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Modded Severin: Struggling to reach high temps with fan over 50%
jamiej723
Hi all,

Having just modded my Severin according to the instructables article below, all is working apart from I am struggling to reach higher temperatures when the fan is above about 50%.

https://www.instructables.com/Arduino-controlled-DIY-Coffee-Roaster/

This is problematic as I really would like the fan on high to keep the beans moving, but am having to turn it down and then stir manually to get over about 120 degrees C and towards first crack.

Just wondering if this is to be expected with the modded popper or if I may have done something wrong in the build.

A bit more info for reference (measured with DMM):
18V PWM going to the fan
230V going to the primary heating coil (42 Ohm) controlled by the SSR

Attached an image of my first roast profile as well. Naturally this went longer than I would have liked but was cut off at about 15 mins. You can see though that I had to keep stepping down the fan in order to push the heat up whilst the heat was set to 100.

Thanks in advance,

Jamie
jamiej723 attached the following image:
screenshot_2021-01-02_at_183554.png
jkoll42
When you are limited by the stock element you want to keep the fan speed low enough so there is good bean movement but not so much air that there is no time for thermal transfer from the air to the beans. This can be done by lower fan speeds or larger bean loads.

You can also introduce backpressure to help retain more heat so you have more of a roast chamber not a roast chimney if that makes sense
-Jon
Honey badger 1k, Bunn LPG-2E, Technivorm, Cimbali Max Hybrid, Vibiemme Double Domo V3
jamiej723
Thanks Jon,

I'll definitely look into adding some back pressure if I cannot get a good balance between heat and air with my current setup. I guess what is confusing me is that before the modifications the popper had the fan on full power (18v from the rectifier) yet blitzed through first crack in about 4 mins.
jkoll42

Quote

jamiej723 wrote:

Thanks Jon,

I'll definitely look into adding some back pressure if I cannot get a good balance between heat and air with my current setup. I guess what is confusing me is that before the modifications the popper had the fan on full power (18v from the rectifier) yet blitzed through first crack in about 4 mins.


Have you confirmed that the heating element is actually drawing the expected amps when operating in the new setup?
-Jon
Honey badger 1k, Bunn LPG-2E, Technivorm, Cimbali Max Hybrid, Vibiemme Double Domo V3
renatoa
The solution for hot air measurement proposed in the instructables article is simply not appropriate for the goal.
A naked junction will never get the right temperature for a gas, it have to be placed in a special shaped sheath like a flute, i.e. a tube with holes.

Anyway, this is one more example of the difficulties you can have trying to rely a roast based on unreliable temperatures measurement.
Just think that the many thousands ikawa machine don't measure beans temp... because don't need Grin

A power profile is much simpler approach, and giving results you can thrust better than trying to drive an illusory BT.

More on this subject in the posts below:
https://homeroast...post_72624
https://homeroast...post_66692
Edited by renatoa on 01/03/2021 3:03 AM
jamiej723
No I haven't confirmed the current to the heater, only voltage, but will measure this next time and do some more experiments with bean mass as well.

I notice too that my ET is actually reading lower than my BT (apart from at charge when the BT drops). I'm not sure why, but assume it must be to do with thermocouple placement although I followed pretty closely the recommendations from the instruct able. EDIT: Is this what you are referring to Renatoa as I have the probe pointing down directly into the airflow?

I don't think I'll be able to follow a power profile if I currently need max heat from the start so will either need to:

Follow a profile of gradually reducing the fan whilst still keeping the beans moving
Better insulate the popper by reducing the heat escaping through the chimney so I can use a relatively constant fan and vary the heater power

Sound sensible?
Edited by jamiej723 on 01/03/2021 2:28 PM
jkoll42

Quote

jamiej723 wrote:

No I haven't confirmed the current to the heater, only voltage, but will measure this next time and do some more experiments with bean mass as well.

I notice too that my ET is actually reading lower than my BT (apart from at charge when the BT drops). I'm not sure why, but assume it must be to do with thermocouple placement although I followed pretty closely the recommendations from the instruct able. EDIT: Is this what you are referring to Renatoa as I have the probe pointing down directly into the airflow?

I don't think I'll be able to follow a power profile if I currently need max heat from the start so will either need to:

Follow a profile of gradually reducing the fan whilst still keeping the beans moving
Better insulate the popper by reducing the heat escaping through the chimney so I can use a relatively constant fan and vary the heater power

Sound sensible?


Honestly I would wait until you confirm that coil is drawing expected current.

That plan would have sounded sensible until you mentioned earlier that prior to the mods it was blowing through 1C in 4 min. To me that's either saying that there is a element issue (unlikely since they usually just fail) or the element isn't *actually* getting 100% of the power it used to be getting.

The ET reading is a totally different issue and is likely a bad TC or placement.
-Jon
Honey badger 1k, Bunn LPG-2E, Technivorm, Cimbali Max Hybrid, Vibiemme Double Domo V3
renatoa
First crack in 4 minutes is massive overheating, so the heater is uncontrolled and works at 100% permanently rather than not drawing enough amps.
jkoll42

Quote

renatoa wrote:

First crack in 4 minutes is massive overheating, so the heater is uncontrolled and works at 100% permanently rather than not drawing enough amps.


