Hi all

So I've been reading about HX machines and how to manipulate their temperature profile. In particular I was reading this http://www.home-barista.com/hx-love-brew-temperature-profiles.html, and have some questions/comments about it.

From what I understand, the temperature "hump" at the start of the pull is due to the hot water that has been sitting in the heat exchanger (and getting quite hot) coming out. The slope then drops because the subsequent water is freshly drawn up from the reservoir and isn't as hot.

Then the author starts talking about how he worked with his HX to get a flat slant "L" Profile, in which the hump is eliminated.

Now here're the parts I don't understand:

1. I have read people saying that they can adjust their brew pressure by modifying their cooling flush amount. But this doesn't make sense to me; changing the volume of the cooling flush should only affect the temperature of the initial few seconds. After that, wouldn't the grouphead start to cooldown as the pull continues (since the water coming through continues to "cool" the grouphead)? Could someone explain this?

2. And for confirmation - is the only way to get a flat temperature profile by flushing and THEN pulling straight away? This would mean that all water is at the same temperature (all coming straight through from reservoir). Is the only way to modify the temperature then to adjust the pressurestat?

1. The flush has nothing to do with the brew pressure. The brew pressure is adjusted at the OPV or bypass on a rotary pump. Perhaps you meant brew temperature though. In that case, yes, the flush volume does affect the brew temperature. Whether the grouphead heats or cools the water depends on your flush tecnique. However, the more you flush, the cooler the water in the HX gets, and consequently the lower the brew temp.

2. I'm not sure about how to get a flat temperature profile. Even using a flush-n-go technique like you describe, I still see a small temp hump. However, I'm not sure that a completely flat profile is something worth seeking. The pressurestat setting has little to do with the brew temp. As you can see here, http://www.home-barista.com/hx-love-man...ature.html, any reasonable pressurestat setting will give you almost the same profile. What matters is the flush routine.

Let me use this post as an excuse to repost my idea for a totally flat temp profile so that maybe someone will tell me it is nuts or maybe worth considering. A small, secondary, insulated "tank" with a temp probe, a small circulating pump capable of the temperature, a small heater element and some software in the controller. Works thusly: the machine fills my special tank from the main boiler. The circulating pump runs this water through the group and back till it is all at the preferred temp using the small heater to get the desired temp. The software holds the brew cycle till this temp is reached or waits if it overshoots (maybe adds a squirt of cold if needed). There is sufficient water to brew a double and it will be exactly the same temperature for the cycle, start to finish, having enough thermal mass between the group and the water in the "special" tank. I doubt this would work in a commercial situation but might be useful in high-end home rigs.

Duke-one,

Your idea certainly could be made to work. But doesn't a "secondary tank" seem suspiciously similar to the brew "boiler" in a double-boiler system?

The recirculation to the group head, as you might get with the thermo-siphon on an HX machine, can keep the back of the group hot, but the front of the group, cooled by the radiation from the portafilter and any exposed group above it, could be 10-20F cooler. So there's still some compensation to be dealt with.

-Ed

Ed: Thats why in my system the "circ" pump would run till the entire group and special tank were at an even temp and they would have enough thermal mass to not change during the shot.

Duke-one,
Sure. But the devil is in the details, and the specifics, and the requirements.
If you define a "flat temperature profile" as staying within a 5-deg F band, and you have a group that doesn't lose a lot of heat out the front, then the odds of success are very high.
On my Expobar Office, when the back of the group and the thermo-siphon are at brew temperature, the water will initially hit the puck at 5-7F below the brew temp, and reach brew temp in 6-8 seconds, if I don't heat the front of the group. I'm sure other machines could vary from this by a wide margin. This is a parameter that's fairly sensitive to "is the A/C on, is the window open, etc.?" if you don't have a saturated group or something similar.
-Ed

Just bringing it back to topic:

Yes, I did mean brew TEMPERATURE, not sure why I typed brew pressure.

But my question is back again - so you flush to a particular grouphead temperature (because the water is cooler than the grouphead), and you pull your shot at that temperature. Doesn't that mean your temperature drops even further (since you are again introducing cooler water) - meaning that you won't be able to get a stable shot temperature??

Thanks
Sing Chee

If you really want to understand how your HX behaves, one thing you want to do is figure out if your HX system is "one way" or not. On my Office Control, when flushing or pulling a shot, water travels to the group through both legs of the thermosiphon loop.... up through the heat exchanger as everyone expects, but also through the "cold return" leg ("backwards"). So the group gets a combination of hot and cold water. I've never attempted to measure the ratio. And the ratio could be different for flushing than for brewing, since they are at different flow rates.

But it can make for interesting dynamics. When "idling", the temperature of the return leg is just below that of the back of the group. Immediately after flushing, the return leg temp will be near ambient. So the contribution of the return leg water to the initial shot temperature could vary considerably with the pause time between the flush and the shot pull.

There's almost nothing constant in getting a constant shot temperature with an HX. The temperature of the group changes during the shot, and the temperature of the water arriving at the group changes during the shot. So it's quite the trick, but not impossible, to get the water coming out of the group to be near the same temperature for the shot duration. -Ed

There's almost nothing constant in getting a constant shot temperature with an HX.

That's putting it correctly and mildly at the same time.



edit - 08/01/2011 - Please note that this graph shows flushing temperatures ONLY until about the 35 second mark. Temperatures after that point show the temperature recovery of the machine at three points. Grouphead temperature is measured with a thermocouple via the thermocouple adaptor and group in/out temps are measured with stick-on thermocouples located on the tubing just as they physically enter the E-61 grouphead. This graph does not represent any "pulled shot".

I have 4 years experience with the VBM DS HX E-61 , and now have had my VBM DD (double boiler, PID brew control) for about 16 days. Not much experience with the DD, but here's what I have seen.

The DD's dedicated brew boiler without the HX has shown excellent temperature control as well as a consistency that was difficult to get from the HX. While the DS HX did operate within a relatively narrow range, the amount of time for a cooling flush, even using Eric's excellent group thermometer, was always a bit of a guess. I could cool-flush to the same temperature, and depending on how long the machine had been idling would change how effective that cooling flush was. The DD seems to operate the same whether I have a 30-45 minute warm up or it sits for three hours idling.

The HX was a bit of a compromise. To get reasonable control over brew temp and limit the cooling flush time I turned the pressurestat down, but that also decreased the steaming power of the machine.

The espresso quality has been better and more consistent with the DD, but we are comparing a rotary DD with preinfusion (because it is plumbed) to a vibratory machine using a reservoir which had a slower ramp to brew pressure.