How can i depict to a chart measurements like: per 15 sec for 2 min

This paper essentially plots what I've plotted above (but with more data points).  Where do your DO values come from?  The dissolved oxygen values in the paper are plotted against temperature; the CO₂ values against time. I can't find your DO values in the paper anywhere.

Ah! I think CO₂ is actually C_O2, for concentration of oxygen.  

Since your values of DO are different from those in the paper, does that mean you have measured them yourself?  If so, is your problem that your times are different from those in the paper or what?  I'm still puzzled as to your specific difficulty.  The data in the paper (that in the last table) is easy to plot in the same way that I've done in the graph in my earlier answer.

I don't know what you mean but there are 8 lots of 15 seconds in 2 minutes.

ok thank you but i'll give some more information i hope it helps you

i have an exercise to which i have to measure dissolved oxygen per 15sec for 2min, per 30sec for 4min and per 1min (the exercise is called cod if you know it).

then i have to depict these measurements to a table e.g

 but i don't know how to depict the measurements of time on the table.... 

That's what i meant

I assume cod is chemical oxygen demand.

You don't say what you mean by "per 15 seconds" etc.  Do you mean you take a measurement of dissolved oxygen, in mg/L, every 15 seconds and then average the values you collected during the two minute period? Then, for the next four minutes measure the oxygen every 30 seconds and average these values, etc?  If so, then I think I would plot the values against the middle of each time period.  That is, plot the first value at 1 minute, the second value at 4 minutes (the middle of the period from 2 minutes to (2+4) minutes) etc.

However, this might be a completely incorrect interpretation, so you need to provide more details of exactly what you are doing.

Thanks a lot for answering! If i give you this table could you maybe explain to me how are the measurements of time depicted like this ( i found it as an example for cod on the internet but i can't get it)

See graph, but can't be sure from this!  Can you supply the URL that points to the place on the internet where you found this data?

 It's the etc. that's puzzling!  Other than that you have only three values to plot:

The oxygen concentration (in mg/L) is plotted against time (in seconds) here (the third column is just the total time in seconds).

Thank you very much for helping me! That's the link but it's in greek. sorry...

http://www.metal.ntua.gr/uploads/4410/askhsh3.pdf

χρονος=time

διαλυμενο Ο2=DO

Actually i think that my times are the same with those in the paper because it's the same type of exercise that has specific time calibration, the problem is i don't get the way of the time calibration. Is the graph based on the last table i sent you??

Whatever.... Thanks a lot for your help!!

Your times are the same, but the DO values aren't.  I've just plotted your three values against the first three times.

How did you determine your three values?

I determine the values of DO with portable dissolved oxygen meter at school but i didn't write the time of every value of DO    The only data i have for time calibration  like i said in the beginning is to measure 8 values of DO per 15 sec for 2min 

Then it seems like you should plot the first of the eight values at 15 seconds, the second at 30 seconds, the third at 45 seconds and so on.

 However, you should really have recorded the time at which you took each measurement.  There is likely to be a lot of unnecessary scatter otherwise!

when you say to plot what exactly do you mean?? Can you give me an example??