# REX C100 PID



## Stan Stalinson (Jul 1, 2020)

I recently bought a second hand Gaggia Classic with a REX C100 PID. So far the temperature seems to drop by several degrees Celsius when pulling a shot and I wish to reduce this. Are there any settings for the PID which can be altered to do so?


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## mr-bean (Nov 26, 2018)

It might not be the PID at fault but merely the PID showing you something isnt quite right with your setup. Say for instance your grind is too coarse or you might be running at too higher pressure, both of which could have a direct bearing on a temp drop, and both are easily put right.


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## MrShades (Jul 29, 2009)

With any coffee machine that has cold water pumped into the boiler as part of the shot making process, you're going to see a drop in boiler temp when pulling a shot.

This is usually 5-10c, depending on the machine and the PID config.

What sort of drop are you seeing? If you're expecting any machine like a Classic to keep boiler temp within 5c of your brew temp during the tail end of a shot then forget it!

Sent from my iPhone using Tapatalk


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## jjstorgaard (Jan 19, 2019)

Some people claim that aggressive PID parameters can reduce the temperature drop during the shot, see this post on Reddit. This guide to installing a Rex C100 in the Classic also lists some parameters. From what I've understood, the aggressive parameters have the effect that it will take it longer to stabilise at the set temperature, which increases recovery time between shots.

JJ


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## Stan Stalinson (Jul 1, 2020)

MrShades said:


> With any coffee machine that has cold water pumped into the boiler as part of the shot making process, you're going to see a drop in boiler temp when pulling a shot.
> 
> This is usually 5-10c, depending on the machine and the PID config.
> 
> ...


 I was getting around 8c. Nevermind then haha


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## phario (May 7, 2017)

jjstorgaard said:


> Some people claim that aggressive PID parameters can reduce the temperature drop during the shot, see this post on Reddit. This guide to installing a Rex C100 in the Classic also lists some parameters. From what I've understood, the aggressive parameters have the effect that it will take it longer to stabilise at the set temperature, which increases recovery time between shots.
> 
> JJ


 Out of curiosity have you read my thread here and the references?

https://www.coffeeforums.co.uk/topic/53056-pid-temperature-stability-issues/?do=embed

If you really want to increase temperature stability during shots, you can try blipping the steam switch. That is the standard recommendation on places like home-barista and GUG.


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## jjstorgaard (Jan 19, 2019)

Hi, yes I have read it and just reread it. I'm in the process of installing a Rex C100 myself (still waiting on a PT100 sensor to arrive) so have no personal experience with it as of yet. I look forward to installing and experimenting it hopefully next week and will definitely be using your thread as a reference.

JJ


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## phario (May 7, 2017)

jjstorgaard said:


> Hi, yes I have read it and just reread it. I'm in the process of installing a Rex C100 myself (still waiting on a PT100 sensor to arrive) so have no personal experience with it as of yet. I look forward to installing and experimenting it hopefully next week and will definitely be using your thread as a reference.
> 
> JJ


 I love the idea to optimising and keeping the temps steady but it's important to remember that the main role of the PID is for consistency, meaning that you can be sure your shots always start at the same value.

If they're consistency decreasing by 5C then that's fine.

Hell the recommended espresso temperatures are in a band of 10+C anyways (see the Scott Rao quote in the other thread).


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## jjstorgaard (Jan 19, 2019)

To be honest, I wonder sometimes if reducing the temperature drop throughout the shot might not be more important than to have the same start temperature within 1 degree every time. As far as I understand, that's how E61 machines work; the starting temperature isn't ridiculously accurate, but it's not gonna change during the shot. I often see people online saying that they have to change the temperature in their more temperature stable machines my several degrees before being and to detect a difference in the taste.

Your posts about how to aim for a consistent average temperature and emphasise reproducibility are very informative, however, and good for reducing anxieties about not reaching some full potential. I'll let you know how I get in with my own PID.

JJ


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## Blue_Cafe (Jun 22, 2020)

Does the V2 with its much larger boiler and straight discharge path suffer from this level of temp drop?


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## phario (May 7, 2017)

jjstorgaard said:


> To be honest, I wonder sometimes if reducing the temperature drop throughout the shot might not be more important than to have the same start temperature within 1 degree every time.


 I am not an espresso connaisseur. However, I think you we should separate the "what is more important" question. I think that everybody agrees that knowing what temperature you're starting at allows you to be much more consistent in your ability to pull a shot. Asking "what's more important" is a trickier line, and it probably doesn't matter.

So essentially your question is: what is the impact on the taste of a drop of 5C versus say, a drop of 1C? You can get even more granular. What is the impact of a drop of 5C where the average of 2.5C is maintained consistently, versus one where it might drop rapidly -5C, hang around there for a while, before going up to -1C.

