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# Compress & External Pressure Charts/Tables

## Compress & External Pressure Charts/Tables

(OP)
I am having trouble correlating a value for 'A' given by Compress and one looked up (and even interpolated) manually when calculating a shell and ellipsoidal head for external pressure.

I was wondering if anyone knows how Compress finds this value?  I would assume that the method used for finding a value from Figure or Table G would be the same method for finding value B from the appropriate chart.

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### RE: Compress & External Pressure Charts/Tables

BigTank,

I believe Compress uses double logarithmic interpolation of Table G.

I have developed a spreadsheet for doing logarithmic interpolation. Tell me the values of L/Do and Do/t, and I will give you the value for 'A'.  See if it matches the one calculated by Compress.

### RE: Compress & External Pressure Charts/Tables

(OP)
Thanks, doct!

L/Do = 1.7037
Do/t = 247.8448

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Fitter, happier, more productive

### RE: Compress & External Pressure Charts/Tables

I get A = 0.000195

### RE: Compress & External Pressure Charts/Tables

(OP)
That is very close to what Compress gives in the calcs.

Interesting...UG-28(c)(1) Step 3 states: '...interpolation may be made for intermediate values of Do/t...'

I used linear interpolation using the tables...probably not the best method.  Looks like I'll be developing your spreadsheet for myself, doc.

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### RE: Compress & External Pressure Charts/Tables

(OP)
Thanks again, doc!

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Fitter, happier, more productive

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