I don't think you are understanding the thread correctly. Prior to mods the uncontrolled heater was blasting the beans through 1C in 4 minutes. Now, modded, the same heater is unable to get the beans to 1C (while showing it is being driven at 100%) unless the fan is turned down so far that the beans have to be stirred manually.
-Jon
Honey badger 1k, Bunn LPG-2E, Technivorm, Cimbali Max Hybrid, Vibiemme Double Domo V3
jamiej723
That's right Jon. I'll try to measure the heater current tonight to see if that sheds some more light on what could be the problem. I don't know what the current actually was before modding, but should be able to calculate this as well as the expected current after modding with some basic circuit theory.
jkoll42

Quote

jamiej723 wrote:

That's right Jon. I'll try to measure the heater current tonight to see if that sheds some more light on what could be the problem. I don't know what the current actually was before modding, but should be able to calculate this as well as the expected current after modding with some basic circuit theory.


Does your popper have 2 coils like is mentioned in that build you linked? Possibly the wrong coil? My only other thought is something in the programming is still cycling the SSR not running it constant? I didn't look into the programming but one flaw is that 70% on 30% off during a second will produce 70% power. It's not going to be a linear curve since the coil takes time to heat up and dissipate heat.
-Jon
Honey badger 1k, Bunn LPG-2E, Technivorm, Cimbali Max Hybrid, Vibiemme Double Domo V3
renatoa
Thank you Jon for post above about the real issue.

But about 70% on - 30% off you aren't right, this switching will led indeed to 70% power/heat, due to the thermal inertia. The heater will average the on-off states.

I am running a fork of his software, that allow me to instantly switch between power control modulation methods, if the ZCD has issues.
Implemented this feature due to very noisy mains encountered in some places, that led to false ZC triggering.
What I want to highlight on-topic is that when switching from a ZC based control method, as ICC, to slow PWM (relay style on-off) the temperature don't move even one degree in the oven.
70% is 70%, about 220 C degrees in both cases, either continuous pale red heater, either pulsing on-off.
You can detect the pulsing only if using a low inertia halogen heater, and a naked junction probe, as can be seen in this video:

https://youtu.be/...
Edited by renatoa on 01/04/2021 8:37 AM
jamiej723
Measured the current at 2.42A into the heater on 100% power, however, I just noticed my DMM only measures DC current...need to scratch my head on what conclusions I can draw from this.

Yeah, it is a two coil heater where I have connected to only the primary coil which measures 42Ohm as per the instructable guide.
Edited by jamiej723 on 01/04/2021 4:14 PM
jkoll42

Quote

jamiej723 wrote:

Measured the current at 2.42A into the heater on 100% power, however, I just noticed my DMM only measures DC current...need to scratch my head on what conclusions I can draw from this.


That it's time to buy an AC/DC Clamp meter?

Sorry....I just had to :)

If you have no beans in there what temp is the chamber at if you let it heat up on 100% and stick the TC right in the air stream?
-Jon
Honey badger 1k, Bunn LPG-2E, Technivorm, Cimbali Max Hybrid, Vibiemme Double Domo V3
jamiej723
OK so done a few more experiments over the past couple of days.
Firstly I did another roast and you can see the profile here: http://imgur.com/...ry/X5Xiujv

I tried to keep the fan on high for as long as possible by covering most of the chimney, eventually had to reduce the fan and stir manually in the 2nd half of the roast.

Also measured the temps without any beans and the fan on full. Tops out at about 115 degrees C. If I almost completely cover the top it gets closer to 160 degrees.

Also did a few more measurements and measured 235V across the 41.5 Ohm heater coil. Although I cannot measure AC current directly, my calculations on the above suggest I am pushing over 5A through the heater and about 1300W. I feel like this should be enough heating power.

So I'm not too sure how to take it forward, in so far as I don't rally want to completely dismantle the whole thing to debug further (it was a right pain to get it back in the original housing).

Perhaps covering the top to create a roast chamber is simplest, not sure if covering completely would be a bad idea? Or maybe putting it a box so the hot air is fed back in.

Could also try larger batches and higher charge temp like 200 to see if that helps.
Edited by jamiej723 on 01/07/2021 5:48 PM
jkoll42
What is your current batch weight?
-Jon
Honey badger 1k, Bunn LPG-2E, Technivorm, Cimbali Max Hybrid, Vibiemme Double Domo V3
renatoa
Not sure if posted elsewhere, but my "vulcanian" logic led me to the following approach to find such machine capabilities:
- set fan to maximum, and load with greens until barely move (with cold air)
- power heater full, and roast to FC, if lower than 6 minutes,you are good, you can start to modulate the power.
- if more than 10 minute FC, then machine is overloaded, lower the charge and try again. This imply lowering the air to have same barely movement at start.

The power level and time are pretty proportional, if 100% heater led you to 6 minutes FC, than you can expect using 75% to have FC at 8 minutes.
Not proportional for the load though... if you have FC at minute 11 and want it at minute 8, this not mean a 30% less charge will do the trick, you should experiment to find the proportionality law.

Notice I didn't used any temperature measurement in this process, FC is the main factor telling me how the roast evolved. I can elaborate more why no temperature measurement need.
jamiej723
I've been working with 65g batches so far which seemed about right before modding.
Did some more esperiments yesterday almost entirely covering the top with aluminium to create more of a roast chamber than chimney and managed to get up to about 170 degrees empty with both fan and heater on full power (and temp was still rising steadily).

Haven't tried this with beans yet, but should give me enough heat on full that I have more control over the roast, whilst still keeping the fan high to agitate as many beans as possible.

Any cons to this approach?
renatoa
I am pretty sure that the air temperature that matters, i.e. those hitting the beans on the roast chamber bottom, i.e. exhausted through slot openings, is much hotter than your measurement.
As a rule of thumb, if you have FC at minute 9, should have about 250 C. Add/subtract 10 C degrees for every minute sooner or later. Just a rough guess, but closer to real than 170.

A brand new popper has a first stage thermostat set at 180 C, to protect the popcorn from burning, do you think it ever trip if only 170 C in the can at full power ?
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