I just got this simple autoclave off eBay (prolly just a chinese remake of the legendary All American pressure cooker).

This one is electrical with a resistive heating element inside. One advantage is that is heats up really quickly and does not need a stove.

There is no power switch or heating control so I plugged it in an extension cord equipped with an on/off switch.

Unfortunately, controlling pressure this way is quite tricky. Once 15 psi (or 121° C) is reached, turning the power off will cause sudden drop in pressure followed by slower decrease then. One has to considerably over-pressurize it and repeat the re-heating every few minutes. This is very daunting since sterilizing grain jars takes 90 minutes or so.

I don't care that much however, since the sterilization time is reduced by using higher pressure so there are not as many on/off cycles.

However, I have an idea of picking a Variac, or variable transformer, to smoothly regulate AC voltage until the pressure gauge stays at place. This will simulate the delicate act of regulating flame on a stovetop when pressure cooking.

Of course one still has to keep an eye on the autoclave (as the pressure can build up very slowly anyway), but there will be less manual labor involved. And it will be more fun to set up.

Another idea is to bring more insulation, maybe thick aluminium foil or some pipe insulation material.

What do you think?

Nice bit of kit there - except for the control circuit! It's quite tempting to get one for myself come pay day. Do you think you could improve the response of the thermostat to even out the pressure a bit? Pressure fluctuations could be a problem when using agar, leading to foaming.

The variac seems like a good option although (not that I'd be confident of doing this myself, mind) some kind of thermostat linked to a control microprocessor, e.g. arduino, should get rid of the guesswork once set up properly.

Insulation is, as you say, probably a good idea too. That should also help level out the temperature/pressure a bit.

I am almost sure it will work properly as variacs are commonly used for heating mantles which are, like the autoclave heating element, nothing more than a resistor.

As for the agar, I use 6L pressure cooker with partially immersed whiskey bottle at a slanted angle in it. When finished cooking, I put the whole PC in a kitchen sink filled with cold water. This quickly removes the pressure so it's possible to open the PC and remove the bottle for quicker cooling before pouring the agar.