help with pooled estimate standard deviation

Hi everybody
I'm not an expert in statistics and I hope that someone could help me
out; the problem is about the "pooled estimate standard deviation".
The question arises from three sets of weighing of gas cylinders:


Observation Cylinder Balance reading (g) Weighing difference DELTAm

1 reference 0.010

mixture 0.022 -0.09
mixture 0.018

reference 0.012

2 reference 0.016

mixture 0.027
-0.011
mixture 0.024

reference 0.013

2 reference 0.012

mixture 0.025 -0.013
mixture 0.023

reference 0.010


Results:
Mean (DELTAm) -0.011 g
Pooled estimate standard deviation sp (DELTAm) 4 mg
Standard uncertainty u (DELTAm) 2.3 mg

What I don't understand is the calculation of the pooled estimate
standard deviation, this is an example found in ISO6142 for the
preparation of gas mixtures through gravimetry; I need to make these
calculation for the gas mixtures that I prepare in my lab.
If someone could help me out I'd really appreciate, here or emailing
me at thepogues74ATyahooDOTit
Thanks in advance

jj

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