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Abhishek Rawal
Abhishek Rawal • Dec 4, 2013

How to calculate time for food in the Pressure cooker ?

Consider the capacity of Pressure cooker to be 5Kg. I have kept 1Kg potatoes in it which I would like to cook.
I know that I need 5 whistles to properly cook the potatoes within it.So, I wanna know how much time will it take for 5 whistles.

Can anyone guide me how to calculate it ? I am working on automatic knob for Cooking gas.
Pressure cooking is chemistry. Neglecting the solids part, the pressure inside a cooker is a function of the vapour pressure of water at that temperature and the air present.

Most well made pressure cookers are designed to get the residual air out before it seals. The implication of this is that the system is a single component one. That is only water is present. (Solids do not contribute to the vapour pressure) What this means is that the temperature inside the cooker is controlled solely by the pressure.

Indian Standard for pressure cookers specifies an operating pressure of 1 bar, which corresponds to a temperature of 121 deg.C.

Cooking is a chemical reaction, which therefore has to follow the Arrhenius Rate Equation. In simple terms what this means is that the rate doubles for every 10 deg.C temperature rise. So cooking rate at 121 deg.C is 4 times faster than at the normal open cooking temperature of about 100 deg.C.

However, there is a catch. The time taken for the entire thermal mass of the cooker and contents must first reach 121 deg.C. That time is variable depending on the cooker (aluminium, steel), surface area of cooker (to account for the radiation loss), and amount of food inside. The curious thing is that once the mass has reached the temperature the time to cook is independent of whether you are cooking one grain of rice or 100 kg.

So the timer that controls the gas has to be started only after the operating temperature is reached.

Now for the whistle.
I am sorry but I have to disabuse you about this. It is complete nonsense. The weight valve everywhere else in the world never whistles. A lesser known Indian brand of pressure cookers came up with this 'brilliant idea' in the heydays of Doordarshan's bahu-saas sitcoms like Humlog and such. The valve was designed to lift when the pressure reached the maximum and remain lifted to emit the 'whistle', which can be heard by the women folk watching the serials in the living room. Once the pressure inside dropped to a designed value the cooker will fall silent. The heat supplied again increased the inside pressure to repeat the cycle.

The implication of this is that during the whistle the pressure (and hence the temperature) falls. All the steam that escapes represents totally wasted heat (hence gas consumed). Worse is to come. When the pressure falls, so does the temperature. Implication? Cooking time increases based on the earlier discussion on Arrhenius rates above.

The rationale for the five whistles?

The time between two whistles is reasonably constant in a given system at that particular cooking activity. It depends on the amount of empty space in the cooker, The amount of heat input and the particular weight valve in use. It is one of the worst inventions in my opinion. The time for five whistles can vary widely between cookers and heat sources.

If you want to develop a control system to regulate the gas consumed, I can give you reference to cooking times for different foods at a fixed pressure.

In my house I have replaced all pressure cooker valves with the non whistling hissing type used abroad. We time the cooking. (As an aside, my wife is a high pressure physicist and goes by actual time under pressure.)

Ideally what has to be done is to monitor the temperature (or pressure) in the cooker. When this reaches 121 deg.C (actually 119 to 123), the heat has to be reduced to slightly more than the radiation loss (rule of thumb 300 W in most of India) either by cutting down the electric power or reducing the gas flow. This should be held for the required cooking time for the kind of food inside.

Unsoaked Kabuli Chena needs about 30 minutes, while rice needs only three minutes.

Sorry that the answer probably takes longer to read than the actual cooking. There is lots more to it. But this will do for starters.
Kaustubh Katdare
Kaustubh Katdare • Dec 6, 2013
Wow! Never thought so much about pressure cookers and whistles. Hats Off to you, @A.V.Ramani sire!
lovebox
lovebox • Dec 6, 2013
A.V.Ramani
Pressure cooking is chemistry. Neglecting the solids part, the pressure inside a cooker is a function of the vapour pressure of water at that temperature and the air present.

Most well made pressure cookers are designed to get the residual air out before it seals. The implication of this is that the system is a single component one. That is only water is present. (Solids do not contribute to the vapour pressure) What this means is that the temperature inside the cooker is controlled solely by the pressure.

Indian Standard for pressure cookers specifies an operating pressure of 1 bar, which corresponds to a temperature of 121 deg.C.

Cooking is a chemical reaction, which therefore has to follow the Arrhenius Rate Equation. In simple terms what this means is that the rate doubles for every 10 deg.C temperature rise. So cooking rate at 121 deg.C is 4 times faster than at the normal open cooking temperature of about 100 deg.C.

However, there is a catch. The time taken for the entire thermal mass of the cooker and contents must first reach 121 deg.C. That time is variable depending on the cooker (aluminium, steel), surface area of cooker (to account for the radiation loss), and amount of food inside. The curious thing is that once the mass has reached the temperature the time to cook is independent of whether you are cooking one grain of rice or 100 kg.

So the timer that controls the gas has to be started only after the operating temperature is reached.

