Error calculations etc..

[xsandralee]

If you measure the pH using a pH measurer (machine) - not pH paper, etc.

what's the ± ___? percent error? on it...

i hate error

[babakren]

errors are usually obtained from various trials..

r u sure what u meant are the errors and not uncertainties?

if it is the uncertainty percentage you are trying to find then you could just look at the digits and the precision of the digital pH meter.

eg. if it gives up to .01 units then the uncertainty is ±0.01 and so the percentage uncertainty is 0.01/value x 100%.

But if it's the error you could just find the maximum deviation from the mean of the trials and then do the same thing like the uncertainty to find the percentage.

[moneyfaery]

For electronic equipment, the uncertainty is usually the last digit of the measurement. Ex: if you're using a mass balance accurate to 0.01g, the uncertainty would be +/- 0.01g. You can see this since the last digit often flickers when you're taking the measurement.

The pH meters I've used were accurate to the hundredths place so it would be +/- 0.01.

[djshah]

Like stated above the uncertainty always depend on the way you are measuring certain things. For example if i was using a centi-metre ruler and finding lengths in teh experiment, I find the lowest value marked in the ruler which is a milimeter. Because even though it is a cm ruler there are mm marks. So for example if my readings are 4.5, 6.7 and 8.0 the uncertainty is "+/- 0.1cm".

BUT be careful when putting in the raw data. It has to be rounded to one decimal. You can never have data that goes 3.86cm, 3.4cm and 3.75cm from a centimere ruler with the smallest unit mm. You can round some of them off to give same decimal points so you would have 3.9, 3.5 and 3.8 and therefore your uncertaint would be +/- 0.1cm.

Most Ph probes(machines) have measurments taken to hundredths like moneyfaery said above. So all your data should have two decimal places example 7.66, 8.52, 9.13. And your uncertainty should be +/- 0.01.

So along with errors be careful how you put in all your data. Never put only 8, but 8.0 or 8.00 depending on how your data was accumulated.

Edited August 30, 2008 by djshah

[pretty_ah_boi]

My teacher mentioned something like , if u were using a digital measuring device, then your uncertainty should be the smallest division of it, divided by two. Or was that for analog measurements? anyone know anything about it?

[babakren]

for digital, its always to its decimal place. For example a pH meter like one mentioned by djshah measures up to the second decimal place so your uncertainty will me ±0.01

for analog, a one meter stick has its measurement up to 1 milimeter, so you divide the 1 milimeter by 2 and thats the uncertainty so in this case it's ±0.5mm

[regulator7]

You are right... even when using digital you are still supposed to divide the smallest by 2. e.g: 0.01 uncertainty would be +/- 0.005

[IBStuck]

sometimes the box the device came in will tell you the uncertainty on it too. my chem teacher alway told me just to be consitent.

[IBdoc]

We were thought that the uncertainty is 1/10 of the smallest division. For example if the smallest division in a burette is 0.1 mL then the uncertainty is 0.1 x 1/10 which is 0.01! Is that wrong? Our teacher told us that there are different ways of finding the uncertainty but it was agreed upon chemists that one tenth of the smallest division is the way to go.

Edited September 11, 2008 by IBdoc

[xsandralee]

Oh, then I think i understand uncertainty, but the real problem is 'error propagation'.

First of all, I don't even know what it is, but my teacher continues to use this term, and whenever I ask him this question, he replies saying, "yeah, you can either use standard deviation or..."

and that's it..

For example, if the lab I'm working on requires consistent amount of data recording, let's say pressure recorded every time the temperature increases by one degree for 15 minutes, that's alot of data, right?

Then, there are uncertainties for 1) pressure reading 2) temperature reading.

Then, if I calculated P/T, then how does the uncertainty work for that?

Also, if I have about 30 values for P/T (becuase I have 30 values for both P and T, each), then what calculations can I use to determine the accuracy of this value? (i dont think its percent error)

[IBdoc]

Error propagation or uncertainty propagation is the fact that the error gets bigger or smaller depending on the operations you do.

When two measurments are added or substracted, then the uncertainty in the sum or difference is equal to the SUM of respective uncertainties. Or when two measurments are multiplied or divided, their relative uncertainties are added. And when measurments are multiplied by a constant, the uncertainty is also multiplied by the constant. And so on...

I hope you got what you want. But can anyone please answer the question I asked in my last post?

[IBdoc]

Please I need an answer desperately since my teacher is not very reliable. She told us that the uncertainty is 1/10 of the smallest division. can this be true?

[xsandralee]

hmm, don't udnerstand what she means by '1/10' of the smallest division'

but i think for UNCERTAINTY, you just do, i.e. 45.6 g +/- 0.1 you kno?

but if you're doing error propagation, thats a totally different story.

[Max]

Please I need an answer desperately since my teacher is not very reliable. She told us that the uncertainty is 1/10 of the smallest division. can this be true?

We were told that it's 1/2 of the smallest increment. For example, you have a mass balance that gives the reading up to two decimal places (e.g. 24.36g) - the smallest increment is 0.01g. As such, the uncertainty is 0.01/2 = +/- 0.005g.

Edited September 29, 2008 by Max