Wikipedia:Reference desk/Archives/Science/2017 April 26

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April 26

Intermodal container

For ocean shipping of Intermodal containers, does the shipping cost depend on the weight of the filled container or not? ECS LIVA Z (talk) 00:51, 26 April 2017 (UTC)

It appears not. A couple of sites that provide information on international container shipping do not mention weight as part of the cost [1][2], and two sites I found to get shipping rates don't include "weight" as a line to fill in, although one will have such a line if you selecting anything but a shipping container as your package [3][4]. However, I do notice that some sites give a fixed weight to every size container. I wonder if shipping companies would refuse to take something over that weight. Someguy1221 (talk) 01:33, 26 April 2017 (UTC)

Rather old OP, I'm afraid, but back in the mid-1980's I was involved in ordering periodical shipments of manufactured items from my employers' factory in South Africa to their warehouse in Hampshire, UK. The quantity of goods could vary significantly from shipment to shipment, and were always sent in a standard shipping container, whose cost of delivery was the same regardless of how full it was. {The poster formerly known as 87.81.230.185} 90.217.249.244 (talk) 14:25, 26 April 2017 (UTC)

In my experience about a decade ago, there was no weight limit on the container for the ship-bound portion of the transit. However, unless you have a deep-sea dock in your backyard, something else is going to be required to get the container to your DC. I do not know about the rail portion, but trucking companies will have upper limits to what they're legally allowed to haul and probably also limits on what they'll haul without extra charge. Matt Deres (talk) 01:04, 29 April 2017 (UTC)

Installing a new thermostat on a 240V household heater

I have an old baseboard heater in a bedroom. It has no markings or ratings on it as far as I can tell, nothing written inside the unscrewable panels on each end. All I know is it's on the 240V household power (3 live wires plus a ground wire) and a 20-amp circuit breaker. I live in the United States.

When the heater is on, my clamp meter shows 6.2 amps on each of the three live wires going into the heater. If I understand the 3-phase AC article correctly, the total current going through the heater isn't 18.6 amps (3 × 6.2), but would actually be 6.2 amps.

I just bought a new programmable thermostat (heater only, no cooling) rated for a maximum load of 14.6 amps at 240V, which is a good margin for the heater.

So I want to know, before I install this thermostat, is my assumption correct that this heater is actually drawing 6.2 amps and is a safe load for the thermostat?

It's challenging to find a programmable heater-only thermostat rated for 20 amps at a decent price ($50-ish). ~Anachronist (talk) 05:31, 26 April 2017 (UTC)

You almost certainly do not have three-phase service. If you're in the U.S., you probably have split-phase. Do not mess around with electricity if you don't know what you're doing. Call a HVAC servicer. --47.138.161.183 (talk) 06:10, 26 April 2017 (UTC)

I've done a lot of work around the house with the single phase wiring. Even helped rewire the whole kitchen during a remodel and the city building inspector approved it. Whether the 240V is 3-phase or split-phase, the question still stands regarding the current I measured versus the load rating on the thermostat. ~Anachronist (talk) 06:23, 26 April 2017 (UTC)

The fact that you're asking the question about the current indicates that you don't know what you're doing and shouldn't be installing the thermostat. Seek expert advice from a qualified electrician. Any wiring mistake that you make that results in a fire could have serious consequences for your household and contents insurance policy. Akld guy (talk) 08:07, 26 April 2017 (UTC)

Is it on a 2-pole or 3-pole circuit-breaker? DMacks (talk) 12:34, 26 April 2017 (UTC)

If you are not skilled in electrical testing and wiring, there is too high a likelihood of injury or property damage to be messing with 240 volt wiring. Installing an inappropriate thermostat would be bad, if you wound up switching a neutral, for instance. Check with an electrician. Also, what kind of electrical device has NO NAMEPLATE? If I were working on the heater, I would have checked the voltage from each of the "live wires" to ground and to each other. Then I would know if there were two wires each with 120 volts to ground and with 240 volts between them, as is commonly seen in US household wiring, and a neutral wire with only a small or no voltage to ground, and 120 volts to it from each of the other two "live wires." If the wires are wye or delta connected, or high-leg delta, other voltage patterns would be seen. Edison 16:13, 26 April 2017 (UTC)

@DMacks: normal 2-pole 20 amp breaker. It's actually a pair of 20-amp breakers with the toggles connected together, so that both will always flip at the same time. They're on a circuit dedicated to two baseboard heaters, one in each bedroom.

