Just a couple of words on Temperature. There is varying temperature throughout the boiler. The probe is positioned to react quickly to changes in temperature. Since the probe is not located close to the exit of the water at the group head there will exist some difference. The offset in the software is there to account for this and also the differences in the probes themselves (+-5%). All of the probes have the same precision but you must have an offset to ensure accuracy. Accuracy in this case is the adjustment until the display matches the exiting water at the group head.

The display temp is actually good for a static condition only. During the brew cycle water enters the boiler and changes the temp. The probe (and the display temp) sees this change much before the exiting water.

Bill C

Billc wrote:The display temp is actually good for a static condition only. During the brew cycle water enters the boiler and changes the temp. The probe (and the display temp) sees this change much before the exiting water.

Thanks for joining in this discussion, Bill. Maybe you can explain what I'm seeing. Why does the temperature on my Scace ramp up 3-4 degrees during the first few seconds of the brew cycle, ending up within 1 degree of the boiler set temperature? My offset is -4 degrees, so I would think that the Scace should show something close to the set temperature minus four degrees after the ramp. But it doesn't.

For the reason you mention, I don't pay attention to the display temp during the brew cycle. However, it's often the case that the display temperature doesn't drop much during the brew cycle -- maybe .5 degree or so. If I run a lot of shots in a row it'll drop a degree or two. The PID and heater seem to do a good job compensating for the colder input water.

I suspect you might find your answer if you pulled your GS3's brew boiler temperature probe out and put it in a steam bath. If you do this, make sure you disconnect the water supply, drain the brew boiler, and physically disconnect the boiler heating element first. You should then be able to remove the probe and turn the machine on to see what it reads without risking a fried element. Then you can see which part of the 'offset' on your machine accounts for the heat loss between probe and grouphead, and which part accounts for how off of "actual" the probe reading is. It'd be an interesting experiment, though I am disinclined to bother myself.

My brew temperature rises on average to right about -3°F of boiler set point (closer if I use a longer warming flush). It came set from the factory with an offset of -5.4°F or something like that. I changed the offset to -2°F of all things, as I found that this offset made the temperature display during the shot read most accurately. In any case, I think little of that and simply set my boiler to 3°F higher than my desired brew temperature, and that's that.

Peppersass wrote:Thanks for joining in this discussion, Bill. Maybe you can explain what I'm seeing. Why does the temperature on my Scace ramp up 3-4 degrees during the first few seconds of the brew cycle, ending up within 1 degree of the boiler set temperature? My offset is -4 degrees, so I would think that the Scace should show something close to the set temperature minus four degrees after the ramp. But it doesn't.

Maybe I'm confused here but what you just described is what you want. You want the temp measured at the group head to match your set temp after the initial ramp up from idle. The offset is added (in your case) to your set temp to make the boiler temp higher than the set temp display to account for the heat loss occurring during the water's trip to the dispersion screen so I would expect the Scace to measure the set temp after warm up if the offset is correct (which in your case sounds like it is).

shadowfax wrote:I changed the offset to -2°F of all things, as I found that this offset made the temperature display during the shot read most accurately.

That's interesting, my offset was set from factory to -4.7 and I adjusted it by taste alone to -2.

mitch236 wrote:Maybe I'm confused here but what you just described is what you want. You want the temp measured at the group head to match your set temp after the initial ramp up from idle. The offset is added (in your case) to your set temp to make the boiler temp higher than the set temp display to account for the heat loss occurring during the water's trip to the dispersion screen so I would expect the Scace to measure the set temp after warm up if the offset is correct (which in your case sounds like it is).

This is strange, but on the La Marzocco GS3 (and I assume the FB80/GB5 as well), the boiler set temperature is not offset-adjusted. When you switch to "editing" mode on it, you see the 'raw' temperature set value of the system, i.e. what it try to will match its raw thermocouple reading to. The offset is only applied on the displayed temperature in normal mode (i.e. when you're not editing temperatures). It's actually kind of convoluted and confusing, but there it is.

