frankmoss wrote:However, I must respectfully disagree

Touche. Yes, good sir, I believe you are correct.

Assuming you don't drop below the threshold where the pumping becomes erratic, (and assuming you allow things to reach a steady state), adding resistance linearly should decrease pressure linearly. I'm not quite sure why I thought this wouldn't be the case. I tested my rheostat with an inexpensive multimeter, and I remember the tolerances being less than 5%. That being the case, specs on the solenoid never appeared after a large amount of research, and I only found one source which gave an inductance value.

These factors combined with the variances in the pumps from the factory mean that a rheostat is still a good idea for finding each machines ideal resistance. Being a perfectionist for accuracy, I will change my initial post to better reflect this reasoning.

If anyone else finds things that I have gotten wrong, please let me know. I want this guide to be the best single-stop how-to possible. Thanks!

My first night of testing was brief, and I didn't make any careful observations, but I thought I would write down a couple of things of interest. I could only pull two shots because I didn't dare have more caffeine at 7:30 with work in the morning. I decided to test the two extremes for a comparison - full resistance and no resistance. I used the same coffee, dose, and grinder setting.

Since I was using a higher dose than I am used to, it was a little difficult to judge how my mod affected flow rate or channeling. Surprisingly to me, both shots flowed. When stopping by time, "with resistance" was about half an ounce shorter. I will certainly have to experiment a lot more to determine the affect on blonding and flow rate.

There were two things of interest in how the pump behaved, however. With full resistance, the pump started to sound a little funky. 50 Ohms appears to be right above the threshold of very erratic pump behavior. My boiler kicked in during the shot, and I really noticed the lack of power. The flow seemed to throb a little bit, and the pump was super quiet. This is clearly not a pressure that I want to pull a whole shot at, although the ramp up to peak (reduced) pressure seemed more gradual than usual.

The second thing of note was that at "no" resistance, the pump didn't act completely normally either. It's possible that the pump needed re-priming after the resistance shot (maybe I inadvertently stalled it). It's also possible that there is resistance in my unsoldered connections, as I was previously advised against. I think the second is the case. When I first tested out my mod, it was after freshly assembling. I think that after a week of sitting, some oxidation has built up or the connections are now loose. I am definitely going to solder my connections soon.

Also, I bought some wire from Ira, and it is vastly superior to what I am currently using. It is a lot more flexible and seems higher quality. I will be changing the wire I use soon as well. Wire is probably the least critical component to this mod - there are lots of different types of wire that would work - but I would recommend purchasing some from Ira at this point. I like it.

Nice work. I've ordered my rheostat, so I'll try out this mod in a week or so. I look forward to hearing about how yours continues to work.

mini wrote:My boiler kicked in during the shot, and I really noticed the lack of power. The flow seemed to throb a little bit, and the pump was super quiet. This is clearly not a pressure that I want to pull a whole shot at, although the ramp up to peak (reduced) pressure seemed more gradual than usual.

I have a switch, so I can turn off the boiler during the shot, because when the boiler turns on it effects my pump pressure. I already had a PID and SSR controlling the boiler, so it was easy to add a switch in series with the SSR control line.

Randy

Thanks for putting together this 'how to' on voltage controlled pressure Matt.
I have a few suggestions based on my use and testing;

- You really should have a pressure gauge somewhere in the brew path to calibrate where you are and what you are doing with brew pressure. Until then, if you have an AC volt meter, 'calibrate' the voltage to the pump to about 107-108 Volts. Put a reference mark on your dial here. This should get you very close to 9bar 2oz in 30 seconds.

- I would recommend for these small boiler 'saturated group' machines that you have the heater full on during the shot rather than full off. (flip on the steam switch when you start the shot) Calibrate your pump voltage for this. Temperatures depend so much on the 'heater cycle' here, but in general, if you start with the boiler in a stable temp state (heater off for 30+ seconds), When you start the shot, boiler temperature drops rapidly. Shortly after this, brew temperatures drop. For the Isomac Venus and Gaggia Espresso; my tests show better brew temp stability during the shot with the heater on. (On Venus, I turn the heater on 10-12 seconds before starting the shot to get the flattest brew temp) Not recommended for Silvia.

- As to the linear response of the pump pressure per voltage change; (this based on some limited testing with a Variac) It works fairly well through a pressure range from 6-7 bar on up. What happens at lower voltage/pressures I believe is some electro-mechanical issues of the various springs and piston mass.... etc.. it no longer oscillates stable.

- It is a good idea to secure those connections better. The connectors on the pump are subject to intense vibration. Solder is best. This may be part of your pump stability problem.

- Have at it to see what the lower pressure limits are here, but I think it is not stable to the low pressure for 'pre-infusion' control. I think pre-infusion is not such an issue with vibe pumps. Briefly open the steam valve before starting the shot to relieve any expansion pressure. Allow headspace. Check your dwell time.
I think it may be more interesting to experiment with tapering the brew pressure a bit at the end of a shot.

I was wondering how long it would be before you jumped into the discussion, Frost. It was probably your posts on CoffeeGeek that got me started after all.

On that note, here is a good resources list I made before I did this mod:

Link #1 - a really good thread off of CoffeeGeek discussing voltage control
Link #2 - has a good disassembled picture of an Ulka pump
Link #3 - high qualitly 3D CAD drawings showing pumping mechanism
Link #4 - CEME's website showing pressure profiles
Link #5 - CEME's website with brief model differences
Link #6 - someone with a solid electrical background repaired a Breville machine (with Ulka pump) and made some measurements. (I used this to determine the required rheostat wattage.)
Link #7 - an older discussion on Coffee Snobs that briefly mentions voltage control

You also made some very good points.

1. I might make a pressure gauge sometime, but one of my primary reasons for doing this mod was to avoid all plumbing if possible. I know it would be beneficial to the results of this topic, but it will probably be a while before I fit one. I might take your advice about optimal voltage after some more experimentation, though.

2. I love toggle switches! Maybe I could use two and have the ability to turn the boiler always off, always on, or back to normal function. I could have a "safety" switch to enable a strictly on/off switch. But back on topic...

4. I would be more inclined to solder all of the connections if my machine hadn't used simple crimp connections inside to begin with. And I like the ability to remove the mod if desired. I did solder the connections on the rheostat when I switched to the new wire today. I checked with my multimeter and found very little resistance between the spade connectors I added. I will revise my how-to to include soldering - I think it's a must.

5. I agree that a true pre-infusion isn't possible. At the highest resistance, flow is still fairly rapid, and I don't think it's a good idea to add more. However, I plan to test a lot of shots starting at this lower pressure end and then increasing to my desired "set point." I see no reason not to make the initial flow as gentle as possible, since channeling was the number one issue I wanted to correct.

Keep up the discussion, it has been really beneficial to me.

If channeling is the number one issue you want to address, I think it would be best to set your brew pressure to 9 bar and dial in your grind and dose to get consistent shots here before employing variable pressure control. I think adding the extra variable (pressure) will only serve to obfuscate.

For heater on option during the shot , you don't need to add a switch as the steam switch will serve for the function.

You really need to adjust the frequency if you want to solve the pre-infusion problem. If you look at the operation of an Ulka you'll realize that what you're trying to do is limit the travel of the plunger but it's really hard to do by limiting the voltage as the voltage required to move the plunger is not linear. Usually enough voltage to start the plunger moving is enough to slam it all the way to the stop. Not to say it won't work, but as you've seen, it will be tricky to control.

Well, unless I'm completely wrong.

Ira