I added a preheater to my Gaggia Classic, between the pump and the boiler, and it is not working as well as I expected.

The problem is that the OPV on a classic is bolted to the boiler, so the OPV is after the preheater, in my machine. I set the OPV to 9 bar, and when I pulled a shot, there was a steady stream of water coming out of the OPV. This was wasting a lot of the preheated water, so the change in temp, measured with an over the lip thermocouple on top of the coffee, was about 3F during a 25 sec shot. I was hoping for better. And, it is dumping hot water into the cold water storage tank, which is probably not good, long term, for my pump.

DanB from the Gaggia forum suggested I add a resistor, to lower the output of my pump, so it would output 9 bar, and then the preheated water would not go back into the cold water tank.

I ended up adding a lamp dimmer on the pump voltage. I just did it last night, and it seems to work. I used my portafilter pressure gauge to check it out, and I was able to vary the pressure with the dimmer. I set my OPV to around 11 bar, and then set the dimmer to get a pump pressure of 9 bar. Now, at 9 bar, no water is coming out of the OPV.

There is a problem with this setup. When the heater turns on, the pump pressure drops, by a couple tenths of a bar. I have a PID controlling temp, and after the machine has stabilized, it comes on for a second, once every few seconds. I am planning to add a relay, in series with the control lines of the SSR controlling the heater. The relay will open the + control line when the pump is on, and leave it connected when the pump is off. I might also add a three way switch into the mix, so I have manual control too. Switch connections would be relay, open (off) and PID.

I was also thinking I could use the dimmer to do a crude version of pre infusion, start the shot with the dimmer set to low for some number of seconds, then turn off the pump completely for a few seconds, then set it to 9 bar for the remainder of the shot. Could even ramp pressure down at the end of the shot.

Installation is pretty straightforward. Dimmer is added, in series, to the wire that goes from the pump switch to the pump. Physically, I installed the dimmer on the rear side of my machine, behind the pump. I did have to remove the plastic body from the dimmer, so it would fit. I also had to cut down the mounting plate about ¾". I wrapped the dimmer with kapton tape, so it won't short anything out.

The shot I pulled this morning was a little slow, I have not really dialed in my Classic yet, need to do a little more tweaking of the grinds. I am also looking at adding a permanent pressure gauge, on the side of the classic. Since I can easily control the pump pressure now, it would be nice to monitor the pressure real time.

Couple other notes.
I have an old classic, it does not use the same Ulka pump they use now, it's an old, obsolete pump from a different manufacturer. Not sure how the current model pump will work with dimmer control.
My house voltage is a little high, I think around 120 or 121, which would contribute to a high pump pressure output, before adding dimmer control.

Randy

I tried that a while back. For me, it worked fine down to about 6 bar. Below that, the pump flow got very low. I think if you drop it below a certain voltage, the pump stops pushing water at any pressure. Based on this, you should be good for pressure control, but may not have much luck with preinfusion.

No word on what it does to the pump's life.

randytsuch wrote:This was wasting a lot of the preheated water, so the change in temp, measured with an over the lip thermocouple on top of the coffee, was about 3F during a 25 sec shot. I was hoping for better.

Better in what way? If you mean a flatter brew temperature profile, you've bought into the theory advanced a few years back by David Schomer that flatter = better. I got over that bias once I had evaluated enough espresso machines that produced highly repeatable temperature profiles and were competitive with the state-of-the-art flatliners. In fact, many lever espresso owners and HX espresso machine owners would argue that a declining temperature profile is superior.

If the goal is to improve your espresso rather than tinker, I would look to better coffee or grinder for improvements.

If you really want to control a vibe pump you'll likely have a lot better luck if you come up with a variable frequency supply and/or put a gicleur between the pump and the OPV and a small accumulator after the OPV.

Ira

Why don't you add a second expansion valve between the pump and preheater, and crank the one on the boiler up to 10-11 bar?

HB wrote:Better in what way? If you mean a flatter brew temperature profile, you've bought into the theory advanced a few years back by David Schomer that flatter = better. I got over that bias once I had evaluated enough espresso machines that produced highly repeatable temperature profiles and were competitive with the state-of-the-art flatliners. In fact, many lever espresso owners and HX espresso machine owners would argue that a declining temperature profile is superior.

If the goal is to improve your espresso rather than tinker, I would look to better coffee or grinder for improvements.

