How raise brew temperature VBM Domobar Super?

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m1ke_adams

#1: Post by m1ke_adams »

Hi,
I just purchased an E61 temperature sensor for my VBM because I think I need to replace the pressure stat and I wanted to know the current setting. This is the 'Coffee Sensor E61 Grouphead' thermometer I have: https://www.mydeal.com.au/coffee-sensor ... er-2102089

My temperatures range between 85 and 89 DegC [185-192 DegF] and after a shot the temperature trends down to 85 for at least 30 minutes before slowly creeping up to 88-89 degC

I understand it should be 92-94 DegC

From the documentation it seems I should increase the boiler pressure via the pressurestat screw to increase the temperature but the boiler is already at 1.55bar (it cycles between 1.3 to 1.55 every 2 minutes or so)

Suggestions? I do think my crema is weak and have wondered if the temp is too low.

Attached temp profile taken at 19 degC ambient, sea level.
VBM Domobar Super Electronique

nahau

#2: Post by nahau »

I guess the question is... do you trust that group thermometer? Is it calibrated, or is there a way to calibrate it? Have you tested it for correctly reading iced water or boiling water? Have you tried manually measuring the group water with another thermometer in a foam cup?

Pressure stat at 1.3 - 1.55 should yield a temp in the range of ~251 - 261F according to the chart


m1ke_adams

#3: Post by m1ke_adams »

Thanks Nahau,
I think your chart is for saturated steam temperature inside the boiler. I'm measuring brew temperature after the heat exchanger and it should be about 92 degC according t the manual.

I will calibrate my new sensor in boiling water because 87degC should not be 1.5bar - it seems that either the boiler pressure gauge is wrong or the temperature sensor is wrong. 92DegC(197F) brew temperature should be about 1.0bar.
VBM Domobar Super Electronique

Jeff

#4: Post by Jeff »

I think you can check with boiling water, but to calibrate requires a saturated steam bath, corrected for your atmospheric pressure of the moment.

The good thing is that a group-head thermometer needs to be repeatable, but not necessarily accurate. On my HX, with my unit and technique, I'd start the pull at 194 or 196 for most coffees. It would typically "idle" around 210 after warming up for an hour in the morning. My numbers won't be your numbers, as group thermal characteristics are different among various E61-style units.

nahau

#5: Post by nahau »

The chart is for temperature of the boiler "water" vs bar as seen on the boiler pressure gauge. I'm assuming your HX runs through your boiler and gets it's heat that way? Boiler pressure will affect the brew temp out of the group so if your boiler is at a high temp, or low, then so will be your brew temps. Why do you think the chart is for steam temperature? Maybe I should have posted this chart for you instead but it didn't go up to 1.55 bar.



I believe we are talking about the same thing.

m1ke_adams

#6: Post by m1ke_adams »

Thanks Jeff and Nahau,
I pretty confident of the new brew temperature sensor accuracy. I get 100.0 DegC in boiling water and 1.1 DegC (still going down) in a bath of water and ice cubes. Both measures are at 1.017 Bar = 0 barG(sea level). You need two points to calibrate and assuming my temp probe is not pressure sensitive (it won't be) then the temp gauge is good.

My brew temperature is too low (87degC versus 92DegC) and my boiler pressure is too high 1.55 versus 1.0 barG.

What to do? If the boiler pressure gauge is correct then increasing boiler pressure to increase brew temperature would be dangerous.

Something else is going on.
VBM Domobar Super Electronique

Jeff

#7: Post by Jeff »

How old is the machine? Is it possible that scale is blocking the thermosiphon? Did the taste of your espresso change significantly after installing it?

nahau

#8: Post by nahau »

Have you tried doing a manual measurement with a thermometer of the water coming out of the grouphead and hot water tap? You need to verify the boiler pressure gauge is reading correctly too. Even if you don't get a really accurate temp reading, it should be close enough to tell you your boiler pressure gauge and water temp is close per the charts. The temps you're seeing now vs bar is way off so you have to do more measurements. It could be a thermosypon problem too.

edit... Jeff, you posted while I was typing! LOL! Yup, the machine might have a thermosyphon stall going on.

m1ke_adams

#9: Post by m1ke_adams »

Thanks Jeff and Nahau,
I'm pretty confident of the brew temperature measurement. I don't know how to check the boiler pressure measurement without replacing the gauge.

So how to check the thermosyphon?
I assume we are talking about possible blockage in the heat exchanger tubes?
Is it a matter of descaling?
It is an old machine but I've never had scaling problems. We are blessed with soft water.
VBM Domobar Super Electronique

nahau

#10: Post by nahau »

To check the boiler water temp all you have to do is get a cup (pref a foam cup), a thermometer (pref digital and fast that goes higher than 250 F), and then measure the water from the hot water tap when it initially comes out. You should see at least 250 since your pressure stat is at 1.55. This is not an accurate measurement, but it'll be close enough to tell you something might be going on with the HX. It'll also tell you that the boiler pressure gauge is reading correctly if the water temp is close to what's on the charts.

Thermosyphon stall doesn't necessarily have to be caused by scale, although it's the most common culprit, but can also be caused by a leak somewhere or maybe even a bad check valve on the HX. EDIT... deleted comment... I must have been sleepy! It'll be hard to determine what's going on without taking something apart. Pull the mushroom and check the brew valve to see if the seal is damaged/pitted or overly compressed. If it is, change the seal... it might have a slow leak. Also check the mushroom gasket that it isn't flattened and may be passing water while you're in there. If they seem ok, then you might want to remove the covers off the machine and see if you find any leaks, or scale build up, especially around anything that has to do with the HX.