Technical Description of Zojirushi NHS-10 Rice Cooker
Kaylan Clarke
The City College of New York
ENGL 21007: Writing for Engineering
Kristine Slentz
January 10, 2024
Table of Contents
Definition 3
Overview 3
Visuals 4
Components 7
Explanation 9
Conclusion 9
References 12
Definition
A rice cooker is a device that automatically cooks rice. The Zojirushi NHS-10 rice cooker, like many others, utilizes curie temperatures and water’s latent heat of vaporization to automatically switch from a “cooking” mode to a “keep warm” mode once rice is fully cooked. The cooking time doesn’t rely on a static timer, but rather a temperature-sensitive switch (Technology Connections, 2020). This means the unit can overcome factors like differences in altitude that create variations in how long a batch of rice takes to cook. It simplifies the process of cooking rice, because the user only needs to know the correct amount rice and water to put in the container. Though it doesn’t cook the rice itself any faster, it does save effort for users by practically guaranteeing well cooked rice with little thought, skill, or supervision necessary. It is very user friendly and intuitive, with only one plug and one switch with only two functions. Its simplistic nature makes it easy to mass produce and cheap to buy.
Overview
The Zojirushi NHS-10 Automatic Rice Cooker is built in a similar fashion to most simple rice cookers. As seen in Figure 2, it has a circular outer body that is mostly hollow with handles near the top on opposite sides of each other, and a plug receptacle below one handle. In the center near the bottom, lamps labelled “cooking” and “keep-warm” indicate the current operation of the machine. Below the lamps is a two-function switch that allows the user to control its operation. On the bottom of the inside of the outer body is a heater, magnetic temperature switch on a spring, and all other electric components. Within the outer body is the removable inner cooking pot, steaming tray, spatula, and measuring cup (Figure 1). Figure 2 shows the glass lid with a knob and steam vent that sits atop the inner pot.
Visuals
Figure 1. The items included with the NHS-10 rice cooker, including the manual (Rice Cooker / Steamer NHS-06/10/18, n.d.; Zojirushi America Corporation, 2024).
Figure 2. Image of the assembled parts of the NHS-10 rice cooker (Zojirushi America Corporation, 2024).
Figure 3. Parts diagram of the disassembled NHS-10 rice cooker (Zojirushi, 2006).
Figure 4. Not an image from the Zojirushi NHS-10 model, but this cooker has a very similar outer body interior to the NHS-10. (Technology Connections, 2020).
Figure 5. View of an upside-down rice cooker similar to the NHS-10, with the bottom plate removed to expose the internals (Common Problems and Solutions for Rice Cooker Micro Switches, 2024).
Figure 6. Diagram of the heating, magnets, and lever on the bottom of the outer body (How Does the Magnetic Temperature Switch Work in the Rice Cooker?, 2020).
Figure 7. An isolated magnetic temperature switch. The spring is visible within the casing (NGeGe, 2022).
Figure 8. A diagram including the circuits and heating mechanism for a general rice cooker (Haase & Wikimedia Commons contributors, 2015).
Components
Accessories[1]
- The provided measuring cup has labels for ¼, ½, and ¾ cup. A full rice measuring cup represents 180 ml, or approximately ¾ of a U.S cup. It can be used to ensure the right ratio of water to rice is added
- The Zojirushi NHS-10 rice cooker comes with a stainless-steel steamer pan for vegetables, eggs, seafood, and the like. It fits within the top half of the cooking pot, held up by an indent in the top of the cooking pot.
- One power cord is included and must be plugged into both the unit and the wall for the rice cooker to work. It consumes a maximum of 500 W and has an electric rating of AC 120V (Zojirushi America Corporation, 2024).
- The included plastic spatula is useful for handling hot food after cooking.
- A user manual is included with instructions for cooking different types of rice, as well as various recipes.
External Components[2]
- The outer body has the switch for controlling the heat settings, a plug receptacle, the lamps to indicate the current setting, and the handles for safe carrying. The bottom of the unit has non-slip stands so it stays in place on countertops. It has dimensions (W, D, H) of 10 1/8, 8 7/8, 8 7/8 inches (Rice Cooker / Steamer NHS-06/10/18, n.d.).