Now for the whistle.
I am sorry but I have to disabuse you about this. It is complete nonsense. The weight valve everywhere else in the world never whistles. A lesser known Indian brand of pressure cookers came up with this 'brilliant idea' in the heydays of Doordarshan's bahu-saas sitcoms like Humlog and such. The valve was designed to lift when the pressure reached the maximum and remain lifted to emit the 'whistle', which can be heard by the women folk watching the serials in the living room. Once the pressure inside dropped to a designed value the cooker will fall silent. The heat supplied again increased the inside pressure to repeat the cycle.

The implication of this is that during the whistle the pressure (and hence the temperature) falls. All the steam that escapes represents totally wasted heat (hence gas consumed). Worse is to come. When the pressure falls, so does the temperature. Implication? Cooking time increases based on the earlier discussion on Arrhenius rates above.

The rationale for the five whistles?

The time between two whistles is reasonably constant in a given system at that particular cooking activity. It depends on the amount of empty space in the cooker, The amount of heat input and the particular weight valve in use. It is one of the worst inventions in my opinion. The time for five whistles can vary widely between cookers and heat sources.

If you want to develop a control system to regulate the gas consumed, I can give you reference to cooking times for different foods at a fixed pressure.

In my house I have replaced all pressure cooker valves with the non whistling hissing type used abroad. We time the cooking. (As an aside, my wife is a high pressure physicist and goes by actual time under pressure.)

Ideally what has to be done is to monitor the temperature (or pressure) in the cooker. When this reaches 121 deg.C (actually 119 to 123), the heat has to be reduced to slightly more than the radiation loss (rule of thumb 300 W in most of India) either by cutting down the electric power or reducing the gas flow. This should be held for the required cooking time for the kind of food inside.

Unsoaked Kabuli Chena needs about 30 minutes, while rice needs only three minutes.

Sorry that the answer probably takes longer to read than the actual cooking. There is lots more to it. But this will do for starters.
Thank you @A.V.Ramani Sir for the wonderful explanation.
Just another question. Might sound a little silly.
Would insulating the sides and lid of the pressure cooker's body from the outside increase efficiency? If yes, then why don't they make such designs?
Abhishek Rawal
Abhishek Rawal • Dec 6, 2013
@A.V.Ramani Thank you very much for the information.

I don't want to develop control system to regulate the gas consumed, I want to make a smart-knob that turns off when the food is cooked.
The function of smart-knob is to automatically turn down to 'OFF' mode when the food is cooked. Consider, I have kept milk for boiling, it really pisses me off when I have to wait for the milk to boil completely & then turn OFF the knob manually.
I want it automatic, I want to develop a knob which detects the food is cooked & automatically turns OFF.
Every food takes different time to get cooked, so I really don't have any idea how to make this 'smart-knob' thingy, but my current focus is only pressure cooker. Any idea ?
Kaustubh Katdare
Kaustubh Katdare • Dec 6, 2013
...and I chip-in my vote for a completely mechanical control system!
Vikram S Bargah
Vikram S Bargah • Dec 7, 2013
Abhishek Rawal
@A.V.Ramani Thank you very much for the information.

I don't want to develop control system to regulate the gas consumed, I want to make a smart-knob that turns off when the food is cooked.
The function of smart-knob is to automatically turn down to 'OFF' mode when the food is cooked. Consider, I have kept milk for boiling, it really pisses me off when I have to wait for the milk to boil completely & then turn OFF the knob manually.
I want it automatic, I want to develop a knob which detects the food is cooked & automatically turns OFF.
Every food takes different time to get cooked, so I really don't have any idea how to make this 'smart-knob' thingy, but my current focus is only pressure cooker. Any idea ?

Well It may Sound silly but, In pressure cooker. we mostly depend on the number of whistles blow to identify that the food is cooked. then why not use some counter in the whistle itself. Say if rice is cooked in 3 whistles then we can set a pressure counter which will increment in every blow and as the counter reaches 3 .it send a signal to electronic regulator & it get off.
@Abhishek Rawal
I want to develop a knob which detects the food is cooked & automatically turns OFF
As the old proverb says: tastes differ. What one considers well cooked may be undercooked to some, overcooked to others. The best that can be done is to use a low cost solenoid operated gas control valve available from China. A thermistor sensor circuit with a user settable on-time delay can be used to shut off the solenoid valve controlling the gas flow.
The user sets the delay required to cook the kind of food he has put inside and turns the gas on. The gas remains on till the circuit senses that the operating temperature has reached, the on delay timer kicks in and shuts off the solenoid afterthe set time.
I must mention that the solenoid valve has a pilot passage bypassing the main passage in case the valve is also used to regulate temperature by adjusting the flow. For your application this by pass must be deactivated. The option of using a regular solenoid valve is of course possible.

I had used this Chinese valve a couple of years back to develop a temperature controlled deep fryer. It worked quite well.

@Vikram S Bargah,
then why not use some counter in the whistle itself.
As I mentioned the problem is that the cycle time for whistling is hugely variable depending on the heat input, empty volume in the cooker and ambient temperature. It is an unreliable quantity.

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