Wikipedia:Reference desk/Archives/Science/2021 August 30

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August 30

Easily obtained references for temperature other than ice water and boiling water?

Are there any other ways to obtain accurate known temperatures (for calibrating a thermometer between 0 and 125 °C) other than boiling water and ice water? Thanks. 02:41, 30 August 2021 (UTC)

I am not sure of other ways, but I do know that the temperature of boiling water is not necessarily 100 °C except at sea level and at normal atmospheric pressure. For example, water boils at about 94.4 °C in Denver, Colorado and about 70 °C at the summit of Mount Everest. The boiling point also varies a bit if atmospheric pressure is higher or lower. So using boiling water for calibration purposes requires accurate elevation data, an accurate barometer and some accurate mathematical calculations. Cullen³²⁸ Let's discuss it 06:08, 30 August 2021 (UTC)

This may sound like cheating, but in both industrial and scientific contexts, the usual procedure for calibrating thermometers is by comparison with high-precision accurate thermometers, such as quartz thermometers. These need to be calibrated themselves, of course; ultimately, the standard is now grounded in combining the Boltzmann constant with measurements of the kinetic energy of a gas, which requires a sophisticated laboratory set-up. In theory, the freezing or boiling points of other substances than H₂O could be used as reference points, but such substances are not easily available in pure or standardized form, or are hazardous, or their phase-transition temperatures are outside the range of interest.  --Lambiam 07:45, 30 August 2021 (UTC)

See also Triple_point#Triple-point_cells. The triple points are much more precisely defined than melting/boiling points. —Kusma (talk) 08:57, 31 August 2021 (UTC)

• Depending of what "easily obtained" means... I worked with a differential scanning calorimetry device whose standard operating procedure called for periodic calibration using small pellets of pure (>99%) indium and gallium. I did not perform the calibration myself but they explained to me that the main was to check that the heat flux sensor does not drift (because you know the enthalpy of fusion of pure metals within a ridiculous value). Looking around the manufacturer’s site it is also used to check the temperature measurement (because you know the melting point within an even more ridiculous value).

I believe for that device the temperature was validated but not calibrated (you just check if the measurement is right and if not you call the maintenance person, you do not adjust a knob to make the measurement fall on the expected value). However looking around I found "How to correctly calibrate a Differential Scanning Calorimeter" on the Linseis website (blacklisted so I cannot link) which implies their DSC devices do have an adjustable knob for temperature.

For OP’s purposes, assuming they are ok with melting heavy metals in their equipment, indium (around 160°C) is a bit too high for the asked temperature range you reference, but gallium (around 30°C) is a fine lower point I think. Tigraan^{Click here for my talk page ("private" contact)} 15:32, 31 August 2021 (UTC)

Melting point for many (but not all) substances tends to be a fairly sharp and easy-to-determine thing under consistent conditions. A melting-point apparatus is a relatively simple device that many chemistry students will be familiar with. --Jayron32 16:01, 31 August 2021 (UTC)

I've done dozens of melting point determinations, usually of new compounds I had just made but I can't think of a readily-available household material with a reliable melting point to act as a standard for the upper end of the 0 – 125 °C range. Access to ice water makes the standard for 0 °C readily available. Benzoic acid is cheap and readily available from chemical suppliers, with M.P. = 122 °C. It's not really "household", however. Does anyone have a better suggestion? Mike Turnbull (talk) 11:51, 1 September 2021 (UTC)

Or maybe use the smoke point of some appropriate oil? (The article does mention that the value can vary with storage, so it will be less precise than a chemistry method based on a pure compound.) Tigraan^{Click here for my talk page ("private" contact)} 15:23, 2 September 2021 (UTC)

How can we balance a ring like a rocks if it's center of gravity present in air?

if center of gravity of any object present in air, doesn't that make object unbalance forever? Rizosome (talk) 05:21, 30 August 2021 (UTC)

The sum of force vectors is the relevant thing, in mechanical equilibrium (balance). If a ring such as doughnut rests on a table, every point of the ring is acted on by a force due to gravity, which is balanced by a normal force due to the surface of the table. The sum of the force vectors is zero, so the doughnut doesn't topple over or go anywhere. However, if the doughnut in the same orientation rests on the tip of your finger, the hole at the center of gravity means it can't be balanced (or is not static).  Card Zero  (talk) 06:46, 30 August 2021 (UTC)

(ec) The question is not entirely clear. Whether an object is balanced or not is not an intrinsic property of the object, but depends on its orientation in a stationary reference frame with respect to the forces acting on it. As a first approximation, for a ring not having momentum touching a horizontal plane beneath it while the only acting force is a uniform vertical field of gravity, there is an equal but opposite force of reaction (see Normal force). If there is only one point of contact between the ring and the plane, and the upward vector from the point of contact passes through the centre of gravity of the ring, the ring is balanced.  --Lambiam 07:18, 30 August 2021 (UTC)

Think about say a funnel, its cg is obviously in the air, yet it obviously has at least one stable orientation on a flat plane (wide end down), and one metastable (lying on its side). Greglocock (talk) 01:36, 31 August 2021 (UTC)

Or consider a tennis ball. Rizo occasionally just skipping thinking. 2003:F5:6F0A:4700:3DAA:6206:2192:B77C (talk) 18:15, 2 September 2021 (UTC) Marco PB

Actually, I'd swap those two. The lying on its side is the stable orientation (though it rolls) given that that orientation places the center of mass at the lowest point; it has the lowest potential energy of any orientation, and as such, by definition it is the most stable. The "resting on the wide end" is metastable because it has a higher COM, and so has a higher potential energy. It is metastable because it requires significant additional energy to knock it over (metastable systems are at a local potential energy minimum, but not absolute potential energy minimum), but presuming a flat surface, the one on its side has lower gravitational potential energy. --Jayron32 17:43, 31 August 2021 (UTC)

But the question is about balancing, not about stable orientations. In colloquial language, having an object be in a stable orientation (in the sense that minor perturbations do not decrease its potential energy) is not considered a balancing act.  --Lambiam 21:14, 31 August 2021 (UTC)