Yoyo, sorry for the long post.
DONT READ UNLESS YOU ARE INTERESTED IN PID or want to argue for fun
wheels wrote:Thanks, that illustrates the difference well.
Have any members got any suggestions for which PID to use? That is, one that will give the accuracy that DiggingDogFarm points out isn't present in the very cheap versions, but which doesn't break the bank. Oh, and either available in the UK, or that can be shipped to, and used, in the UK.
Phil
For simplicity and cost...buy this;
Re: Sous Vide anyone ?
Postby quietwatersfarm » Tue Dec 17, 2013 1:03 pm
Just to say that these
http://anovaculinary.com/ are the business. Wouldnt swap for anything, running three units here in big plastic cambro tubs.
Seriously, unless you've got cash, time and nothing else to do..it is the way
That's what I'm gonna do!
I used to work for these guys;
http://www.mtl-inst.com/One of the things they specialize in is process control for the hazardous materials industry.
I'm gonna go in into more detail (sorry
)...
So if you DIY guys are looking for the easy way out...see above. If you are really bored and have nothing else to read...go on....
Okay..lets start by understanding how a thermostat works... For simplicity, it is two different types of metal strips glued together. Each metal expands at a different rate according to temperature. This causes the strip to bend...and press on a switch. On or Off. That's it. Depending on the metals used, the range, and/or rate/accuracy of the switch will vary. However, for our purposes...it switches on or off. This is considered digital and can be considered I or O. The dial you turn just pre-loads the strip.
Now for the PID quick lesson. The definition of PID is here;
http://en.wikipedia.org/wiki/PID_controllerWhat does that mean for us meat makers?.. And to the point Sous Vide?
In practicality, most variables (ie temp, flow, PSI etc) can be measured two ways. As an analogue variable or a digital variable. A digital variable describes a state, either on or off, I or O. Or an analogue variable describes the condition. For example.... a pump switches on....
Digital.....the pump is on
Analogue... the pump is flowing 10 gpm
And thus analogue was born. Analogue measures a range, digital monitors a condition. Both have a roll in true PID. How does analogue work? The most common version is 4-20mA. i.e a temperature sensor is set to measure between 0-100F...the sensor will out put 4mA for 0F and 20mA for 100F. The controller will see the same.
So, the same can be said for an output...Digtial being...on or off, or analogue can be a variable. For the same example...
Digital...turn pump on or off
Analogue...run pump at 8gpm
Okay, cool, we figured out how to measure and control things. What know? Well, the second part of the renowned "PID" solution is the controller. Wonderdave mentioned controllers...well he is 100% correct. In our market they come in many forms...the most common are; "Smart Relay" and a "PLC" ...(Programmable logic controller). What they do is interpret the data from the sensors. Either Digital or Analogue and apply the "algorithm" that Wonderdave mentioned, and apply an output. PLC's ....you need to be a rocket scientist to program (not actually true..but pretty close)...Smart relays....a lot easier to program these days with free software available.
So..taking our pump example.... the pump can flow anything from 0-100gpm...our set point is 100gpm....the controller turns the pump on and assesses the flow (via algorithms and 4-20ma sensor). Initially it sees the flow as 0 gpm (4mA as the pump hasn't started), so in an ideal world speeds the pump up to the set point of 100gpm (20mA). So if we wanted a 50gpm flow...we would set the controller for 12mA (16mA availble for 100gpm span) and the controller would slow the pump speed down to 50% or 12mA on the sensor.
The behavior of the controller and the pump are a function of the software in the PLC. Depending on the function...pump speed, temp, power factor etc might need to be controlled also. And thus to my point....all of this conjecture, and boring reading, could be avoided by purchasing a ready made sous vide for a couple of hundred bucks....or you can undertake a process engineering degree and do it your self.
I know this was long winded...but the degree is many years. I'm not an engineer by any means...but I've played, and am still playing, with PID solutions every day. If you have a question let me know. I probably wont know the answer, but i will figure it out.