Two poles means it's two hots (presumably split-phase at 180°, each 120V for 240V total, as Edison says) and the third current-carrying conductor is neutral. "Three-phase" (three hots at 120°) would require three poles of circuit-breaker. DMacks (talk) 19:09, 26 April 2017 (UTC)

@Akld guy: I've installed thermostats before, we have other baseboard thermostats in the house but those thermostats are mechanical ones rated at 22 amps. Electricians check my work — and I may call one anyway in this case. The fact that I'm asking a question doesn't indicate I don't know what I'm doing, it indicates that I am reluctant to cut off the heater wires to measure the voltage (they aren't connected with screw-off caps, but the crimp kind). But I did manage to measure the voltages by shoving the probe down into the back of the caps, see below. The clamp meter was an easy non-contact way to compare the current in the heater with the current rating of the breaker and the thermostat. If I were replacing the mechanical thermostat with a new 22 amp one, there would be no need to ask questions. But I want a digital programmable one in one particular room, and those don't seem to be available higher than about 15 amps. What gave me pause is that the rating on the thermostat is less than the circuit breaker (I can replace the circuit breaker with a 15 amp one if needed).

@Edison: Right, there is NO NAMEPLATE. Maybe if I pulled it off the wall (I see very large nails fastening them at the back, so I might damage the heater pulling it off) there might be a plate on the back. These baseboard heaters here are nearly 50 years old. So I have to resort to measurements to figure out its specs.

The voltages between the wires are: white-red: 210V; white-black: 8V; red-black: 12V; ground-red: 120V, ground-black and ground-white: 1V. Those low voltages fluctuated as the heater warmed up. But seeing that gave me the idea to put the clamp meter around the red and black wires together, and it read 12.5A or 0A, depending on the relative direction each wire passed through the clamp loop. That indicates that the red and black are in phase, and the heater is likely drawing 12.5A. Is that a reasonable deduction? If so, I'm reluctant to put a 15A thermostat on this thing; it's a bit too close for comfort. ~Anachronist (talk) 19:02, 26 April 2017 (UTC)

• Your unit is a 220 VAC 6.2 Amp resistive heater. Red and black are on the opposite sides of your single-phase (i.e., split-phase) service. This is just like any other big 220VAC appliance. If you have a thermostat that can handle 220 VAC at 6.2 Amp, you can in theory use it as a switch in either the red wire or the black wire, but I think this is a really bad idea, unless you are sure that the unit is designed to work that way. If you are replacing an existing thermostat, how is it wired? Does it interrupt just red, or just black, or is it DPST and interrupts both? How is your digital thermostat powered? If it expects 120 VAC and is designed to interrupt 120VAC, it is likely to to fail in the 220VAC system even if its internal relay can handle 6.2 A. If it were me I'd replace the entire unit. If this is infeasible, I would use a DPST 20A relay and use a 24 VAC thermostat to actuate the relay. I'd also think twice before accepting advice from random people on the Internet. -Arch dude (talk) 05:12, 27 April 2017 (UTC)

  @Arch dude: Thank you. I'm not following advice, just gathering information. In this case, the existing thermostat (a mechanical 22 amp thing similar to this) has been failing to keep the room at a constant temperature anymore; the room alternates between too hot and too cold, so I want to replace it, preferably with a digital programmable one. The one I bought is a 240V thermostat (this one) rated at about 15 amps, can be used in either a 2- or 4-wire installation. Thanks for confirming my heater is 6.2 amps, but still, I found myself uncomfortable with putting a 15 amp device on a 20 amp circuit (I don't want the thermostat to be the weakest point in the circuit), so I returned it to Home Depot today. I wonder why the only 22-amp heater thermostats I can find are mechanical? ~Anachronist (talk) 06:25, 27 April 2017 (UTC)

