Maintaining accurate temperature control is important when making biodiesel. Most of us use a simple digital temperature reader and when the oil in the reactor gets up to the required temperature we manually turn off the heater. Conventional thermostats can be used to control the temp but they are only accurate to about 5°C. I personally prefer to set my thermostat at about 70°C so that it acts as a safety device rather than a processing controller.
A PID temperature controller is a useful and very accurate way ( + or- 1°C) to control the processing temperature and they have now come down in price so much that they very affordable. I use 6 PID controllers in my workshop, 3 in my Plastic to oil pyrolyser, 1 in my warm cabinet, one in my methanol recovery still, and one in my small batch processor.
Some people are put off by the apparent complexity of setting them up initially, made worse by the terrible Chinese/English manuals the accompany them.
So for anyone who is interested in using these devices I am going to put up a series of posts explaining what ones to buy, how to wire them up, how they work and how to initialise them.
What one to buy?
There are lots of models on sale on Ebay under lots of brand names, Rex , Sestos, Mypin Omcron etc but they all basically work the same way. I have several makes but I prefer the Rex C100 because its a little simpler to set up than some of the others. It will come supplied with a Ktype temperature probe that is suitable for tapeing to the outside of a metal reactor but if you want to install the probe inside your reactor you will have to look for another type of probe. The kit shown above also includes a solid state relay SSR which you will need, 20amp minimum, 40 amp is better. Another item that is useful is a heatsink that helps to dissipate heat from the workings of the SSR but if you are mean like me you can make one from a piece of sheet metal.
In future posts I will refer to the Rex C100 model that is shown above, the instructions will be a little different if you have a different model.
If anyone has experience of these items they want to share please feel free to post here.
Here is another one, same model but complete with a heatsink.
You can buy these devices even cheaper direct from China but buying it from the UK will mean that you can return it if it is faulty and delivery from China can take several weeks.
This is an example of a probe that can be installed with the probe inside the reactor. A small hole is drilled in the the wall of the reactor and the probe is bolted into place. This should work with both metal and plastic reactors. Since it is designed to survive inside the exhaust manifold of a high performance car it should have no problem inside a biodiesel reactor.
Thanks that'll be a great help.
pid's have been very popular with the guys in the UK for years especially those that use the wbd method or need more accurate control of temperature 8i when distilling their methanol.
"Often they have two displays,one marked SV and the other PV. SV means the Set Value or the temperature at which the user requires the process to run, while PV is the Process Value meaning the actual temperature presently."
I remember looking into them before but was put off as you say by the complexity of installing one.
A slight correction, a PID is no more accurate at measuring temperature than any other temperature controller, they are only as accurate as the temperature sensor being utilized be it a thermocouple (TC), capillary tube, by-metalic, etc... The difference with a PID compared to a standard temperature controller is a PID uses algorithms to try and anticipate the over/under shoot of system in order to maintain a more precise control of the process. A standard aqua-stat or thermostat just has a temperature offset and usually also has a heat anticipator which determines when to close. For example you set the control to 160F and set the offset to 15F, when the controller hits 160F it opens and when it drops to 155F it closes again.
In my experience a simple aqua-stat is plenty accurate for anything related to producing biodiesel, in fact its all I use now on my big reactor, including for recovering methanol.
FWIW I made up a schematic with how to wire one up along with a high limit which is also very important. Besides this just being common sense the problem with SSR's is when they fail they typically fail closed which will result in a runaway heater if no high limit is used.
Edit: I see I still have a link in my sig for the circuit
Simple schematic for a pump and heater control with a high limit
Sensor for the biodiesel/glycerin layer
Hi Jon, I accept that the sensor in a typical immersion heater is just as accurate as the one used in a PID controller. But the end result is what matters and in the case of a PID controlled heater set at 60°C the temp will swing between 60.5 and 59.5°C. With the typical fixed offset thermostats we use here it will swing between 62 and 48°C.
I notice that you use the alarm outputs of the PID controller as a safety cutout in case the SSR fails. I propose to retain the thermostat in the heater unit, set slightly above the processing temperature so that if the SSR failed, the thermostat would cut off the power before the oil got too hot. I know the alarm setup works, in fact I use it to control a kerosene burner in my plastic to kerosene unit but I want to keep the setup procedure as simple as possible as this seems to be the part that people have most trouble with.
Thanks for the link to your posts on PID controllers, theres a lot of good info there and if you dont mind I might use some of it especially the parts on safety and wiring.
Here is how to connect a PID controller into a biodiesel processor.
