Tare on analytical balance

[chhubert]

Greeting all,

Wondering if there is any difference between weigh-by-difference and tare before weighing when weighing a substance on the analytical balance. I was told that weigh-by-difference gives more accurate measurement.
Hubert

[sam.pedraglio]

Weigh by difference is accurate if you don't reach the weigh limit of the balance.
If you tare your balance you can work along the entire range of the instrument.

[lmh]

depends on the balance. Ask the manufacturer.

[Peter Apps]

Hi Hubert

If you weigh by difference (weigh container, remove from pan, add material, re-weigh, subtract) the zero appears twice in the uncertainty budget. If you tare the container the zero only appears once, which is better as long as other sources of uncertainty are independent of where in the balamce's range you are weighing (which they will be for good quality balances).

In any case, unless you are trying to weigh more precisely than the balance allows (mg on a three-figure for e.g. !) the relative uncertainty from weighing will be in the parts per million, while for the rest of the method it is likely to be orders of magnitude higher, so you can safely use whichever method is the easiest, which in most cases is to tare the container.

Peter

[chhubert]

Thanks, Peter. And how are you doing? I'm glad that you no longer need to do "very very precise" work that sometimes makes me crazy.

Hubert

[HW Mueller]

There are scales out there which don´t just correct the display when you push the tare button??

[Peter Apps]

Hi Hubert

I'm fine thanks, are you still in metrology ?. I'm analysing African wild dogs scent marks to identify the active constituents so that we can make artificial territorial boundaries to stop the dogs wandering out of conservation areas and getting killed by cattle farmers. As an echo from my metrology days I recently had a paper on bias when weighing volatile solvents accepted by Accreditation and Quality Assurance.

Hello Hans

If I understand correctly analytical and micro-balances work by measuring how much electro-magnetically generated force it takes to zero the position of the weighing pan. Each time you tare, the force is adjusted to bring the pan position back to zero - and of course at the same time the display reads zero. The repeatability and reproducibility of weighing is at least three orders of magnitude better than anything else that goes on in a chemistry lab.

Peter

[DR]

There's also the manipulation (transfer) of material to consider.

If you are likely to have any residue sticking to the container, weighing by difference may be more accurate (assuming you spill nothing while you do your transfer).

If you're doing a quantitative transfer, taring before weighing would be much easier and a better way to approach it.

[AICMM]

Peter,

Something occurred to me from your recent dog scent post. The US Fish and Wildlife service just re-introduced the Lobo in my area and your work would probably prove interesting to them as a means of minimizing livestock contact. Something you are already doing with someone in the US?

Best regards.

P.S. could not agree with you more about the power of the nose as a detector, although calibration is seemingly a difficult issue....

[Peter Apps]

Hi AICMM

In principal the scent-mark BioBoundary would work with wolves, since they clearly demarcate their territories with urine marks. As far as I know nobody is looking at it at present. The main technical problem is the mind boggling complexity of mammal scent marks and the almost incredible LOD of a dog's nose - 10 to the minus 18 molar for some substances, try matching that with a GC-MS , then there are all sorts of strategic issues about what is the most effective way of finding active signalling compounds among all the others in the scent mark, getting analytical chemists and field zoologists to talk to one another, etc.

Peter