Cargo spacecraft visiting the ISS

Why do cargo spacecraft other than the Progress (i.e. the Dragon, Cygnus, H-II Transfer Vehicle, etc.) require the use of the Canadarm instead of using automated docking like the Progress? Given that the Progress, and previously the ATV, had automated docking capabilities, why weren't these just used for the other cargo spacecraft? Narutolovehinata5 ^tccsdnew 07:37, 26 April 2017 (UTC)

International Space Station#Docking discusses the advantages and disadvantages of automatic vs. manual docking. --Jayron32 10:51, 26 April 2017 (UTC)

• This is because they use different ports. Progress and ATV use(d) the Russian SSVP docking system, the rest use the American Common Berthing Mechanism. The latter is a bigger hatch, easier for supplies, but is not suited for automated docking, due to different acceleration requirements at contact. Nitpick, when using Canadarm to place these VVs on a CMB, the term is berthing (as the name suggests), they don't dock. Fgf10 (talk) 07:54, 27 April 2017 (UTC)

Stowaway on a trip to space

The International Space Station is serviced by a variety of unmanned cargo drops. Supposing that someone could sneak onto one of those cargo flights without being noticed, would such a flight be survivable? Are the cargo modules pressurized, kept at a reasonable temperature, etc.? And would the additional weight be sufficiently within the safety margins that the rocket could reach its destination without running out of fuel (assuming no cargo was removed)? Dragons flight (talk) 14:20, 26 April 2017 (UTC)

Could a human survive? Sure, if you don't care about comfort, safety, or permanent health harm, and accepted very tiny margins of confidence. For example, Progress is not functionally very different from Soyuz, a manned Soviet spacecraft. In principle, they are the same vehicle, but engineered to different standards of safety, comfort, and confidence.

Would you be comfortable berthing in a vehicle that is essentially a fifty-year-old Soviet design, only built for carrying cargo with tolerances that are relaxed below the standards originally set for cosmonaut safety and comfort?

So if the question really is whether the journey might be livable, I think we can say yes. This obviously glosses over some of the practical difficulties. How would the stow-away crew get in? How long would they have to hide inside the module before the launch? How much kit could they bring with them - food, water - among other biological needs? Could they bring a spaceflight-safe seat or harness? Do they care about comfort and safety in even the slightest fashion? What rate of probable mortality or morbidity would be acceptable?

Nimur (talk) 16:21, 26 April 2017 (UTC)

Here is a short-course, from 2015, whose lecture notes are published on NTRS: ISS Payload Thermal Environments, taught originally by a team from NASA and Boeing to help train engineers who intend to deliver cargo payloads to ISS.

As always, if you learn to search NTRS, you'll find almost everything you need to know, in full-form.

Nimur (talk) 16:24, 26 April 2017 (UTC)

• Two obvious questions would be temperatures and also CO₂ buildup. Although the payloads (well, half of them) are pressurised with a breathable atmosphere, this doesn't include the life support needed for a payload that's also breathing. Andy Dingley (talk) 16:57, 26 April 2017 (UTC)

Another issue might be the accelleration of the rocket, which is usually kept down to an human sustainable level - not the most efficient way to put a mass into orbit with a rocket. Also these cargovessels have no heat shield. So anything goes wrong up there - you are toast. Also they would have to provide one extra seat for you to come back, in a vessel with an heat shield.

Anyway, if you think they let you sneak near one of these or even sneak something or someone in, you have probably seen to many James Bond or Jackie Chan-Movies. --Kharon (talk) 01:21, 27 April 2017 (UTC)

@Kharon: James Bond might only do that in a fictional scenario, but if Megan Rice were to try I would put money on her to win. The more sure everyone is that a place must have great security, the more vulnerable it is to people who don't believe in security. Wnt (talk) 00:25, 28 April 2017 (UTC)

Well, the Dragon capsule made by SpaceX certainly has a heat shield and full environmental systems on board. It has been used to take live animals to the ISS. The weight difference that one person would make can certainly be negated by the launch vehicle. I think it would be possible, from what I know of the Dragon capsule. The real question would be whether there was space that a human could fit in, as they are packed pretty tightly with supplies, and if the CO2 build-up was being controlled by the atmospheric system. The maximum journey time would be three or four days, so you would need to have enough water with you to survive that time. This is assuming the mission contained live animals. I don't know if they keep the environment sustained to the same levels when it is a mission that is not taking live animals to the ISS. Whatever the circumstances, I don't think it would be very comfortable, and I highly doubt that SpaceX would not notice a person on board. Zzubnik (talk) 03:26, 27 April 2017 (UTC)

video

How does this[5] work?

