Coffee machines don't typically have pumps, they use heat to push the water up.

Is it possible? Absolutely yes it is possible. I work on Heat exchanger systems that do exactly what you are describing.

Problem is, it is not economically viable. (For the minor convenience of controlling the flow to your coffee machine, you would be adding so much additional cost to the machine. Additionally, you would be adding alot of additional parts which could break and cause increased repair costs. Its just not worth it)

My systems i work on are for defense use. (Keeping radar systems cool so electronics dont overheat) in those cases it totally makes sense to spend thousands of dollars installing and maintaining pressurized systems that react to (temp, pressure and flow rate, and increase or decrease rpm's based on the additional cooling or pressure needs of your system during certain modes of operation. - ie if you are in southern california its much warmer than alaska. If you are operating at high power there is much more heat dissipated than in standby or low power, etc etc)

In the case of a coffee machine - you would be creating a machine that costs $800-1000 plus upkeep costs, and most folks would never want to pay that for some specialized coffee production.

The machine I have is quite customizable and cheap. Users are installing pressure sensor. I was wondering if I could skip the pressure sensor which is used as a feedback to modulate the pump current, just by knowing what the pump's RMP/current is.

You can look into a dc voltage pump, an actuated valve, or a few other configurations. What kind of flow and pressure variations are you looking for? Are you pumping hot or cold water?

I don't think he's talking about designing a commercial coffee machine, I think he's talking about a one-off hobby project.

And people do pay that for coffee machines. A Jura Giga 6 is about $6000, a Z10 is around around $4000. Neither of those is considered to be a "Pro" machine.

Depends on how static the other variables are. Hint they probably aren't or others wouldn't be using pressure control.

Also please explain how you intend modulate the current? Most speed controllers adjust voltage, frequency or resistance which do change the current but it isnt a direct correlation the load on the motor at a given speed changes the current.

We evaluated using current to control pressure in automotive fuel systems. The only thing I recall was with constant current the pump pressure would vary a significant amount. It's been too long to recall what the cause of the variation was/were.

Really what you're trying to do is vary the motor speed and torque, not so much the current.

At that point, the best way to do this is with a VFD (though you'll need a rated motor). Variable Frequency Drives are sometimes called Variable Speed Drives for just that reason.

That's not correct.

Cold water to a boiler. When you do the coffee shot, there's a variation of the restriction. The goal is also to push with higher pressure initially (preinfusion), then lower during the infusion.

Consumer drip machines use boiler tubes, high end espresso machines have pumps. Given the number of drip machines vs espresso machines I gotta say it is.

Of course, I am being pedantic. Give me specific context and I will admit you are correct.

VFD - frequency

VSD - voltage

I would assume the coffee machine pump is a DC motor not an AC one.

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I read his post a bit too fast, missed the "typically". The context of my post was pretty darn specific.

Is it possible to "guess" the flow/pressure just knowing the RPM and supplied power ?

Right, it's pretty similar to my system. Coolant going into heat reservoir. Thanks for sharing your experience.

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That is what i was thinking. It's possible to estimate flow and pressure based on pump demand (current, etc), but you'd probably want to do so real testing and trending to understand the actual correlation. This is really the only shortcut around modeling every electrical and mechanical stage of the process, accounting for losses, variability, etc. Either way, it becomes a bit of a science project.

You are correct.

It's possible to calculate it, yes. But under most circumstances not with enough accuracy to use it as the control input. There's just too many variables.

What you really want to know is either flow or pressure, and measuring current is too far abstracted from that.

I can see wanting to use current in place of adding a flowmeter, but pressure transducers are cheap and super reliable, so if that's what you want to control for just measure it directly.

Thanks, you basically answered it. Just out of curiosity, why is it so variable? Shouldn't you expect power and RPM a direct measurement? For the sake of knowledge ;)

A good question.

There's going to be a lot of variation in current vs pressure because there are 3 energy form changes (electric to mechanical to fluid flow) and many outside influences.

Starting with electrical- is the operating voltage 115V? Or is it 121V or only 109V because someone is running a toaster simultaneously on the same circuit? All those will effect current and yet have nothing to do with pressure.

Is the motor warmed up or in a cold room? The shaft and pump seals, how does temperature, mineral buildup etc effect them? How often does the operator clean the machine? Does the grind of the coffee or make of filter influence back pressure or temperature or something else? All this, and yet even if you figure all that out it will change as the parts wear over time.

So what you will run in to is that there are so many things that effect current that you will never be able to entirely trust it as a measurement for pressure.

So the best possible solution is the simplest - avoid all the guessing and go right to the thing you want to know. It also allows you to build a check into your system if you want - of actual known pressure.

That can be helpful for safeties, interlocks, warnings etc.