# Gaggia evolution - help with REX C100 PID - settings



## bluelemon (Aug 23, 2014)

Hi guys! I am in little trouble with the REX C100 settings. Before cleaning the machine all worked just fine. Then I auto tune the PID and now the temperature its not stable and I have more steam then water when I start to brew. I use the PID only for brewing and got PT100 sensor. I am not familiar with PID settings and will appreciate if someone explain to me. Thanks!


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## NJD1977 (Dec 16, 2014)

A few questions for you:

1. What cleaning did you do?

2. Have you checked that the PT100 sensor is still securely attached to the boiler casing?

3. Is your set point on the REX still the same as it was, and what is it?

4. Have you checked the PT100 and SSR connections into the back of the REX are secure?

Unstable temperature would seem to indicate some kind of instability in either the sensor or the delivery of heat to the boiler.


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## bluelemon (Aug 23, 2014)

I read today on the internet about the settings and think that now temperature is back to normal. About questions. Regular cleaning with citric acid to clean the boiler. The sensor is still in place. I check it. Later i will add my settings. Today i change the alarm from 5 to 2 and SC (dont know what is) from - 16 to 0 (i read about it) and now seems that all its OK. If you can explain about meaning of diferent settings will be glad to hear. Thanks!


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## NJD1977 (Dec 16, 2014)

bluelemon said:


> I read today on the internet about the settings and think that now temperature is back to normal. About questions. Regular cleaning with citric acid to clean the boiler. The sensor is still in place. I check it. Later i will add my settings. Today i change the alarm from 5 to 2 and SC (dont know what is) from - 16 to 0 (i read about it) and now seems that all its OK. If you can explain about meaning of diferent settings will be glad to hear. Thanks!


SC is a self correction adjustment to account for any inaccuracy between the probes true temperature and the read out on the REX. i.e. if you measured boiling water and the REX told you it was 116degC on the display, you'd have to apply an SC value of -16 which would then make the display read 100degC. If your SC reading had been set to -16, then the temperature of your boiler could have been ~16degrees higher than the value shown on the REX - i.e. if you'd set your brew temperature to 93degC it could actually be around 109degC at the probe which could explain why there's steam in the boiler on a brew cycle.

You shouldn't be using the alarm function if you're only using it for brew temperature control, so whether you set it to 5 or 2 is irrelevant. The output is controlled by your setpoint and relay or SSR coming from pins 4 and 5 on your REX.

None of this would explain the instability in the temperature though - whatever these values are set at, you should still be able to hold a very stable temperature. Note that during autotune mode, the REX purposefully varies the temperature quite wildly to learn the temperature profile of the machine. So when you refer to the temperature being unstable, do you mean during the auto-tune period or after?


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## bluelemon (Aug 23, 2014)

The problem was that the temperature was to high near the boiling point. I think that this SC was the problem and I see on the display 96° but real temp is 112°. When I lower that value tha things get ready now.

This is my settings for now. What number should be on the AL1?

AL1 - 2

P - 5

I - 100

d - 20

Ar - 50

Г (Proportional cycle) - 1

SC - 0


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## NJD1977 (Dec 16, 2014)

Most of those settings are set by the Rex during the auto tune. The SC should be pretty close to zero if you're using a PT100. My SC is set at +2.2deg but I calibrated it against boiling water. AL1 is irrelevant if you're only controlling brew temperature. If you were controlling steam temperature via the AL1 output then you'd typically set AL1 at +45deg which would trigger at 45deg above your brew temperature set point, which is about right for steam.

I would set AL1 at something much higher than you have though to prevent the relay clicking in and out all the time. Just put AL1 equal to +45 then if you ever do add the steam control its ready to go.


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## MrShades (Jul 29, 2009)

Contrary to the above - the SC is usually used so that the display / set temperature accurately reflects the actual brew temperature at the puck. With a Classic this usually results in an SC of around -8

If you leave it at 0 then you'll have to set the display / set temp to 102C if you want a brew water temp of around 94C.

As most people make espresso by reference to the brew water temp - ie 92, 93, 94 - it makes a great deal of sense to use the SC parameter to achieve this... allowing the set temp to be the brew water temp.

Mine is set to -8


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## NJD1977 (Dec 16, 2014)

I agree it can be used to achieve that effect but I don't think that's the original intention of the SC function in the Rex. I believe it's there to tweak the readout after calibrating the sensor against a known temperature.

Either way I think we're saying the same thing in essence. The use of the SC value will display a constant offset to the value being read by the sensor. How you choose to use that function is up to you really.


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## bluelemon (Aug 23, 2014)

Thank you very much guys!

Regards!

Dimitar


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