Low group head temperature on Rocket Giotto

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#1: Post by lachlanr »

I am hoping someone may be able to help explain what is going on with my HX machine and group head temperature reads.
- I have a Rocket Giotto V2 with PID set at 122 degrees Celsius, which gives a Bar pressure of 1.25
- I predominately use a bottomless portafilter
- I recently purchased a "Coffee Sensor" screw in for the group head which even after long warm-up only reads a temp of 85 which increases to max 87-88 when I open the group head lever and run water through.
- Coffee quality has always been good, using VST baskets & shower head filter, weigh and timing shots.

I am trying to work out if this is due to the bottomless portafilter (but when i swap it out for the regular portafilter the temperature only increases 1 degree), the PID limiting fluctuation or potentially an error with the Coffee Sensor measure (felt it was unlikely). Or an internal plumbing issue/backleak
- Ambient room temperature is about 20 degrees Celsius.

The coffee sensor i am using is: https://coffee-sensor.com/product/tempe ... ermometer/

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#2: Post by ira »

Well, I'd start by boiling a small pot of water and measuring that temperature, that will tell you if the thermometer is off by that much. Once we know the thermometer is accurate we'll have a better idea of what's going on.


#3: Post by HeinK »

Well, what can I say. Hmm. Anyhow, there is good news.

The good news is that you have a machine that makes decent espresso. And now for the bad news.....

125°C is not a brew temperature. It would burn your coffee. And generally speaking espresso is made with a pressure around 9 bar and not with 1.25 bar. So there is nothing wrong with your machine, but there is a slight learning curve for you. Read the manual, understand what the display really means and you will be fine!

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#4: Post by homeburrero »

Your grouphead thermometer is useful, but needs to be carefully interpreted.

When the machine is idle, it tells you the temperature at a point inside the heavy brass grouphead. This temperature can be especially useful in alerting you when you have a problem like a thermosyphon stall, where the group goes unusually cold due to water not circulating through the HX and thermosyphon loop of your machine.

The idle group temp can also be useful in telling you whether you need a cooling flush, and whether the system has properly rebounded after pulling a shot.

When the pump is on and water is flowing, however, the sensor of this temp gauge is reflecting the temp of flowing HX water at a point inside the group before it reaches the coffee bed. The water may heat up or cool down before it reaches the coffee bed, so depending on your flush routine, the displayed temp midway through the shot may be above or below the temp of the water hitting the puck. This thermometer is based on a system invented and made by Eric Svendson (HB member erics) which is often referred to as an EricS or Eric's thermometer. There is a plethora of discussion here on HB about that thermometer and how it relates to brew temperature with different flushing techniques.

It can be pretty overwhelming. One of my favorites for seeing some good Giotto V2 graphs is this one: How to systematically develop HX flushing routines per target brew temperatures?
Temperature Differential between Eric's Thermometer and Brew Temperature
Brew temperature offset for Erics thermometer
Thermometer Adaptor and Brew Temperatures
Rocket HX espresso machines with Eric's thermometer
Erics e61 adapter, how to pull consecutive shots?

All this info is for the EricS thermometer, which I believe has a slightly deeper probe than your coffee sensor gadget*, which does affect the readings. Also I'm not sure if the coffee sensor thermometers are as well calibrated as the EricS, so you may have to allow for that. Your readings, both at idle and during a pull do seem lower than the EricS readings would be for a boiler at 1.25 bar. Ira's suggestion, to check the calibration of the thermometer is a good one. There in Bendigo it should read 99.3 ℃ with the tip (but not the thermometer body) submerged in boiling water. It should read 0 ℃ with the tip immersed in water with a slush of crushed ice. If you have a well-calibrated probe thermometer around you can just compare the coffee sensor against that.

* The basic Coffee Sensor has a shallower probe depth than the EricS at the recommended depth. The Coffee Sensor Pro, like the EricS, allows you to set the probe depth.
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#5: Post by cafeIKE »

Eric's thermometer tip is isolated from the group mass with the nylon compression fitting and it extends further into the water flow. The 'Coffee Sensor' probe appears to be one piece and thus the group mass will have more influence.

Relative to Eric's, the tip on the 'Coffee Sensor' is likely to read the group mass idle temperature rather that the water temperature which flows to make the coffee, necessitating some offset.

On my Vibiemme DB Eric's thermometer reads 189° @ idle with a boiler temperature of 220°. Temperature rises rapidly to ≈201° when pulling a shot, then droops a couple of degrees depending on the flow.

I use the thermometer almost not at all and just replaced the battery after about a decade.

Jim's Espresso 101: How to Adjust Dose and Grind Setting by Taste is far more useful than all the sensors extant.

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#6: Post by homeburrero »

There is a basic and a more expensive "Coffee Sensor Pro" version of this, and the Pro version allows you to adjust the depth like the EricS thermometer.

In the OP's case, his idle numbers do seem low. My Rocket Evo V2 has no PID but should be thermally similar to his machine (HX, 3mm thermosyphon restrictor) and with my steam boiler at 118 ℃ (1 barg at 5000ft asl) it idles between 93 ℃ and 96 ℃ depending on ambient drafts near an open window. My EricS is at the recommended 3/8" depth.

I could live without mine but did find it very useful when developing my HX flush techniques - I do a longish flush-and-go on an idle machine, and very brief flush when making shots in succession, using the thermometer to let me know when the group temp has rebounded. And if I've done something dumb and caused an HX stall it lets me know. I also use it as a crude postmortem for a shot's brew temperature - - in my case I figure that my brew temperature was roughly the average of the temp reading midway through the shot and the temp a few seconds after stopping the shot. Then if I taste it and find it noticeably good or bad I have a clue as to whether it might have been due to the brew temp.

I think that if I had a DB machine it would be far less useful.
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