How to Set USL and LSL in Process

Six Sigma – iSixSigma Forums General Forums Tools & Templates How to Set USL and LSL in Process

This topic contains 4 replies, has 3 voices, and was last updated by  Strayer 9 months, 2 weeks ago.

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  • #55948

    Hi All,

    I need your help,

    My company just fabricated clog test machine to detect clog in the tubing. Basically this machine has digital flow rate (in L/min) to determine pass/fail criteria. The output of air pressure is controlled by air pressure gauge that can be set prior to use.

    Here are some my observation of this machine
    1. flow rate that pass through the tubing can change as the air pressure change.
    2. the air pressure is not at constant pressure though we already installed the air tank to stabilize the pressure. This makes inconsistent reading if the air pressure up/down.

    Based on above conditions, How do I set the USL and LSL as pass/fail criteria based on flow rate?. How do I determine the air pressure as reference to determine pass/fail criteria?. Do I need to calibrate the digital flow rate since we use its measurement to determine pass/fail criteria?

    Thanks in advance


    Chris Seider

    ummm, record the data numerically and not as pass/fail and shouldn’t a customer be asked for specs?



    Per @cseider. USL and LSL come from customer specifications. Sometimes the “customer” is the next step in the process (internal specification that the end customer never sees). But it sounds like your difficulty is measurement variation and/or machine performance variation. I’d suggest that you do Gage R&R so that you know how much of the variation you’re seeing is due to your measurement instruments and then look at ways to reduce machine variation. Do not reset specification limits for pass/fail. What you need to do is fix whatever is causing variation outside these limits.


    Aufa Fadhli Pratomo

    Hi @cseider and @Straydog,

    Thanks for your response,

    Yes, actually this is about internal specification that I need to set up. Costumer just want unclog tubing. But in the production process, I still need parameter (in this case usl and lsl for flow rate) to determine pass/fail. As you say, I have difficulty on the measurement variation, machine performance (variation) or maybe variation on material dimension (e.g Inner Diameter of tube). I have been thinking about performing gauge R & R. But all the variation make me confuse.

    I have tried to make fix setting on pressure gauge. But the pressure is inconsistent that may affect to the reading of flow rate. Plus, the variation that come from measurement, machine or material dimension make difference on flow rate for each tubing.

    Could you give me some advice what tools do I need to reduce the variation or methodology to set this thing right?

    Thank you



    If I understand your situation correctly, you know what problems are occurring, with both the machine and your measurement system, but you don’t really know why. I’d suggest that you do RCA (root cause analysis) on each identified cause of variation, beginning with 5-whys and fishbone. You won’t fix this by dealing with the symptoms. You need to drill down to find the sources of the problems.

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