I thought that buoyancy only depended on 1. the object's weight and 2. the amount of fluid the object displaces. My (admittedly poor) understand is that how that weight is distributed within the object has nothing to do with the buoyancy force. ECS LIVA Z (talk) 20:43, 26 April 2017 (UTC)

Is Buoyancy#Stability helpful? In non-uniform systems, mass distribution can affect the practical implications of buoyancy even if the magnitude of buoyancy force isn't affected by such things. — Lomn 20:54, 26 April 2017 (UTC)

The center of gravity doesn't change in this case though. The hourglass stays rotationally symmetrical throughout the video so its center of gravity stays in the center. ECS LIVA Z (talk) 22:46, 26 April 2017 (UTC)

I suspect the video is fake. The hourglass seems to be staying put on the bottom after the tube is flipped over, then there's an obvious edit in the video, after which it starts to rise. They probably just subbed in a lighter hourglass in a tube and resumed filming. StuRat (talk) 21:18, 26 April 2017 (UTC)

The floating hourglass paradox is a well known physics puzzle. You can buy these things in the right kind of novelty stores, and they behave exactly like this. There would be no point to faking the video. ApLundell (talk) 14:53, 27 April 2017 (UTC)

• I would expect that there is the usual buoyancy on the hourglass within the tube, but if that's quite close fitting then viscosity through the narrow gap between glass and tube will limit the speed it can float upwards at. Andy Dingley (talk) 21:44, 26 April 2017 (UTC)

This doesn't seem impossible, but I don't know if it is genuine. The key would be to have a flexible membrane at the bottom of the hourglass. As the sand in the hourglass lands on the membrane, it may reach a point where its weight can effectively counter the pressure of the liquid and push the membrane outward (or reduce the degree to which it bulges inward). This would increase the volume of the hourglass and make it buoyant. The classic demonstration of this type is the Cartesian diver, but in that case an external source delivers the pressure. Wnt (talk) 22:11, 26 April 2017 (UTC)

I suspect this is just a one-element version of the Galileo thermometer. The obvious cut may conceal an interval while the overall apparatus (or the water) is warmed so that the float rises, and the hourglass sand inside the latter is a deliberate red herring. {The poster formerly known as 87.81.230.195} 90.217.249.244 (talk) 05:20, 27 April 2017 (UTC)

This is a valid explanation. I was already somewhat suspicious that the liquid was not water (something about the refraction... it's hard to say); a liquid other than water likely has a much lower heat capacity or may change volume to a greater degree. The liquid would need to start off at an elevated temperature, making it less dense, and the hourglass would rise as it cooled off. My gut feeling is that the obvious cut here does not conceal chicanery, if only because that would make the riddle too easy. ;) Wnt (talk) 12:23, 27 April 2017 (UTC)

The floating hourglass puzzle can be immediately turned upside down to perform again after it has performed.

And you can make it yourself with a tube, some ordinary water, and an hourglass (Or rather, an egg timer. An actual hour glass would be unwieldy for this kind of demonstration.). You may need to add weight to the hourglass. It should be buoyant, but not very buoyant. ApLundell (talk) 14:58, 27 April 2017 (UTC)

@ApLundell: What's your source for that? I think my solution with the membrane could meet this specification, assuming there are flexible membranes on both the top and the bottom of the hourglass. Wnt (talk) 02:40, 28 April 2017 (UTC)

I'm sure there are lots of ways it could be gimmicked, but you don't need to. The physics puzzle famously works with an ordinary hourglass.

It was apparently first introduced by Martin Gardner in a 1966 article in Scientific American, but I don't have the issue so I can't confirm. It was very popular, zillions of people wrote in about it. He included it in his book Mathematical Carnival.

