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lttlscamp
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Joined: 25 Dec 2009
Posts: 57
Location: Toronto
Expertise: I like coffee

Espresso: Rocket Giotto Evoluzione
Grinder: Baratza Vario
Posted Sat Jun 15, 2013, 5:48pm
Subject: Group head water temp declining
 

I have a Rocket Evoluzione (the older model) that's a few years old now. It has a group head thermometer. Up until recently when the machine was sitting idle the water temp was around 204F. When I did my cooling flush it would jump up to 212-215F and I'd let it settle around 208 before prepping and making my shot.   Usually that gave me around 204 during my shot.

Recently the group head temp idles around 200F and drops down to 194 or so after a flush. It doesn't even hit 200 during the shot.  

Anyone have any suggestions about what could be the problem?  Bad pressure stat?  The boiler pressure gauge still reads around 1.1 bars.

Thanks.
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calblacksmith
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calblacksmith
Joined: 25 Nov 2007
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Posted Mon Jun 17, 2013, 6:53am
Subject: Re: Group head water temp declining
 

Thermometers can drift off calibration over time.
I am not saying that yours is for a fact, but you do need to have another thermometer to check against. It takes 3 devices be they thermometers, pressure gauges or whatever to find the truth, unless one is certified recently for accuracy.

The reason you need 3 is that if you have two, and they have different readings, which one is bad? When you have two out of three that agree, the one with error is clear to see.

Off hand, I can't see why there should be a drop in the temp of the brew system if your brew pressure/temp still reads the same as it always has.

 
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steamer
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steamer
Joined: 11 Feb 2005
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Location: socal
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Posted Mon Jun 17, 2013, 7:23am
Subject: Re: Group head water temp declining
 

If you can test the thermometer, test it in boiling water, unless your in some high mountainous area, it should read 212 deg. Its that simple.
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dana_leighton
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dana_leighton
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Drip: Technivorm; CCD; Melitta
Roaster: Poppery I w/PID controller
Posted Mon Jun 17, 2013, 8:35am
Subject: Re: Group head water temp declining
 

steamer Said:

If you can test the thermometer, test it in boiling water, unless your in some high mountainous area, it should read 212 deg. Its that simple.

Posted June 17, 2013 link

Not quite.

I am not in some "high mountainous area," but rather in the hills of Northeastern Pennsylvania. Elevation 932 feet. Boiling point 210.4 on a "normal" day.

(edit)
According to this site barometric pressure is not the only influence, since it's reported as a corrected value. So at 932 feet, with pressure 29.92, the boiling point for me right now is 209.7

 
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steamer
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steamer
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Posted Mon Jun 17, 2013, 3:11pm
Subject: Re: Group head water temp declining
 

Boiling point calc

Click Here (www.engineeringtoolbox.com)
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frcn
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frcn
Joined: 23 Dec 2001
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Location: Northern California
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Posted Mon Jun 17, 2013, 9:43pm
Subject: Re: Group head water temp declining
 

Scale in the thermosyphon loop could cause this. Have you pulled the mushroom to check for scale?

 
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lttlscamp
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Joined: 25 Dec 2009
Posts: 57
Location: Toronto
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Espresso: Rocket Giotto Evoluzione
Grinder: Baratza Vario
Posted Tue Jun 18, 2013, 12:14pm
Subject: Re: Group head water temp declining
 

Thanks for the suggestions.

I compared it with two other thermometers and all three were within one degree of each other.  

I've also descaled in the last few months and the thermometer looks clean.
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Freddo58
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Joined: 16 May 2013
Posts: 36
Location: New Zealand
Expertise: Just starting

Posted Tue Jun 18, 2013, 2:18pm
Subject: Re: Group head water temp declining
 

lttlscamp Said:

Thanks for the suggestions.

I compared it with two other thermometers and all three were within one degree of each other.  

I've also descaled in the last few months and the thermometer looks clean.

Posted June 18, 2013 link

If the boiler pressure cycles normally say from 0.9 to 1.1 bar (approximately) then boiler temperature and pressure can be assumed to be correct (it's unlikely that both the pressure gauge and the pressurestat would "fail" at the same time - so if both are appearing to function normally - then you can reasonably safely assume that both are okay).  This is unless the pressure gauge started over-reading and an adjustment was made to the pressurestat to reduce boiler pressure - but doing this requires intervention, so you'd know that this was a possible cause of lower group temperature.  So if pressure is 1 bar in the boiler, the water temperature must be 120 deg C / 248 deg F.  There's a vacuum breaker, so it's very unlikely that there could be a problem from air in the boiler, but if you wanted to check this then run some steam for a while, then monitor temperature and see if it rises.

So if boiler water temperature / pressure is normal, and you trust your thermometer, then it doesn't leave many possibilities, as the group is heated only by heat transfer through the thermosyphon, and brew water heated by the heat exchange circuit - it must be a heat transfer problem.  Either the thermosyphon flow is restricted, or the HX coils are scaled up and not exchanging heat efficiently, or both.
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CoffeeRon
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Posted Tue Jun 18, 2013, 5:26pm
Subject: Re: Group head water temp declining
 

I'm probably grasping at straws here but is it possible your boiler probe is acting up and your boiler is really low on water? It would possibly cause some odd temp. problems like yours. Might be worth pulling the probe and checking if it's clean and checking the boiler level- fairly simple to do at least.
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Freddo58
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Joined: 16 May 2013
Posts: 36
Location: New Zealand
Expertise: Just starting

Posted Wed Jun 19, 2013, 2:21pm
Subject: Re: Group head water temp declining
 

CoffeeRon Said:

I'm probably grasping at straws here but is it possible your boiler probe is acting up and your boiler is really low on water? It would possibly cause some odd temp. problems like yours. Might be worth pulling the probe and checking if it's clean and checking the boiler level- fairly simple to do at least.

Posted June 18, 2013 link

Indeed, that is another possibility.
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