I'm stymied and need some brainology regarding a Gaggia Group question

[SJM]

I am having a persistent challenge come up over at the Gaggia Group that I can't seem to meet without Tex's expertise, and so I need some help from y'all.

New owners of old Gaggias are often (very very often) convinced that their pumps are faulty because nothing is coming through the brew head.  More often than not it is scale and/or....something wrong between the pump and the brew valve.  Tex posted what he calls the Water Debit Test*, which simply means disconnecting everything after the pump and testing the flow of water directly from the pump to see if enough flow is generated in enough time.  Now I am getting some flak from a member who seems quite sure that the pump can pass the WDT but not generate the 9-10 bar necessary to produce espresso.   I need to know 1) is that feasible and 2) how to devise a test which will prove or disprove the thesis.

I'm thinking it shouldn't be too hard to put a gauge directly onto the outflow from the pump?  That is if the answer to #1 above is even "yes"


*To test the pump’s effectiveness, I disconnect the high pressure tube between the pump and boiler at
the pump. I then slip a piece of plastic tubing over the output and clip it in place so no water can leak
onto the electrics.
Place the unattached end in a graduated measuring cup, then turn the pump on and measure the
output: If it’s less than 100 milliliters in 15 seconds the pump is bad, but maybe not irrevocably.
If your pump is sluggish, try descaling it by dissolving 2 tablespoons of citric acid in 2 cups of water. Place
the input and output tubes in the container, and switch the pump on for 30 seconds followed by a 60
second wait period. Repeat the on/off cycle 30 – 40 times, and then repeat the water debit test. If
there’s no noticeable improvement, then assume the pump is beyond salvage and replace it.

[John F]

Water debit is water debit and brewing pressure is brewing pressure.

You can debit water at any given pressure depending on the factors in play. You can generate 9 bars (or however many) with much more/less water debit that you want.

The only thing I can think of to prove/disprove pressure is to mount a pressure gauge like I did or use a portafilter thingamagig like Tex uses. 

So yeah, your member is right that pumping X oz of fluid is possible to do and be out of range for proper brewing pressure but 1) why would he do a water debit to to test pressure anyway? 2) didn't he test pressure to set the OPV?

[SJM]

Water debit is water debit and brewing pressure is brewing pressure.

You can debit water at any given pressure depending on the factors in play. You can generate 9 bars (or however many) with much more/less water debit that you want.

The only thing I can think of to prove/disprove pressure is to mount a pressure gauge like I did or use a portafilter thingamagig like Tex uses. 

So yeah, your member is right that pumping X oz of fluid is possible to do and be out of range for proper brewing pressure but 1) why would he do a water debit to to test pressure anyway? 2) didn't he test pressure to set the OPV?

Okay, I didn't explain this correctly. 
I/we understand perfectly that a gauge (damn, it takes me three times to spell that word correctly) is the only way to test the pressure at the brew head.
He/we/no one is using the WDT to test brew pressure.

The question is:  can a pump pass the WDT and still not generate enough pressure to brew espresso.   And, if so, what would be a good test to determine that that is the case.   

There is a member at GUG who keeps suggesting that pumps be replaced even when the WDT has been passed, and I suspect that that is wasteful.  I'm trying to find a way to deal with the discrepancy between what I think is true and what I can prove.

Susan

[John F]

I guess I see your question like this...

If a car can drive one mile how can we prove it isn't driving at 25MPH without measuring the speed of travel?

This is what I see you asking. The water debit is measuring an amount of water (like the driving one mile). You are asking to prove the pressure exerted without measuring the pressure but using the debit measure to prove/disprove what pressure was exerted for the debit.

What am I missing?

Just use water debit to measure the debit and pressure gauge to measure the pressure...all problems solved.

[SJM]

Let's forget that I asked.

I obviously am not explaining the question correctly, and I can't figure out how to do a better job, so....just fergeddaboudit.    

Susan

[milowebailey]

Susan

Here is my understanding

two tests

Water Debit, aka Volume  A machine MUST be able to produce between 60 and 100ml of water from the group in 10 seconds. Less than this means that it is impossible to produce an espresso shot within the standard parameters.  (Brew time = 25 seconds).

Pressure -  The “correct” pressure at the group is 9 –10 bar, and in theory any machine fitted with a standard Vibration pump should achieve this. The only way to actually test this is with a portafilter mounted manometer, so it’s unlikely that the average joe would be able to check it.

If the Water Debit test does not pass then check for scale..... if it's scale free and still fails, replace the pump.  If the Water Debit test passes, check the pressure, it could still be a bad pump that can flow water but not under much pressure....

From what I've seen if a pump is bad neither test passes, but that's not to say it can't pass the debit and fail the pressure..... and assumes there is no OPV in the loop that is adjusted wrong.