There's no shortage of examples of this device. (Search for "Floating Hourglass" or "Rising Hourglass") Here's a video of one constructed with a wider tube, so the explanation of the trick is more obvious [6].

ApLundell (talk) 04:03, 28 April 2017 (UTC)

@ApLundell: I'll grant that looks very similar .... but there's a key difference. The hourglass in that video does not sink on its own! It simply stays in place at first when the whole tube is flipped. So no matter what it looks like, I don't think it works the same way at all. Wnt (talk) 09:45, 28 April 2017 (UTC)

@Wnt: I don't know what to tell you except to encourage you to rewatch the video that started this thread. In it, the video begins with the hourglass floating at the top of the tube until a hand manually rotates the entire assembly. Exactly as in the classic rising hourglass demonstration.

By the way, The tubes can be made with an hourglass of positive or negative buoyancy. (That is, once the static friction is released, they will either sink of rise.) Here's a video where there's one of each.[7] One that performs like the classic rising hourglass as seen in the video that started this thread, and one exactly opposite. In either case, the fascinating part is that the hourglass refuses to move until its center of gravity has lowered a bit. ApLundell (talk) 13:59, 28 April 2017 (UTC)

Wow... I totally misremembered what I'd seen happen. OK, so none of my fancy hocus-pocus is actually needed. Although the membrane thing actually *would* help anyway (the hourglass would float once the sand was down against the membrane, and refloat later) this is indeed just as you said, a simple matter of friction from an unstable center of gravity. Wnt (talk) 01:01, 29 April 2017 (UTC)

It is possible that when the sand is at the top of the hourglass, it's exerting a slight pressure on the air in the lower chamber, compressing it slightly, and as the air flows to the top of the hourglass it decompresses. But this would work only if the volume of the hourglass somehow changed with this difference in pressure, maybe using a membrane as Wnt suggests above. Another explanation: Notice that you never see the hourglass rising with the rounded end on the bottom? The flat end is starting out at the bottom. So that flat end may be 'stuck' momentarily to the end of the cylinder, and its own buoyancy pulls it up.... in which case the sand is just misdirection. ~Anachronist (talk) 06:51, 27 April 2017 (UTC)

The centre of mass (CM) of the hourglass stays on the axis of symmetry, but it does move down. Just after the thing is inverted, the CM is in the upper half, but when some of the sand has fallen, the CM moves to the lower half. The centre of buoyancy (CB) always remains at the exact centre of the hourglass. When the CM is above the CB, the hourglass is unstable and tries to rotate, thereby wedging itself against the walls of the tube, so that friction can overcome the force of buoyancy. When the CM gets below the CB, the hourglass is stable and gets free of the walls.

There is an additional effect, which I didn't completely calculate here. Because the CM of the hourglass not only moves, but even accelerates, the effective weight of a running hourglass is variable and different from the weight of the same hourglass when all sand is stationary at the bottom. It may depend a bit on the exact shape of the hourglass. If you wish to calculate, there are four components: the mass of the sand in the upper half, which is a cone (or maybe not exactly a cone) of shrinking mass, accelerating down; the mass of the sand in the lower half, which is a cone of increasing mass, accelerating down; the sand currently falling, which is in free fall and therefore does not contribute to the weight of the hourglass; and the sand hitting the lower pile, which rapidly accelerates up, slightly overcompensating for the sand in free fall, because the length of the free fall changes. PiusImpavidus (talk) 11:04, 27 April 2017 (UTC)

• The standard answer to this puzzle is the static friction caused by the top-heavy hourglass trying to rotate, and therefore pressing firmly against the side of the tube [8].

ApLundell (talk) 14:53, 27 April 2017 (UTC)

• I think that Martin Gardner covered this in Scientific American decades ago. It has to do with the friction of the hourglass against the wall changing as the sand goes down. See this. The Martin Gardner article was reprinted in the book Mathematical Carnival, in chapter 14 titled "The Rising Hourglass and other Physics Puzzles". Hardback: Knopf, 1975; softcover: Vintige, 1977. Bubba73 ^{You talkin' to me?} 14:40, 29 April 2017 (UTC)