- The inner pot has a capacity of 6 cups or 1.2 liters. The inside of the pot is nonstick and has measurements for the correct water to white rice ratio (Rice Cooker / Steamer NHS-06/10/18, n.d.).
- The lid is made of glass and has a plastic knob on the top for safe handling and a steam vent.
Internal Components
- The heating element is the thin, circular metal the heats the inner pot when the unit is on. It surrounds the magnetic temperature switch, as shown in Figure 4. It comes into contact with the bottom of the inner pot to transfer heat to the food inside the pot.
- The switch is attached to a lever and copper contacts. When the switch is set to the “cooking” option, the copper contacts connect. The switch and lever also control which of the lamps are lit up (Technology Connections, 2020).
- The thermal fuse shown in Figure 4 is a safety feature that prevents the unit from overheating or catching fire. It will disconnect the current if it is triggered by dangerously high temperatures (What Is a Thermal Fuses, 2024).
- Magnetic temperature switch is shown in isolation in Figure 7. It consists of two magnets within a casing and a spring. As shown in Figure 6, one magnet is on top of the unit, while the other is attached to the lever on the bottom. The magnets have a curie temperature slightly above 100℃, the boiling point of water (Technology Connections, 2020).
- The thermistor in Figure 4 is a voltage resistor that prevents the heaters from reaching their maximum temperature in “keep warm” mode (Technology Connections, 2020).
Explanation[3]
The Zojirushi NHS-10 rice cooker functions similarly to most two-function rice cookers. The user must first measure out the correct ratio of rice to water in a quantity less than the maximum capacity. Then they must combine the rice and water in the inner pot and put the inner pot inside the outer body. The weight of the pot will depress the spring within the magnetic temperature switch, allowing the user to toggle the “cooking” setting on the switch. Once the rice is done, the unit will automatically toggle the “keep warm” setting, and the user can remove the cooked rice.
The circuitry for a rice cooker like the NHS-10 is shown in Figure 8. Wires connect the lamps, thermistor, thermal fuse, heaters, and lever to power and ground, creating a closed circuit. The copper contacts connected to the switch determine where current flows, and whether they are connected influences if the thermistor is bypassed or not. The critical concepts to understand for the rice cooker mechanisms are curie temperatures and latent heat of vaporization, specifically pertaining to water. Most magnets have curie temperatures, meaning there is a temperature where they lose their magnetic properties. The latent heat of vaporization for water is 100℃, which is also it’s boiling point. Water cannot get hotter than 100℃ because it will begin to evaporate at that temperature. Any additional heat energy added to water once it reaches 100°C will convert the water into steam, not raise its temperature.
To begin, the cooker is automatically set to “keep warm” when plugged in. For safety reasons, the switch cannot be set to “cooking” if the inner pot is not weighing down the spring within magnetic temperature switch. It also cannot be set to “cooking” if the pot is too hot (if there is already cooked rice in the pot, for example). In “keep warm” mode, the copper contacts (shown in Figure 5) are not touching, which means the circuit includes the thermistor, limiting the voltage and therefore the heat of the heaters.
However, when the magnetic temperature switch is weighed down by the pot (by raw rice and water, for example), it is possible to toggle “cooking” mode. This is because, as shown in Figure 6, the magnets come close enough together to attract and attach to each other. This magnetic attachment is what holds the lever, and therefore the switch, in the cooking position. When “cooking” mode is successfully engaged, the lever causes the copper contacts touch, bypassing the thermistor and allowing the unit to run at maximum heat. Because of the latent heat of vaporization of water, the temperature of the contents within the pot will never exceed the boiling point of water (100°C), so the magnets will continue to be magnetic.
Rice is done cooking once it has absorbed all the water. Once all the water is gone, the pot can exceed 100°C. The magnets that hold the switch in the “cooking” position are carefully chosen to have a curie temperature slightly above the boiling point of water. Once the pot exceeds 100°C, then all the water must be gone, meaning the rice must be done cooking. This is the logic behind the automatic switch, and why the magnet must have a curie temperature slightly above 100°C. Once this temperature is exceeded, the magnets will lose their magnetic properties, and the switch will be flipped to “keep warm” by the spring.
