"MET" Probe - ET2 or XT (Exhaust Temp) Slightly More Accurate Name?

[JavaBuzz]

Hi everyone,

Wondering on people's thoughts on below. Note that this is only with the perforated drum; I haven't tried the solid drum yet...

PREMISE:
From what I can gather, the "MET" probe might be more accurately labeled as an Exhaust or "XT" probe, at least if left in its default position (not bent).

EVIDENCE:
I have had a Huky for a couple weeks now. During testing it out, I've noticed that the "MET" temp reading is actually often lower than ET and/or BT (though it should never or very rarely fall beneath these temps). This is often true both when heating the Huky and when the Huky is charged with beans.

At first I thought my temp sensor might be wrong, but I manually placed a second temp sensor as close as I could near the 3 probes, and the reading on that one was pretty close to what the Huky probes were reporting (granted this second probe was connected to the same TC4 meter as the other probes).

Another thing I noticed, if I take the exhaust pipe off the Huky (no beans charged), the MET probe tends to go to its correct highest-temp position. Once I attach the exhaust pipe, the temp typically drops below the other temps (I've even seen this happen even without the fan running).

Increasing/Decreasing fan speed on the exhaust seems to affect this probe quite a bit as well. From everything I can tell, the "MET" probe is definitely not measuring "Max Environmental Temp," and though not a 100% accurate label, Exhaust Temp (XT) might be a better label for this probe location.

THOUGHTS:
Anyone else have a similar experience and/or thoughts on this? Please post if you are using the perforated or solid drum as well, as I think this could affect the reading.

[JavaBuzz]

Here's a post on HomeBarista that appears to be someone else with a similar experience as well:

http://www.home-barista.com/home-roasting/huky-500-data-logging-setup-and-modification-t30805-10.html

[SusanJoM]

I'm not convinced that changing terminology is going to do anything except confuse people.  Whether MET describes accurately what that temperature is or not, at least we know that it is the temperature taken at that location in our HUKYs.  If some people start calling it something else, all we are going to have is a whole lot of "well, I mean the location we used to call MET and now we call..."   It's a label.  We all know where it is on our roasters. 

I'm not arguing that there might not be a better description (just outside the drum at the top of the HUKY), but unless you put a probe in the exhaust pipe itself, I don't thing Exhaust Temp (oh dear ET has already been used) is going to cut it.

My pre-caffeine two worthless cents....

[JavaBuzz]

I definitely agree that trying to change what people call it would probably cause more confusion than it's worth. Plus I don't believe it's in the right spot to get a proper reading of Exhaust Temp (XT) either.

I am still interested if other people have seen the same issue, to at least least confirm that I don't have a bad probe or some other problem:

Do other people also have the problem of the MET probe temp dropping below ET and/or BT, when it should almost always be a higher reading than these temp probes?

If you do or do not have this issue, are you using the perforated or solid drum?

[SusanJoM]

My MET is definitely lower  than ET or BT during the warm-up.  It's the farthest away from the heat source so until beans are in the drum and moving that heat upwards and outwards, it stays cooler.

Here is a picture of the 15 minute warmup with the perforated drum

[JavaBuzz]

Thanks SusanJoM! Yes, MET looks similar to mine as well (below everything else), so I'm not alone.

Here's an example where I had already done one roast before, and I didn't start the roast until about 10 minutes after the first roast (and had the heat on low in-between).

Perforated drum. "MET" is green. Would be interesting to see if solid drum users experience the same.

FYI, just starting out on the Huky, so still learning how to control it.

[SusanJoM]

This isn't the roast, but a similar 15 minute warm up with the solid drum. 

And, yeah, something looks weird with your temperatures.
I'm not sure what, but BT should remain lower than ET which should remain lower than MET once the roast is in progress.

Can you remind me what your thermometery and datalogging involves? 

[JavaBuzz]

I had a suspicion that on the solid drum, because of the added mass and no perforations for heat to exit, the MET probe would act more like an MET probe. From the graph you posted, it does look like the MET probe could be acting more like its name suggests.

Thanks again Susan! Would be interesting to see other people's experiences too.

Maybe "ET2" (second Environmental Temp) or ET2/MET (perforated/solid drum) would be an even more accurate description than XT (Exhaust Temp)?

[JavaBuzz]

Just noticed your question Susan. You're right, it is weird that BT went higher than ET.