I am under the impression that flatter = better. I can't think of a post, other then this one, where someone said that a declining temp is better. The small boiler, and the declining shot temp because of the small boiler is one of the knocks I have seen against the Gaggia's.

I was hoping that I would be able to implement the preheater so it would be easy to take it out, to compare with and without, but there's not enough room in the gaggia to reach in there and disconnect the tubes, so it's kind of hard to go back and forth.

And I'm an engineer, I have to tinker, it's what I do

I just bought my vario, so a new grinder is not on my list of changes, but I am going to start trying different fresh beans, from the better roasters.

ira wrote:If you really want to control a vibe pump you'll likely have a lot better luck if you come up with a variable frequency supply and/or put a gicleur between the pump and the OPV and a small accumulator after the OPV.

Ira

I was trying to come up with something relatively easy to do, to prove it would work. I have considered a zero crossing circuit to control pump speed, maybe in the future. And, I'm really not sure how the gicleur/accumulator would work here, and how to design it.

cubastreet wrote:Why don't you add a second expansion valve between the pump and preheater, and crank the one on the boiler up to 10-11 bar?

That was actually my first plan. There is a writeup at the Gaggia forum from someone who did it. I still may, but it will cost close to $100, by the time I buy the parts required, with shipping. This change was basically free, since I had the dimmer already.

Randy

randytsuch wrote:I am under the impression that flatter = better.

I'm interested in blind taste tests demonstrating this, if you're aware of any.

I can't think of a post, other then this one, where someone said that a declining temp is better. The small boiler, and the declining shot temp because of the small boiler is one of the knocks I have seen against the Gaggia's.

Here's some discussions I found by searching on declining brew profile. I authored a number of the posts, but others have mentioned the same points. The thread Do ultra precise brew temperatures really matter? is an oldie but goodie.

PS: Many levers have declining brew temperature profiles and you'll have no trouble finding owners who will argue those are superior, if you're willing to wander into the Lever Espresso Machines forum.

randytsuch wrote:That was actually my first plan. There is a writeup at the Gaggia forum from someone who did it. I still may, but it will cost close to $100, by the time I buy the parts required, with shipping. This change was basically free, since I had the dimmer already.

At best a zero crossing frequency divider would give you .5 .25 .125 steps in your pump duty cycle with full pressure pulses at the on cycles.

Instead of 60 full pressure pulses per second from a vibe pump you would end up with 30, 15, or 7.5 much below that you would start to see more flutter in the pressure, and even worse I would guess you would start to see stresses on OPV's and parts downstream from the pressure shock.

You would need to do a dual conversion switching power supply if you want to frequency control and voltage control a vibe pump. In that case partial power clipping like a dimmer would do is just about as effective without the added money.

-Cecil

randytsuch wrote:I ended up adding a lamp dimmer on the pump voltage. I just did it last night, and it seems to work. I used my portafilter pressure gauge to check it out, and I was able to vary the pressure with the dimmer. I set my OPV to around 11 bar, and then set the dimmer to get a pump pressure of 9 bar. Now, at 9 bar, no water is coming out of the OPV.

I really like this idea, and am somewhat surprised it has not "caught on" more. There was quite a bit of discussion of it awhile back on this Coffeegeek thread.

randytsuch wrote:When the heater turns on, the pump pressure drops, by a couple tenths of a bar. I have a PID controlling temp, and after the machine has stabilized, it comes on for a second, once every few seconds. I am planning to add a relay, in series with the control lines of the SSR controlling the heater. The relay will open the + control line when the pump is on, and leave it connected when the pump is off.

I ran into this problem during development of a PID system for the Alexia. Solved it the same way you did. It is more of a problem on the machines with higher wattage heaters. Haven't noticed to be quite as noticeable on the Silvia, but I'm not surprised it popped up with your Gaggia.

Jim

A vibe pump in it's basic mode is a gated relaxation oscillator. With the pump membrane being the fixed known.

You can either vibrate it faster, or slower to regulate the vibration of the pump and it's flow, depending on the gating of the one way valve on either side of it, or vary it's distention to vary the flow by it's point displacement.

One is Frequency based, one is Amplitude based. A Frequency division method is lacking without dual conversion. And a Amplitude would not be linear for the output.

The basic fact is trying to do a reliable set pressure from a modulated vibe pump is a dual or more variable problem that would be a total PITA.

Yes it can be done. But there may be easier ways to do it. That is all I am saying.

-Cecil