So when Dick is saying that the Scace reads ~0-1°F less than his set temperature, he means the raw reading, not the offset-adjusted one. The offset just makes the display read much lower than the temperature at the group.

Peppersass wrote:Thanks for joining in this discussion, Bill. Maybe you can explain what I'm seeing. Why does the temperature on my Scace ramp up 3-4 degrees during the first few seconds of the brew cycle, ending up within 1 degree of the boiler set temperature? My offset is -4 degrees, so I would think that the Scace should show something close to the set temperature minus four degrees after the ramp. But it doesn't.

For the reason you mention, I don't pay attention to the display temp during the brew cycle. However, it's often the case that the display temperature doesn't drop much during the brew cycle -- maybe .5 degree or so. If I run a lot of shots in a row it'll drop a degree or two. The PID and heater seem to do a good job compensating for the colder input water.

The reason for the rampup is that the dispersion screen and the ambient side of the group lose heat to the surroundings, and they gain heat from the hot water as soon as brewing commences. That is why we throw away the first 3 secs of readings when evaluating machines for WBC.

Also, it looks to me as if your offset value is close to right if the reading from the group ends up being the same as the value on the machine's display.

FWIW, there is a convection loop within the neck of the saturated group. Relatively hot water from the boiler flows up the group along the top of the group neck. It's cooled within the group as heat is given off through the group metal and out to the room. Then the relatively cooler and denser water flows along the bottom of the group neck back to the boiler.

-Greg

gscace wrote:Also, it looks to me as if your offset value is close to right if the reading from the group ends up being the same as the value on the machine's display.

In the portion of text from Dick that you quote, he says that the Scace ramps up and the temperature approaches the boiler set temperature, i.e. not the offset-adjusted value. If his offset is -4°F, then the display ought to be reading ~3°F lower than what the machine display (which is offset-adjusted) is reading. Dick, care to clarify further?

shadowfax wrote:In the portion of text from Dick that you quote, he says that the Scace ramps up and the temperature approaches the boiler set temperature, i.e. not the offset-adjusted value. If his offset is -4°F, then the display ought to be reading ~3°F lower than what the machine display (which is offset-adjusted) is reading. Dick, care to clarify further?

Um, what you say in the first sentence is correct: the temperature shown on the Scace-driven digital thermometer approaches the boiler set temperature, not the offset-adjusted value that's displayed on the GS/3 screen. I don't quite get what you're saying in the second sentence, so let me provide an example:

Let's say I want to brew at 196F. Since my offset was programmed by LM to be -4F, I must set the boiler temperature to 200F to achieve the desired brew temperature of 196F. This is because, supposedly, LM measured a difference of -4 degrees between the temperature of the water in the boiler and the temperature of the water exiting the group.

But here's what happens: The GS/3 display shows 196F, which is my desired brew temperature. I run a 3-second flush and then attach the Scace. I start the shot, and as soon as water reaches the dispersion screen the temperature shown by the Scace rises quickly. Within about 10 seconds, the temperature shown by the Scace shoots past the GS/3 display temperature of 196F and stabilizes at 199F-199.5F for the rest of the shot. This is very close to the boiler set temperature, not the desired brew temperature.

I'm interested to read that at least two other GS/3 owners have changed the offset to be closer to the boiler set temperature, just as I would need to do (assuming there's no measurement error going on.) This would imply that there's not as much heat loss in the water's trip through the group as LM seems to have measured. Perhaps they measured right at the start of the shot and not when the temperature stabilizes.

As I stated in another thread on this subject, the ramp shows that the temperature is not constant throughout the shot, so a fixed target temperature is a little misleading. You're not really brewing the entire shot at the temperature displayed by the GS/3. The reality is that the GS/3 has a temperature profile which, though very reproducible, results in brewing at different temperatures at different times during the shot. So, in the end, you probably have to do what shadowfax and tekomino did, and change the offset so you get the best shot when the displayed temperature is close to what roasters or reviewers with LM machines recommend.

But here's a far more important question, Nicholas: how do you get those little degree symbols to display in your posts?