I have a TC4 meter using the stock probes and locations from Mr Li.

I've been doing pretty small batches (230gr) to start out, so that might be affecting some of the readings, though first crack did appear to start right about where it should have in that roast (400F). I think I might have dropped the heat too much too fast on this roast too (which might have caused the ET drop).

This was only my 3rd roast with the Huky, so the drum probably isn't completely seasoned either (though I ran a couple seasoning roasts through it) and I definitely have a lot to learn. I do think the BT probe, especially when no beans are in the drum, might be picking up heat from the center post and that might be raising the temp as well.

I'll have to do a few more roasts and see what happens. Could still be I have some other problem (grounding issue, meter acting up, etc.).

[JavaBuzz]

So I did some more testing (and a couple more roasts), and it looks like fan speed is the primary culprit affecting my temp measurements. If I run the fan around 60V for a bit, the temp readings (esp MET) tend to run out of norm (MET drops significantly, often below ET and BT).

However, if I keep the fan mostly around 30V (ijust above its minimum), the temp readings line up as expected when a roast is charged in the drum (MET top, ET middle, BT bottom).

Thank you for the information Susan, it was helpful! Hopefully that was the only issue.

[leftism]

I'm experiencing the same low MET temperature and also use the perforated drum.
At first I though the same as you that the probe was faulty, because it showed so much lower temperature than the BT.
But later became aware that the perforated drum must be the reason that the temperature was so low. This must be a good thing or?

Wouldn't the risk of scorching decrease when the MET temp is low? I just have the analog ET gauge for now and from what I've seen it hasn't been higher than 240-250 which also seem pretty low.

Here is a roast I did today that shows the low MET.

[SusanJoM]

Hey, I have a couple of questions about your graph/roast.

How did you warm the roaster up?  Fan on or off.  Motor turning or not.  Grate open or closed?  What gas level? 

I have come to a tentative conclusion that warm up time is (I could still turn out to have this all wrong) long and slow and important with the perforated drum .  Because of the more diffuse airflow, the various parts warm more slowly and never reach the same temperatures.  What they do reach is  stable relationship between the three.  For some reason I think it's important not to charge the roast until that stability is reached.  All of which harks back in some way to Chang's Challenge (I think).

 
And, are you saying that the line that says it's ET is really MET? 

[JavaBuzz]

The fan speed/pull really seems to affect the "MET" readings a lot with the perforated drum (and sometimes even the ET a bit).

It seems to me that with the perforated drum, if you're using a Variac/speed control and not the damper, you probably don't want the fan running faster than slightly above the minimum voltage for any substantial length of time. Basically only turn it above this speed when you want to do a quick "chaff removal" or temperature adjustment.

However, this assumption is from someone who has only done 6 or so roasts on the Huky now, and only 1 roast using the theory above. Also, this is just looking at temperature probe readings and not actually results in the cup (and the roast that used the above theory won't be tasted until at least tomorrow).

Below is the graph of the aforementioned roast I did with the fan running the whole time, but the voltage on the exhaust fan was just above the bare minimum (~30V) for a majority of the roast. I did very briefly raise the fan speed a couple times to ~60V. This graph looks much closer to normal to me.

For me, every roast I've done I've had a substantial warm up time, somewhere around 20-25 minutes. I've usually been keeping the heat during warmup around 2kPa, then I drop to 1kPa later on when temps get high (after 10-15 minutes or so I'm guessing).

I turn the motor on right away when I start the heat, and the fan on early in the warmup as well (was during somewhere around 60V at first, but will probably do 30V from now on). I have always been leaving the damper wide open (though I might start playing with that some as well eventually).

[SusanJoM]

T I have always been leaving the damper wide open.

What about the grate?

[JavaBuzz]

Grate/Shutter was closed the whole time on this roast (I thought it might provide more even heat distribution).

The gas output at the start should have been around 3kPa (and I lowered as the roast progressed). I'm still only doing 230gr (1/2 lb) charges as well (which I know can be more difficult to control, but gives me more chances to practice).

Less bean mass in the drum could very likely cause the temps to fluctuate easier too. So far BT has seemed pretty accurate though (when beans are in the drum; empty can be a different story), going by 1C start and such.

I think when beans aren't in the drum, some metal support bars in the center of the drum (that are very close to the BT probe) might make the BT readings increase a bit from the heat they absorb and emit.