[SJM]

Susan

Here is my understanding

two tests

Water Debit, aka Volume  A machine MUST be able to produce between 60 and 100ml of water from the group in 10 seconds. Less than this means that it is impossible to produce an espresso shot within the standard parameters.  (Brew time = 25 seconds).

Pressure -  The “correct” pressure at the group is 9 –10 bar, and in theory any machine fitted with a standard Vibration pump should achieve this. The only way to actually test this is with a portafilter mounted manometer, so it’s unlikely that the average joe would be able to check it.

If the Water Debit test does not pass then check for scale..... if it's scale free and still fails, replace the pump.  If the Water Debit test passes, check the pressure, it could still be a bad pump that can flow water but not under much pressure....

From what I've seen if a pump is bad neither test passes, but that's not to say it can't pass the debit and fail the pressure..... and assumes there is no OPV in the loop that is adjusted wrong.

Okay
The WDT as Tex has defined it is a test of the water flow right at the pump. Now maybe this is heretical, but it is the WDT that every Gaggia Group member is directed to when s/he suspects that his pump is not working.  Tex's point is to remove the issues that arise after the pump so as to eliminate it as a suspect and to hone in on the more probable causes of problems.   And he has always insisted that if the pump passes the WDT that it should not need to be replaced.  Now it seems that that may not be the case, that a pump might pass the WDT (at the pump) but still not have enough pressure to brew espresso.  Dang me.  Where is that man ?

Thanks both of you.

Susan

[jspain]

Where's Tex when we need him???!!!! I hope he's doing OK.....

[SJM]

Where's Tex when we need him???!!!! I hope he's doing OK.....

I get first dibs !!!!!

[ecc]

Susan,
I would say Tex is right.  If the pump can produce the correct volume in the correct time, it is very unlikely it is bad.

The reason is in the way vibration pumps work: a core between springs is pulled back and forth by a magnet with a one way valve at the end.  Water is allowed in when the core is back, and forced out the valve when the piston moves forward.  This method provides just as much suction (filling the cylinder) as it forces out, so it works well with reservoirs.

When it fails:
If the springs on either side break, the piston path won't be as long and less water will be pushed past the valve.

If the electromagnet fails, or overheats and piston seizes/breaks seal rings, no water comes out.

If the one way valve fails closed, and or the pump is clogged, less water will be pushed past the valve.

The only scenario that I can think of that allows for the full water delivery on the WDT test and not under pressure is if the one way valve leaked horribly but without failing completely.  Possible, but not likely.

If the machine is equipped with an OPV, I would think that would make minimum pressure testing a little easier, too.

[John F]

I agree if it passes a debit test chances are the pump is good.

But it doesn't tell us if it's brewing at 7 bar, 8, or 12.

[ecc]

I agree if it passes a debit test chances are the pump is good.

But it doesn't tell us if it's brewing at 7 bar, 8, or 12.

Not the question asked.  If it makes the volume test it should have no problem producing 9 bar.  As to what the exact brew pressure is, that is an entirely different question.

[SJM]

Me, still, trying to hold up my end of an argument/discussion over at GUG....

I think that there is some ball park amount of pressure that must be needed to move a specified amount of water in a specified amount of time, right?

If Tex's test had just been "move 100 ml of water" that would not have indicated any pressure because heck it could have been running down hill;  but because he said "in 15 seconds", some minimum pressure must be necessary, right?

Someone keeps telling me that the WDT involves no resistance, and I think that can't be quite right, but I am inadequately educated to deal with the argument.

Susan

[dickcoffee]

I guess this isn't proof of anything, but my own experience is this: a lady in South Bend advertised a Gaggia Coffee for free on account of "it has a bad pump".  I took it, on first try could get no water through the group. It was filthy, cleaned it up, descaled the boiler and it now makes good espresso. I'd bet a lot of folks have a similar experience and your own experience, which is far greater than mine, suggests the same. So, it can be proved with a pressure gauge, but it's not worth the cost. People should just take your word for it, or replace the pump, but odds are that would be an unnecessary expense.

[John F]

I agree if it passes a debit test chances are the pump is good.

But it doesn't tell us if it's brewing at 7 bar, 8, or 12.

Not the question asked.  If it makes the volume test it should have no problem producing 9 bar.  As to what the exact brew pressure is, that is an entirely different question.

I thought it was the question..

Can the pump pass WDT and be pulling shots under 9 bar...I thought that was the question anyway and I figure the answer is yes.

Like Susan mentioned there is no load restriction on the pump during WDT.

Anyhow, we can come at it from a hundred angles but for me pressure is a separate factor to a WDT. Debit alone tells us nothing about the (brewing) pressure and that I think is the GUG members argument. My argument to them would be just check both and be done with it.