Conclusion
Before the use of automatic rice cookers, kitchens in Japan were built with a large stove called a kamado. These not only required large spaces, but they were very difficult to use and required vigilant supervision and hard labor because of the wood fires they used. The first automatic rice cookers, emerging in 1955 in Japan, used a double boiling method in which an outer pot would boil water to generate heat to cook rice in an inner pot. Like the Zojirushi NHS-10 and other modern rice cookers, the machine turned off automatically once all the water evaporated (The Kitchen Revolution – Rice Cookers, 2020). Machines reminiscent of the modern electric type emerged in the 1950s through the Japanese company Toshiba, which released the first one-pot cooker. Induction cooking was developed in the 1980s and is the method that the Zojirushi NHS-10 uses to this day (Ewbank, 2020).
The mechanisms for this rice cooker are relatively simple, but effective and reliable. Other products, like the newer models of Toshiba cookers, have more functions like fuzzy logic and programmable functions. Although there may be more complex machines out there, their multitude of options means many more components and more opportunities for things to malfunction and break. Any product requiring a microcomputer, display, or software is susceptible to more issues like crashing and software bugs than one relying on simple hardware like the NHS-10. Finally, basic safety should be exercised when using and examining this product. Users should never touch any of the internal components when the unit is plugged in, as it can cause serious burns. The thermal fuse should never be removed, as it is an important safeguard for malfunctions.
References
Common problems and solutions for rice cooker micro switches. (2024, July 20). Unionwell. https://www.microswitchsupplier.com/blog/rice-cooker-micro-switch.html
Ewbank, A. (2020, July 31). The Battle to Invent the Automatic Rice Cooker. Atlas Obscura. https://www.atlasobscura.com/articles/rice-cooker-history
Haase, H., & Wikimedia Commons contributors.(2015, July 18). File:Rice Cooker 115V.svg. Wikimedia.org; Wikimedia Commons. https://commons.wikimedia.org/w/index.php?title=File:Rice_Cooker_115V.svg&oldid=501050759
How does the magnetic temperature switch work in the rice cooker? (2020, May 30). vectormagnets.com; Hangzhou Magnets Co., Ltd. http://www.vectormagnets.com/f728592/How-does-the-magnetic-temperature-switch-work-in-the-rice-cooker.htm
NGeGe. (2022, August 30). Below 1200W General Rice Cooker Temperature Limiter,Rice Cooker Center Thermostat Accessories(2PCS). Amazon.com. https://www.amazon.com/General-Temperature-Limiter-Thermostat-Accessories/dp/B0B9K11DR2
Rice Cooker / Steamer NHS-06/10/18. (n.d.). Zojirushi Online Store. Retrieved January 10, 2025, from https://shop.zojirushi.com/products/nhs?variant=42422576906462
Technology Connections. (2020). Old-fashioned rice cookers are extremely clever. In YouTube. https://www.youtube.com/watch?v=RSTNhvDGbYI
The Kitchen Revolution – Rice Cookers. (2020). Kids Web Japan. https://web-japan.org/kidsweb/hitech/rice/rice01.html
What is a Thermal Fuses. (2024). Www.swe-Check.com.au. https://www.swe-check.com.au/editorials/thermal_fuses.php
Zojirushi. (2006, June 16). Zojirushi 6-Cup Rice Cooker. Amazon.com. https://www.amazon.com/Zojirushi-NHS-10-6-Cup-Uncooked Cooker/dp/B00004S576?th=1
Zojirushi America Corporation. (2024). Operating Instructions for Automatic Rice Cooker/Steamer. Zojirushi.com. https://www.zojirushi.com/servicesupport/manuals/manual_pdf/nhs.pdf
File:Rice Cooker 115V.svg. (2020, October 25). Wikimedia Commons. Retrieved 15:57, January 9, 2025 from https://commons.wikimedia.org/w/index.php?title=File:Rice_Cooker_115V.svg&oldid=501050759.
Zojirushi Owner Manual
[1] All accessories are shown in Figure 1
[2] All external components shown in Figure 2
[3] All information for the explanation is according to Technology Connections (2020).
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