Addressing Inconsistency

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Shot times above and below the shot time target.

This can look like either (reference variance chart):

  1. Shot times that are loosely grouped and fall above and below the target time band with no correlation to time (low shot to shot consistency)
  2. Shot times are tightly grouped but drift in and out of the shot time target band over time (high shot to shot consistency but low accuracy)

Before you can solve the second issue you must solve the first.

Low shot to shot consistency

Low shot to shot consistency can be caused by a number of issues, including: 

  • Variation in coffee dry dose (the amount of coffee in the basket).  If you have an electronic dosing grinder check that it is delivering a consistent dry dose into your portafilter (be aware of any spillage, this will throw your dry dose out).  If you have a manual dosing grinder check your levelling off process is delivering a consistent dry dose every time
  • Uneven distribution of coffee in the basket.  Check distribution technique/distribution of coffee being delivered to the basket by the grinder.  You may need to reposition the portafilter below the grinder outlet to deliver coffee more uniformly to the basket.  The coffee bed should ideally be flat before tamping. Whatever you do, do it the same every time
  • Crooked tamping.  Ensure coffee is being tamped straight up and down
  • Equipment.  Temperature/pressure variations, leaky grouphead seals and incorrectly programmed volumetrics will throw shot times out

Low shot time accuracy

If shot to shot consistency is high but shot times are drifting out of the target band you need to improve shot time accuracy.  Issues that can affect shot time accuracy are:

  • The grinder not being adjusted often enough.  Adjust the grinder as soon as shot times trend away from the target time
  • The grinder being adjusted in too large an increments.  Make smaller grinder adjustments more frequently to get a feel for how adjustments are affecting shot times.  Keep adjusting towards the target shot time
  • Bean variation.  Changes in bean condition will impact on shot times.  Make sure your stock rotation is enforcing beans are used from the oldest roast date to the newest.  Mixed up stock will wreak havoc on consistency
  • Beans too fresh/too old.  Very fresh coffee can cause instabilities in shot times due to large amounts of CO2 being released during the extraction.  Conversely old coffee can require such a fine grind to achieve the shot time target that it’s very easy to jam the extraction completely leaving little margin for error.  Keep an eye on stock dates and adjust ordering to ensure your using beans of an appropriate age

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