacr.hc360.com/2012/05/041002440575. shtml.