Mahuta ♥ Posted March 5, 2009 Report Share Posted March 5, 2009 I kind of need help on finding the uncertainties for the measuring cylinder and such..and dropping pipettes. I'm doing my biology practicals and desperatly need them.Anyone has an idea? I need them soon if anyone's willing.Thanks alot. Reply Link to post Share on other sites More sharing options...
Max Posted March 5, 2009 Report Share Posted March 5, 2009 Usually the uncertainty is stated on the apparatus. However, if that's not the case, the uncertainty is half the smallest increment. For example, if the measuring cylinder measures 50cm³ with increments of 1cm³, the uncertainty is 1/2 = +/-0.5cm³. Reply Link to post Share on other sites More sharing options...
IBdoc Posted March 7, 2009 Report Share Posted March 7, 2009 We never take the uncertainty that is stated on the apparatus. We always take one tenth of the smallest incriment. For example, if the smallest division on the cylinder is 0.1mL, then our uncertainty will be +/-0.001mL. Does it depend on the teacher? Or does IB require a specific way of finding the uncertainty of an apparatus? Reply Link to post Share on other sites More sharing options...
Vvi Posted March 7, 2009 Report Share Posted March 7, 2009 We do the same as what Max said, take half of the increment. It makes sense because when you round up measurements, you round up or down from the middle of something. And that middle is the uncertainty.So if you have a beaker that can hold 500 ml of liquid and the beaker is marked in units of 100 ml, the uncertainty would be +/- 50 ml. Which sounds ridiculous, but it's the largest error that you could make when estimating at what level the fluid is at. Reply Link to post Share on other sites More sharing options...
hanz Posted March 7, 2009 Report Share Posted March 7, 2009 the uncertainty is sometimes marked on the apparatus if not, i see if the apparatus i use is analogue i take half of its smallest digit ! and if its digital i take its smallest digit ! i think its wrong though =[ Reply Link to post Share on other sites More sharing options...
IBdoc Posted March 7, 2009 Report Share Posted March 7, 2009 No it's not wrong. When it's digital you are supposed to take the smallest digit. For example, if the mass is given as 1.43g using and electronic balance the uncertainty will be +/-0.01g. At least that's how we do it. Reply Link to post Share on other sites More sharing options...
IB_sucks Posted March 14, 2009 Report Share Posted March 14, 2009 I kind of need help on finding the uncertainties for the measuring cylinder and such..and dropping pipettes. I'm doing my biology practicals and desperatly need them.Anyone has an idea? I need them soon if anyone's willing.Thanks alot.this is the best way to find uncertainties for everything. If you are using something analouge (doesnt work on electricity) you take half the limit of the reading as the uncertainty. for example, if the smallest reading on a measuring cylinder is 1 ml, then the uncertainty will be 0.5 ml. for digital instruments, you take the limit of the reading itself as the uncertainty. for example, if the smallest reading on a digital balance is 1 gram, then the uncertainty will be +/- 1g, hope this makes everything much more simple Reply Link to post Share on other sites More sharing options...
Sandwich Posted March 14, 2009 Report Share Posted March 14, 2009 If you can't find it on the equipment, you should give uncertainty to the smallest part of data you can take. So like a 10ml measuring cylinder which has 0.1ml markers on it should be given as having an uncertainty of +/-0.1ml because that's the smallest part you can see (:To be honest, provided your uncertainties are consistent with the data you present (i.e. you can't really say that something has +/- 0.1ml uncertainty and then give your data as 3.42 ml, and vice versa you can't say something has 0.01g uncertainty and then give a measurement of 0.1g -- it'd have to be 0.10g) nobody at the IBO is going to check the instruments in your lab. They only want to see that you paid attention to potential errors in your data. Reply Link to post Share on other sites More sharing options...
Forget-me-not? Posted March 20, 2009 Report Share Posted March 20, 2009 What I've learned:digital devices - to the smallest increment i.e. a centigram balance measures by 0.01g, so the uncertainty is +/-0.01ganalog devices - half of the smallest increment that can be measured with: i.e. there are 10mL graduated cylinders at my school that have 0.2mL increments, so the uncertainty is +/-0.1mLEXCEPTION: a cm ruler's uncertainty is 0.1cm, NOT 0.05cm (according to 2009 bio syllabus)I have a question, too.If you're finding the mean value, i.e. volume, is the uncertainty the same as each individual value's uncertainty, or the sum of the uncertainties of all the values?Because I was taught 2 different ways (chem vs. bio?) Reply Link to post Share on other sites More sharing options...
Sandwich Posted March 20, 2009 Report Share Posted March 20, 2009 I can't say this is definitive, but because the errors are technically accumulating (if you were out by 0.1ml on the first volume, and out by 0.1 on the second one too, you couldn't really give a 0.1 error if you have the potential to be out by 0.2ml) I'd say that it's the sum of the uncertainties of all the values.Do you need to give errors for mean values, though? Given that it's already an approximation in that it's the mean, you couldn't really give a cumulative error, even, because the cumulative error would be balanced out by the fact that it's all being divided over the number of results you have. I was under the impression that errors are necessary only for measurements-- when it comes to things like averages, I'm not 100% sure they're necessary. Reply Link to post Share on other sites More sharing options...
moneyfaery Posted March 20, 2009 Report Share Posted March 20, 2009 I was under the impression that errors are necessary only for measurements-- when it comes to things like averages, I'm not 100% sure they're necessary.Percentage error is used here... so for this case, it's the same as using the average of the uncertainties. The reason you need to propagate uncertainty is because the error from measurements is carried on in calculations, meaning that the final calculated value for whatever isn't exact, precise or accurate. Reply Link to post Share on other sites More sharing options...
Hedron123 Posted March 30, 2009 Report Share Posted March 30, 2009 All my teachers say it's half of the smallest increment, just like Max said. The measuring cylinder has a +/- 0.5 cm3 error while the pipette has a +/- 0.1 cm3 error. Reply Link to post Share on other sites More sharing options...
Tilia Posted March 31, 2009 Report Share Posted March 31, 2009 This is out of topic, I'm sorry, but it felt silly to start a new thread for such a short question. Can I write 8.5 J K-1 mol-1 ± 1.2 J K-1 mol-1 or should it be9 J K-1 mol-1 ± 2 J K-1 mol-1?Help appreciated! Reply Link to post Share on other sites More sharing options...
ivy12003 Posted March 31, 2009 Report Share Posted March 31, 2009 This is out of topic, I'm sorry, but it felt silly to start a new thread for such a short question. Can I write 8.5 J K-1 mol-1 ± 1.2 J K-1 mol-1 or should it be9 J K-1 mol-1 ± 2 J K-1 mol-1?Help appreciated! I'm assuming that 8.5 has the correct number of significant digits (and the uncertainty is correct); so the first one is fine. Reply Link to post Share on other sites More sharing options...
Tilia Posted April 1, 2009 Report Share Posted April 1, 2009 I'm assuming that 8.5 has the correct number of significant digits (and the uncertainty is correct); so the first one is fine.Thanks, lab reports give me anxiety! Reply Link to post Share on other sites More sharing options...
IBdoc Posted April 4, 2009 Report Share Posted April 4, 2009 In a bio lab report, if we have different volumes with a different number of significant figures but the same number of decimal places, how does the number of significant figures and uncertainty change when calculating the average of the volumes? Reply Link to post Share on other sites More sharing options...
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