I don't know the answer to that, and I wonder how many people here are capable of answering that question. And how many people who claim they are can recognise the difference in a double blind test.


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## Zeak (Jun 12, 2017)

jjstorgaard said:


> Some people claim that aggressive PID parameters can reduce the temperature drop during the shot, see this post on Reddit. This guide to installing a Rex C100 in the Classic also lists some parameters. From what I've understood, the aggressive parameters have the effect that it will take it longer to stabilise at the set temperature, which increases recovery time between shots.
> 
> JJ


I'm using the second preset from MrShade's list and during the shot I only lose 3C (goes from 95 to 92). After the shot is finished it dips to upper 80s.


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## jjstorgaard (Jan 19, 2019)

phario said:


> I am not an espresso connaisseur. However, I think you we should separate the "what is more important" question. I think that everybody agrees that knowing what temperature you're starting at allows you to be much more consistent in your ability to pull a shot. Asking "what's more important" is a trickier line, and it probably doesn't matter.
> 
> So essentially your question is: what is the impact on the taste of a drop of 5C versus say, a drop of 1C? You can get even more granular. What is the impact of a drop of 5C where the average of 2.5C is maintained consistently, versus one where it might drop rapidly -5C, hang around there for a while, before going up to -1C.
> 
> I don't know the answer to that, and I wonder how many people here are capable of answering that question. And how many people who claim they are can recognise the difference in a double blind test.


 So what you're saying is that having a relatively high level of precision with starting temperature is the most important, as I read it.

Your point about the temperature possibly dropping and then recovering is good, I hadn't thought about that.

I've finally taken delivery of a PT100 sensor, done the wiring and found out that my cheap eBay PID doesn't work with the sensor. A new PID should arrive tomorrow, I'll let you know how I get on, even though I don't see myself doing any blind tests...

JJ


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## jjstorgaard (Jan 19, 2019)

Zeak said:


> jjstorgaard said:
> 
> 
> > Some people claim that aggressive PID parameters can reduce the temperature drop during the shot, see this post on Reddit. This guide to installing a Rex C100 in the Classic also lists some parameters. From what I've understood, the aggressive parameters have the effect that it will take it longer to stabilise at the set temperature, which increases recovery time between shots.
> ...


 I don't have the MrShade kit so am afraid I'm not aware of the presets. I don't know if the drop in brewing temperature is likely to be higher or lower than that seen by the PID.

JJ


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## Zeak (Jun 12, 2017)

jjstorgaard said:


> I don't have the MrShade kit so am afraid I'm not aware of the presets. I don't know if the drop in brewing temperature is likely to be higher or lower than that seen by the PID.
> 
> JJ


Gotcha. Well, guessing it should be (somewhat) similar in terms of settings. If so, my P, I and D values are these atm:

Set 2: P=4.5, I=64, D=16


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## phario (May 7, 2017)

jjstorgaard said:


> So what you're saying is that having a relatively high level of precision with starting temperature is the most important, as I read it.


 I found that after I started using the PID, I became very uncomfortable not using one just because you're a lot less certain where the starting temperature is---this is particularly important on the Gaggia and similar machines that have such a long warmup. It allows you to remove one more uncertain element from the equation. You don't have to stress out about how long to wait until warmup is achieved, or how long you need to wait after the first shot to get the second.

For an amateur like me, the key to dialing in an espresso is just removing as many variables from the usual list of I guess six elements (grind size, brew time, grind weight, pressure, tamping technique, temperature).


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## jjstorgaard (Jan 19, 2019)

I've been using the PID for a bit now and am generally quite happy with it. Before, I used to flush a bit of water and wait for the boiler to come on and off again. I think that was pretty consistent, but having to time it like that was a bit of a pain.

The PID is a lot more fuss-free. After letting it auto tune a couple of times, I've noticed something quite strange: the temperature displayed goes up during a shot. That makes me wonder if the graph from Auber with the correlation between set temperature and average brew temperature won't be way off. I'll just have to experiment more to see if I should lower the temperature.

I also tried copying settings reported from others, which seemed to cause oscillations by several degrees while idle. My PID is an Inkbird model, which seems to be identical to the Sestos one. From what I can tell, it does not have a decimal point for the P-value, unlike the Rex one, and the values reached by auto tune seems to be very different than what's reported online.

JJ


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## phario (May 7, 2017)

I don't know how PID values compare between models. One way test is to simply remove the I and D values and work entirely with the P value. You can get near equilibrium and just turn off the device for a few moments and turn it back on, then measure the rise and time.

The P value provides the proportionality constant that multiplies the error between desired and current temperature. So it is surprising that P would be different between devices.

The fact that you claim your temperature increases on the shot is really weird. The first question here is what P value you're using. The second is to just test what happens if you take a P value that is a half or a quarter.

i also don't know how the hot ambient temperatures change this currently.


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