PID selection

I am new here and need help selecting a pid. I need to be able to easily switch between two temperature setpoints at will. Maybe I should explain exactly what I am doing. I am collecting used motor oil, boiling it to remove any water, then centrifuging it to remove particulate contaminants. The end product is blended with diesel fuel for use as fuel in my tractors. I accomplish this with a 5500W, 220VAC water heater element in a 55 gallon drum. Temperature control has been a problem from the beginning. After I boil for a few hours, I need to let it cool to 160 to 180F to run the centrifuge pump for a couple more hours. I will run the pump manually because I need to stop it to clean the centrifuge periodically. I may boil one day, then centrifuge a couple days later. Is there a way to do this with a single pid? I am thinking a dual display unit with a ssr output to power an external relay to run the heating element. I have selected a j type thermocouple in a stainless steel well. The pid selection is my problem. Didn't they used to make pid's with a toggle switch for this purpose? How would you guys approach this and what would you use?

What software platform are you using? What do you have for a PLC, or any user interface hardware/software?



There should be no reason why you can't just change the setpoint of the PID loop at different time during your processing.

So during your boiling phase, you'll have a high setpoint.

During your cooldown, you will input a lower setpoint.

Obviously you will need to have an HMI or some operator interface available to write to the PLC and change the setpoint. Also, your PID loop will need to be tuned to ensure proper controllability a both setpoints.

Is that cheaper than buying new diesel fuel?? Sounds like and awful effort.

I think he's looking for a standalone PID temperature controller with a simple means of switching between the setpoints.

What device are you using for temperature feedback (Thermocouple, RTD, other)?

Are you using time proportional (on/off) type switching for the heater element? Or an analog signal to a controller?

I think there are units with remote setpoint intputs but they're susually analog inputs...what you need is a unit with a digital input so you can flip a switch to select between two separate programmed setpoints.

Here are some units that you may want to dig into to see if they have your needs covered:

https://store.redlion.net/store/comersus_listItems.asp?idCategory=566
http://www.omega.com/ppt/pptsc.asp?ref=cn77000
http://www.mod-tronic.com/Minco_ct16.html
http://www.lightobject.com/Temperature-Controller-C7.aspx

I am looking for a stand alone pid, dual display and dual setpoint. OkiePC is on the right track. I'll be using a j type thermocouple. I've looked at many pid's but haven't found one that can be easily switched from one temperature setpoint to another without reprogramming. I'll check out the links from OkiePC when I get home from work. Thanks for the input. Oh, at almost $4.00 per gallon for diesel fuel, I expect the savings my little fuel processor will generate to cover my investment very quickly.

If you are actually boiling the oil then PID control cannot work during this part of your process as the energy is used as latent heat and the PV will not change. To achieve this you only need to put the loop into manual control with the CV at 100% (or whatever value you need to keep the liquid boiling) for this part of the process and then turn the loop into auto with the lower setpoint once the boiling phase is complete.

There are PID controllers available with a 2nd setpoint option built in. One is the Watlow EZ PM series of PID controllers. Honeywell also makes one although it is more expensive than the Watlow model. These use a digital input to trigger the alternate setpoint.

I don't think he's boiling the oil, just raising it to some value above 100deg.C to boil off any water.

use a thermostat out of an electric cooking oven, it has a sensor and can switch the power directly without difficult ****.
you dont need pid only on/off.
setting can be done by your wife.
ask the old iron collector in the street for one of them.

or you can buy a temperature controller for 1000 dollar and a SSR dor another 40 bucks.