Autoclave thermal controller

I'm attempting to make a project that uses an Arduino Uno to control two to four autoclaves at a time. I want to get away from the constantly failing and unreliable thermal switches that currently run them.

Currently the design is an Uno with a 16x2 lcd display (RGB LCD Shield Kit w/ 16x2 Character Display - Only 2 pins used! [NEGATIVE DISPLAY] : ID 714 : $24.95 : Adafruit Industries, Unique & fun DIY electronics and kits) taking temperature measurements with a type-k thermocouple (Thermocouple Type-K Glass Braid Insulated Stainless Steel Tip : ID 3245 : $9.95 : Adafruit Industries, Unique & fun DIY electronics and kits) via the max31855 breakoutboard (Thermocouple Amplifier MAX31855 breakout board (MAX6675 upgrade) : ID 269 : $14.95 : Adafruit Industries, Unique & fun DIY electronics and kits) to control the heating element with a normally open powerswitch tail (Powerswitch tail 2 : ID 268 : $25.95 : Adafruit Industries, Unique & fun DIY electronics and kits).

Ideally I would be able to control two autoclaves per Uno and be able to control their operating temperature on the LCD screen and have it also display a countdown timer for the sterilization cycle.

At some point I would also like to replace the mechanical pressure gauges with pressure transducers as well but, I still haven't found a transducer that can operate at the pressure/temperature I need (250 F+). Right now I just need to have a working prototype for a single autoclave that only uses a temperature reading to operate.

Any help with this is greatly appreciated.

I know the pressure transducers to work at 250degF exist, I have seen them on fully automated autoclaves. They exist, but may not exist inexpensively.

You haven't posed a problem. What do you need help with?

You sort of implied that you need to control the temperature of the autoclaves by simply turning the power on and off. Sounds good so far. So what is the required temperature profile? Just constant temp for a period of time or something more complex?

There are many Arduino temperature controller projects published online. Pick one that is close to what you want and modify it.

The solution for the pressure sensor is to mount it on a tube connected to the autoclave. Then the sensor electronics can be away from the hot part.

Hopefully your autoclaves have some kind of mechanical failsafe that will kick in if your code turns out to have a bug or two in it.

Looking for a cheap pressure sensor that can withstand 250 degrees liquid?

Likely you could find solution in a automotive oil pressure sender.

Your project is very much in the realm of a uno to develop and test with.

Very likely the unit is going to have a existing built in thermal cutoff switch, be sure to keep that in your heater circuit.

adwsystems - The current problem is making sure I have selected the correct hardware for the solution I plan on implementing. As far as the pressure transducer goes I will look into the automotive pressure sender Slumpert suggested.

MorganS - You are correct I merely need to control the on/off state of the 120v heating element via a digital temperature reading. I would like to implement a stepping heat stage at the begining to help reduce wear on the heating element then it has to maintain a set temperature for 4 hours then shut off.

wildbill - Yes there is a rubber plug built into the lid that will pop out and release the steam pressure before critical PSI is reached. I hope that in combination of a normally off relay and the plug will prevent any failures resulting in damage to the equipment.

Slumpert - Thank you for your suggestion of the oil pressure sender. As far as I can tell there is not a thermal cutoff built into the autoclave. It uses a thermal switch (http://www.goodmans.net/i/9537/all-american-4159-thermoswitch-50x-75x.htm) to control power to the heating element.

I am trying to modify an All American 50x sterilizer (http://www.allamericancooker.com/allamerican50xsterilizer.htm). We use them to sterilize various growth mediums for mushrooms that are sold to local restaurants. With the current physical thermal switches we are plagued with unreliable temperatures and switch failures. Which is why I am wanting to move to a digital controller to reduce maintenance costs from parts and losses from production runs being contaminated from inadequate sterilization because of the hit or miss physical controllers.

What is damaging the heaters now? Overheating because the heat takes a while to reach the sensor? Maybe just reposition the sensor? Change procedures so it's not overloaded or underloaded?

I believe it is just wear from thermal expansion that pushes against a plate that operates the internal relay in combination with poorly made parts from the factory that causes them to become unreliable then fail.

I'd be inclined to get the thermocouple and breakout board (and an Uno of course) and see whether you can get decent temperature results. Worry about the power switch tail and LCD later.