The sensor probe has two wires marked red and blue. The red is usually the positive but if you get them the wrong round it will show a - sign before the room temp when switched on and you can reverse the terminals. In this case the sensor is simply taped to the outside of the metal reactor with a thick patch of insulation taped over it. It should be positioned roughly 1/3 to 1/2 way up the tank. If the pump is used to mix the oil while heating this will give a quite accurate reading. Fitting the probe into the reactor will give a more accurate reading but carries the risk of leaks.
The wires between the PID and the SSR carry only 12 v and can be light thin cables.
The SSR is installed on the live supply to the heater element. The existing 20a double pole switch should be retained and the existing thermostat in the heater should not be removed or bypassed. It will act as a safety cutout if the SSR fails.
The wires carrying 240 v to the PID should be 5A rated twin flex.
On a safety note, no 240v connections should be left exposed, plastic or metal boxes are the best way to house these units. Most important of all, the supply to your processor must have a RCD ( residual current device fitted). All modern houses and workshops in Ireland and the UK should already have one, but make sure, it could save your life.
In this diagram the immersion heater and the PID controller share the same 240v supply. This will mean that if you turn off the heater at the 20A switch the PID controller will also go off and you will not be able to read the temperature in the processor. A better arrangement is to have the controller on its own 13a plug, plugged into the spare socket on the IMB150 processor.
In the next diagram I will show the connections on the back of the Rex C100 unit. If you purchase a different unit they will be different but if you refer to the manual you should be able to work out which connections are which.This message has been edited. Last edited by: imakebiodiesel,
I am not sure what you mean by "thermostat in the heater unit" but if your talking about a standard water heater t-stat then it will also have a high limit built in. Over here they have a manual red reset button to reset the snap disk. These work great for a high limit and are more reliable than using the PID alarms, I believe I mention this in my write up as well as where to put it in the circuit.
Another good method of mounting the TC is to use a hose or gear clamp to clamp the TC to the steel or copper piping in the mixing loop.
Feel free to use any of the ramblings I post.
Simple schematic for a pump and heater control with a high limit
Sensor for the biodiesel/glycerin layer
I have had a few immersion heaters with the reset button you mention but most heaters here have only an adjustable thermostat probe. This should be set to the maximum 70°C and will act as a high limit safety device.
Here is the diagram for the connections to the Rex C100 controller.
As you can see the connections are very simple. Other brands will be similar but not necessarily the same, consult the manual that came with the unit.
Here is the front panel of the Rex C100, Other makes will have the same controls but may be laid out differently.
The displays and controls on a PID controllers allow many different adjustments to the unit some of which are very complex, the good news is that this model and most others come factory set for our purpose so there is no need to make any adjustments.
As soon as you switch on your controller it displays the input type and range. It only stays on for four seconds so you need to read it quickly.
The upper PV display will read InP this just means input.
The lower SV display will read °C k this means that it will measure in degrees centigrade and is set for a K type temp probe.
The display will then change.
The upper PV display will read 400 this is the max temperature range
the lower SV display will read 0 this is the min temperature range.
The display will then change to the normal readings.
The upper PV display will show the current temperature at the tip of the probe.
The lower SV display will show 0000.
Press the Set button momentarily once. The SV display will start to flash, use the Shift, Up and Down buttons to change the SV to your desired processing temperature. ( example 0058 = 58°C.) Now press the set button again and the Sv will stop flashing. Your PID controller is set, the heater will bring the oil up to 58°C and hold it there for as long as the unit remains switched on.
The controller will remember your SV setting and do this every time you switch it on.
In the next post I will explain how to use the autotune function that makes the controller even more accurate.
The autotune function allows the controller to adjust its settings to suit the power of your heater and the volume of oil you are heating.
Switch the controller and heating element on and if if you havent already done so, set the SV temperature to the desired processing temperature.
Now press and hold the set button for 5 seconds.
The PV display will show "Cr ", we dont need to change this so press the set button momentarily to move on to the next setting.
The PV display will show "AL1", we dont need to change this so press the set button to move on.
The PV display will show "AL2", press the set button to move on.
The PV display will show "HBA", press the set button to move on.
The PV display will show "LbA", press the set button to move on.
The PV display will show "Lbd", press the set button to move on.
The PV display will show "ATU", this is the Autotune function. Change the SV setting from 0 to 1. Press set to move on to the next PV display which will be "P". Make no more changes and the display will automatically change back to normal display in a few minutes.
When you get back to normal display you will notice that the autotune light is on. It will stay on while the controller collects information about your setup. Continue to heat the oil up to the desired temp and dont turn the heater off until the autotune light goes off. When it has enough information it will automatically turn the light off and return to PID control. It will now remember these settings every time you use the unit.
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