How Electric Pressure Cookers Work
An electric pressure cooker consists of three parts, the lid, the inner pot and the housing. See the diagram below to the right. The key internal components are the safety valves and the smart control box.
The Inner Pot
The inner pot is a removable cooking pot. It is generally made from aluminium or, in higher quality machines stainless steel. The aluminium inner pots are pressed and polished or coated or anodised, but this metal is unsuitable for the dishwasher. Higher quality stainless steel inner pots are made with sturdy, three-ply, or copper-clad bases for uniform heating.
The size of the inner pot determines the size of the housing, and to some degree the price of the cooker.
The inner pot capacity typically ranges from 3 litre (5.3 pints) to 6 litre (10.5 pints). Over 6L it becomes difficult to manufacture and the cooker becomes prohibitively expensive to some consumers.
The Lid Lock
The lid has a gasket or sealing ring. When the lid is placed on the cooker in the sealed position, the lid and the inner pot form an air-tight chamber. The pressure inside this chamber increases when heat is applied to the inner pot. This would be an unsafe operating situation if the lid were not in the locked position when a significant amount of pressure is generated inside the chamber. Some electric cookers have the pin lock mechanism to prevent the lid from being accidentally opened while there is significant amount of pressure inside. The pin is essentially a float valve. If there is enough pressure inside the inner pot, the float valve is pushed up by the pressure. Once pushed up, the pin of the float valve serves as a latch lock and prevents the lid from opening, even under force.
Instant Pot also adds further safety assurance with a safety power switch. If the lid is not in the desired fully closed position, the electronic control system of Instant Pot can detect the situation and will not switch on the power for heating.
Similar to the conventional pressure cooker,in the electric pressure cookers the pin of the float valve can be self-destroyed by excessive temperature or pressure. The float valve without the pin becomes a pressure escape hole, and the pressure inside the chamber is released through this hole. Although in such a case the float valve is permanently damaged and it’s possible a lid replacement may be required, this is an effective last line of defence from the safety perspective. The modern electric pressure cookers also use other safety assurance measures before this last line of defence kicks in. Instant Pot employs the patented innovative push-down pressure release mechanism to release excessive pressure, even in the unlikely case where the pin of the float valve cannot be destroyed. Instant Pot provides multi-level in-depth defence systems to offer unprecedented safety assurance to the consumers.
Normally, the only way the pressure can escape is through a pressure release regulator valve on the lid. The pressure release valve has an anti-block shield inside the lid. The pressure release regulator valve is designed not to release the pressure under normal operating pressure range when in the sealed position. If the pressure increases beyond the safe operating range, the pressure release regulator valve will be pushed up, similar to that on a conventional pressure cooker, to release excessive pressure built up inside the chamber. The difference from the conventional pressure cooker and Electric Pressure Cookers is that the pressure release valve only kicks in to release excessive pressure due to possible failure of electronic based sensing circuitry.
The Housing: Sensors, Heating Element & Control Box
The housing unit contains a heating element, pressure and temperature sensors and a control box. The functional diagram of the housing unit is shown on the right.
The control box is the heart of the intelligence of the electric pressure cookers. It monitors the temperature and pressure of the inner pot with sensors. It is equipped with a microprocessor to control the timing, heating and complex cooking cycles. This is how a positive feed-back system is formed to achieve precise cooking conditions. If an unsafe operating condition is detected, it will sound an audible alarm or cut off the power supply to the heating element or both.
The control panel, being the user interface of the control box, allows one-key operation to activate pre-programmed cooking cycles for various dishes. For instance corn, peas, cauliflower and broccoli can be steamed in 1 minute. Frozen corn, peas, cauliflower and broccoli will been done in 2 minutes. Dry beans will take only 30~40 minutes.
Intelligent Pressure Cooking
Electric pressure cookers normally operate at a pressure range of 70-80 kPa (kilopascal) or 10.15~11.6 psi (pound-force per square inch). This translates to the temperature of 115°C~118°C or 239°F~244°F. Initially pressure may reach 105kPa/15.2 psi, due to the heat dissipation delay from the heating element. This is comparable to a stove-top pressure cooker reaching 15 psi and then the heat being turned down.
By switching on and off the heating element and varying the heat intensity at different temperatures and pressures the best cooking results can be achieved in an intelligent way. The following diagram shows the pressure curves of Instant Pot.
Some electric pressure cookers, such as the Instant Pot have intelligent cooking capabilities to achieve the best result, such as:
- automatically altering the cooking time and pressure based on the chosen food type , i.e. rice, soup, meat, vegetable, etc.
- adjusting cooking cycle based on the amount of food in the cooker, by measuring the pre-heating duration.