John Cuthber

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John Cuthber last won the day on September 9

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About John Cuthber

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    Chemistry Expert
  • Birthday 11/10/1965

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    England
  1. Which is more accurate when calculuating current/voltage/resistance?

    Nobody said you did. However the phrase "1 part per million" is a sensitivity, not a precision. I'm still not sure you understand the difference, or you wouldn't have said that, And you are also wrong in assuming I don't understand the effects of loading by the meter. If you do electrical measurements badly, you get the wrong answer. The same is true of analysis. So what? In the circuit you drew, when the multimeter is connected the voltage really does fall to about half a volt. The meter records that voltage accurately. If a cheap meter was all you had, it would be better to use a potentiometer to measure the voltage- in principle, that has zero loading on the circuit. No. Are you refusing to understand a fairly simple idea?
  2. Which is more accurate when calculuating current/voltage/resistance?

    You say things like this which is simply wrong, because an off the peg cheap multimeter will generally give better precision ( nominally a part in 2000) than most analytical chemistry (Typically a few parts in 100). and which is true, but irrelevant because it muddles sensitivity with precision. So, it's not a matter of my understanding what you say. I wonder why you say things that are wrong or irrelevant. I certainly don't feel I'm doing any harm by pointing out what you are doing. My point is that you can't (usually) model, calculate or simulate what will happen as accurately as you can measure it. So the answer to the OP's question (the one they posted, rather than the one you imagined) is that measurement is more accurate. I think this sums it up pretty well
  3. Which is more accurate when calculuating current/voltage/resistance?

    Get what? I still get that being able to measure something at 1% is not the same as measuring it to 1%. Well, actually, they will. If, I breathe on it I will add water, but not chromium. That will reduce the chromium content. The change will be tiny- a few milligrams of moisture on a few tens of grams of metal will change the % chromium by a few parts in ten thousand. That's a much smaller change than I can measure the % chromium to in the first place. But I can measure the water content of the steel to that precision- I just weigh it, dry it, and weigh it again. Better yet, I can heat it in a stream of dry air and pass that air over a sorbent which traps the moisture (given up by the metal). And, since the mass of the trap can be small, I can measure the change in moisture quite easily- even if the original metal block is a kilo. I can measure moisture on a kilo of metal to a few parts per billion. But I still can't measure it to much better than 3 digits. Do you understand that difference?
  4. Which is more accurate when calculuating current/voltage/resistance?

    As far as I can tell, I have to congratulate you on either your mind reading, or your time travel, since the OP has yet to explain what those needs are. However, to kick the issue back to the topic. There wouldn't have been a debate if you hadn't muddled up sensitivity with precision... Yes; we do, but it's got nothing to do with the topic. However, once you introduced it I felt it was sensible to point out why you were mistaken in thinking that "Chemists often work in parts million" in some way invalidated my point. Fundamentally, somewhere along the line, someone has to do the measurement. You can often rely on the manufacturer to do that well enough. If you don't like the E24 series, you can pay more, and get better precision.
  5. Presidential Alert system

    If one person in 10 switches the TV on "to see what's happening" that will be a big power spike. If it knocks the power grid out (even locally) and that causes something important to fail (traffic lights would be an obvious possibility) then Trump's vanity project will kill people. Are there any Trump supporters here who would like to comment on why they think he' doing a good job?
  6. Presidential Alert system

    If hundreds of millions of people reply saying "Where is the 'unsubscribe' button?", even he might get the message.
  7. Presidential Alert system

    I'd like to think that even Trump would realise that spamming the whole nation would be counter-productive. I'd like to think that...
  8. Which is more accurate when calculuating current/voltage/resistance?

