slofox Senior Member Joined: 28 Nov 2012 Posts: 32 Location: Hamilton, New Zealand Expertise: I love coffee
Posted Wed Mar 20, 2013, 12:17pm Subject: Gaggia Classic OPV
My apologies for asking a question that has, no doubt, been asked and answered before but I can't seem to find a ready answer with the search function. Please feel free to refer me to an existant answer if there is one. Otherwise, any suggestions would be appreciated.
I don't have a pressure gauge to check the accuracy of the OPV setting in the Gaggia Classic, nor enough readies to go out and buy the stuff I'd need. I have done the "approximate" adjustment by backing off the original setting by about 270 degrees.
So. The question. What would I look for to tell me if I am in the ball park? Are there obvious symptoms of either too high or too low a setting? If so, what are they?
In practice, the machine does a pretty reasonable job. But I do think it tends to produce coffee a little on the bitter side, compared to coffee from the same beans at my roaster's shop. (Course they have a commercial machine there which may well have a bearing on the end product...)
FYI. Grinder is an Ascaso, flat burr model with the stonking great brass mounting and the micro-adjustment worm screw.
The only other test I know of is a flow rate test through the OPV - with a blind basket in place or the steam switch on you should get about 125ml in 30 seconds flowing out through the OPV outlet into the water reservoir area. You'll need to set the machine up to draw water from a glass or similar and to deposit the overflow from the OPV into a measuring glass/jug, then turn the pump on with a blind basket in place (or maybe with the steam switch enabled as well, to keep the group valve closed), and measure the flow rate out of the OPV return.
The test is described here, someway down the page. This test relies on your pump being about average for these machines, and there being no kinks etc. in any of the lines, and so it's not as accurate as using a gauge.
Similar, The flow rate of a ULKA pump is about 600-650 cc/min with no back pressure and at 9 bar, the flow is about 250-260cc/min. If you remove the water reservoir and put a glass of water for intake and an empty glass for output, then you can measure the output. Use a blind basket to completely obsrtuct the group and run the opv/return tube into the empty outflow glass. Time from first drip out the tube for 15 seconds and measure. About 60-65 ml is good for 15 seconds. This is an approximation, but is an easy way to check your blind adjustment.
Flow vs pressure for various ULKA pumps. The E5 series is commonly used.
According to the flow test, I had the OPV too low. But after winding it back up a little to get the flow rate right, the taste test was a dismal failure - very bitter coffee. So I have reduced the setting back again and have much nicer end product.
Look for how to #7 Brew Temperature. The post is a few years so digital thermometer was not shown, but is necessary. Cut the cup short and put the thermometer through the side. When you get ready to use, heat the cup and particularly the thermometer with boiling/near boiling water. The reason for that will become fairly obvious when you use it. The temperature of the brew water in the cup drops fairly quickly as you try to read it so preheating lets the thermometer read more quickly. Styrofoam is best for the cup because of the low thermal mass, paper can do in a pinch.
BB&B and you can use 20% off so cheap. Buy 2, so that you can take one apart. Many brands have the thermistor or wiring glued in so get the Polder.
For the second Polder, and surfing, this post. You take the thermometer apart. You may need to heat the steel probe to soften the plastic where it holds the probe. You can find a place to put the wire through the front or top and get the thermistor the boiler. Sometimes you can wedge it in a spot by an existing fasterer, or try high temperature foil tape, glue ...
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