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ChemTrak7 SPC Training. Control Chart Details Distribution Chart Details Step 1 : Stay Inside the Control Limits Impact of tests that are outside of limits Step 2 : Adjust the Limits with Experience Living with the manufacturer’s TDS Squeezing some more margin

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chemtrak7 spc training
ChemTrak7 SPC Training
  • Control Chart Details
  • Distribution Chart Details
  • Step 1 : Stay Inside the Control Limits
    • Impact of tests that are outside of limits
  • Step 2 : Adjust the Limits with Experience
    • Living with the manufacturer’s TDS
    • Squeezing some more margin
  • Step 3 : Show Some Improvement
    • Scrolling back to show the old days
control chart details
Control Chart Details

Test below LCL and post-add value at UCL.

Vertical bars show range of daily test results.

USL (Upper Specification Limit) (0.85)from manufacturer’s TDS

One test on this day.

Range of test results this day.

UCL (0.81) and LCL (0.75) must be set by you to keep test results far away from USL and LSL so Cpk > 1.33.

Starting date for Control and Distribution Charts.

Ending date for Control and Distribution Charts.

LSL (Lower Specification Limit) (0.65)from manufacturer’s TDS

Each vertical line is 1 day. Chart covers ~ 4 months. Use the scroll bar on the Test Log to move dates.

distribution chart details
Distribution Chart Details

USL, UCL, LCL and LSL on vertical axis

Maximum value on the horizontal axis is set by the maximum count (131 in this example). Other bar lengths are relative to the maximum.

5 readings just above the UCL

131 readings just below the UCL

27 readings below the LCL

µ (average value of all readings) is 0.788

Cpk = 0.82 for the time period from 11/14/11 to 3/5/12

Cpk should be at least 1.33

If 3σ tail is too near the USL, “Check UCL” will show.

263 samples during time period from 11/14/11 to 3/5/12

If 3σ tail is too near the LSL, “Check LCL” will show.

step 1 stay inside the control limits
Step 1 : Stay Inside the Control Limits

Too many results below the LCL

This is the worst result in the past 4-month period.

Note: Cpk = 0.82

step 1 stay inside the control limits1
Step 1 : Stay Inside the Control Limits

Too many results below the LCL

Without this one test result ...

… Cpkimproved to 0.84

step 1 stay inside the control limits2
Step 1 : Stay Inside the Control Limits

Now there are no results below LCL

With all of the low test results removed …

… Cpkimproved to 0.91

… 0.91 < 1.33 so process still not under control.

step 2 adjust the limits with experience
Step 2 : Adjust the Limits with Experience
  • Current experience shows σ = 0.025 and µ = 0.788 oz/gal
  • TDS Sets USL at 0.85 oz/gal and 0.8 oz/gal is optimum.
  • For Cpk = 1.33, if µ = 0.80 then σ = (0.85 – 0.80) / 4 = 0.0125
  • For Cpk = 1.33, if µ = 0.79 then σ = (0.85 – 0.79) / 4 = 0.015
  • So, we must decrease the UCL, but not below 0.8 oz/gal, and we must increase the LCL to reduce σ from 0.025 to ~0.015.
step 2 adjust the limits with experience1
Step 2 : Adjust the Limits with Experience

Lower the UCL to 0.8 and raise the LCL to 0.76 oz/gal.

After 4 months, the Cpk is improved from 0.82 to 1.38 -even with 7 readings < LCL.

This is a huge improvement, the tank control looks good, and yield has increased.

The cost of this improvement is more frequent testing and smaller, more frequent adds.

step 3 show some improvement
Step 3 : Show Some Improvement

To show your continuous improvement, scroll down to an earlier date.

Note where the limits changed,

the wider spread,

and the old Cpk.

step 3 show some improvement1
Step 3 : Show Some Improvement

Then scroll back to the top ...

… you can show

a tighter spread

and a better Cpk.

step 3 show some improvement2
Step 3 : Show Some Improvement

+ opportunity to improve for the next audit.