    Yes, I can measure the uranium in steel- it's probably something like a part per million. But I can't measure it to better than a few percent. It is important to distinguish how small an amount of stuff you can measure - which may be in parts in 10^12 or 10^15, and how small the error margin is on that measurement which is likely to be "within a factor of 2" at that kind of level. I know there's somewhere between 0.95 parts per million and 1.05 parts per million of uranium in my steel. And I might check and find there's somewhere between 1 and 4 parts per billion of plutonium in it, but I can't measure the plutonium any better than that. Imagine that God makes some stainless steel. It is exactly 12% Chromium all the way through. Say I take a gram of it. I can weigh that gram to a part in a million on a good analytical balance- as long as either the balance is in a vacuum chamber, or I know the density of the air to a sufficient accuracy (something like 1 in 1000) that I can correct for buoyancy. Otherwise I only get the mass correct to about 1 part in 10,000 (Measuring steel is fortunate in this regard- weighing water or salt would have a much bigger error) I take some, weigh it, and dissolve it in boiling mixed nitric and hydrochloric acid. I'm almost bound to lose some as spray from the bubbles formed. I take the solution, let it cool and make it up to 1 litre in a volumetric flask. OK, I now have 1 gram dissolved in 1 litre- but the flask isn't perfect. The tolerance of a "grade A" 1 litre flask is 0.4 ml. That's a 0.04% error margin. And, if I'm good I should be able to transfer nearly all the solution to the flask- but inevitably some will be left in the beaker- even though I rinse it loots of times. I can probably transfer better than 99.99% of it, so that's another 0.01% error to add. And then I take a suitable volume of it and , for example, titrate it against some reagent. Well a typical burette holds 50 ml and is calibrated in 0.1ml, With luck I can estimate to 0.05ml. So that's a 0.1% error margin. So the overall error is something like 0.15% Repeating the measurement a few times might improve that slightly perhaps as far as getting the imprecision down to 0.05% or 1 part in 2000; that's the same precision as a high street store multimeter, and it depends on me being very good at my job (as well as having time to do repeat analyses) I would get a measured concentration of Chromium that was in the range 11.994 -12.006% That's about as good as I could hope to get. Realistically, I'd be lucky to get error margins ten times worse. The very best measurements I ever made were of formaldehyde concentration in water. You can get results on that which are repeatable to about 1 part in 10,000. (For those who are wondering, you add a known mass of formaldehyde solution to an excess of a solution of silver oxide in tertiarybutyl amine- they react to produce silver which you filer off and weigh.. Because the calculation is based on the ratio of two masses you don't need to worry about volumetric errors). My meter will give me 10 times better precision than that. And the analytical result may have been wrong.
  9. Which is more accurate when calculuating current/voltage/resistance?

    My point was that you can calculate it- for example if you know the voltage and the power. But you need to measure those (or buy things where someone already made the measurement. Regardless of precision needed, you don't get anywhere without measurement. Not often. That 61/2 digit meter will measure voltage much more precisely than I can measure, for example, chromium in steel.
  10. natural phenomena should be not named after people

    Is your torque wrench calibrated in Joules?
  11. What does dark beer taste like?

    I'm not sure Kosher makes that much sense. Ask anyone who likes cheeseburgers or lasagna or.. well lots of stuff. The usual proscription of milk and meat is that it's forbidden by the Book. Thou shalt not seethe a kid in its mother's milk. Who won? And before it picks up the flavours from the barrels (etc) it tastes awful.
  12. Which is more accurate when calculuating current/voltage/resistance?

    It depends. How can you calculate, for example, the resistance in a circuit? However, I'm willing to bet that, in most cases, my 61/2 digit multimeter will be better than your calculation or simulation. Even the cheap ones you buy on the high street are pretty good. For example they are good enough to tell you that the mains voltage probably isn't exactly 110, 220, 230 or 240 volts (or whatever other value you were expecting).
  13. What does dark beer taste like?

    I presume you have lumped the Calvados in with the brandy.
  14. How can it be that we can still see the CMBR?

    "How can it be that we can still see the CMBR?" Where could it hide?