CO2 nose tingling

While drinking Pepsi from the bottle, the carbon dioxide I inhaled had a tingling effect on my nostrils. Why is that? I thought tingling only occurs in dissolved form. 212.180.235.46 (talk) 20:51, 26 April 2017 (UTC)

I notice little bits of soda or champagne splashing out the glass as bubbles near the surface burst. I suspect that some of these land in your nose. StuRat (talk) 21:51, 26 April 2017 (UTC)

@StuRat: you can get the tingling effect even when the bottle is away from your mouth. I have always had two tiny holes in the hard palate behind the upper two front teeth; linking the mouth to the nasal cavity (they're mostly blocked but on some days I can suck air through them). I think carbonated water can go up these holes, still bubbling, and give the nose a tingling sensation. ~Anachronist (talk) 07:04, 27 April 2017 (UTC)

If carbon dioxide dissolves in water, some of it gets converted to carbonic acid. Your respiratory tract is moist, so this happens. This is the same mechanism by which things like chlorine gas have their deadly effects: the molecules dissolve into the secretions lining your eyes and respiratory tract, forming acids, which then attack the tissue. --47.138.161.183 (talk) 21:58, 26 April 2017 (UTC)

AC Transformer Ratings

My power supply (built from a kit) says that it needs a 2x9VAC transformer to output 5V DC: is this the same as 18 VA? Can I use an 18 V transformer that outputs 1 A? I don't understand why they list the specifications as 2x9 instead of 18, when all the transformers I find for sale just say 18. OldTimeNESter (talk) 23:19, 26 April 2017 (UTC)

Your kit should have some kind of circuit diagram to show how things connect I suspect it needs more than two connections to the power supply, and that you have to connect them correctly, not at random. Your circuit may need a centre tap, where the two 9V windings are connected in series. Some transformers can also be connected in parallel (in phase) to double the current output. Graeme Bartlett (talk) 23:50, 26 April 2017 (UTC)

No, 2x9VAC is not the same as 18 VA. The reason why the kit manufacturer has specified a transformer with two secondary windings of 9V each depends on several factors, and it's impossible to say why they have done so without seeing the circuit design. It may help to understand that when AC is rectified in a four-diode diode bridge, the DC voltage that results is very close to 1.4 times the AC voltage supplied by the secondary. Similarly, when using a two-diode full-wave rectifier circuit with center-tapped transformer, the DC voltage that results is very close to 1.4 times the AC voltage supplied by half of the transformer secondary. Here are some configurations, from which we can make some deductions:

• 1. Two 9V windings in series (18VAC) applied to a diode bridge result in 25.2 VDC, far more than required.
• 2. Two 9V windings connected in series (forming a common center tap) to a full-wave rectifier result in 12.6 VDC, about right for deriving 5VDC via a regulator device.
• 3. Two 9V windings in parallel (in phase!!!) connected to a diode bridge result in 12.6 VDC, about right for deriving 5VDC via a regulator device, and able to deliver nearly twice the current as that of configuration 2.

We can almost certainly discount configuration 1, leaving either of the other two options. Therefore your 18 V transformer is almost certainly not suitable. It will definitely not be suitable if the kit uses option 2's half wave rectifier, unless your transformer has a center tap (you didn't mention one). The kit should describe how to connect the two 9VAC windings, from which you could deduce what type of rectifier is employed. Or you could just count the diodes. Akld guy (talk) 06:54, 27 April 2017 (UTC)

Check whether the 2-diode circuit [9] described at Rectifier#Full-wave rectification (B2U) is used in your power supply. The transformer in this circuit can be described as having an 18 V AC center-tapped output, which is the same as 2 x 9 V AC outputs connected in series. Please do not write "18 VA" unless you really mean "18 Volt-ampere". The 2-diode circuit can be replaced by the 4-diode circuit also shown at Rectifier#Full-wave rectification (B2U) which would allow a simpler transformer with 9 V AC output. Blooteuth (talk) 13:44, 27 April 2017 (UTC)

The circuit diagram has 4 diodes: it shows the transformer having four inputs from mains, labelled from left to right as Va, 0, 0, and Vb. The two "0" inputs are tied to ground, and each of the Va,Vb connects to two of the diodes. Both pairs of diodes are wired in parallel. I don't think it's a center tap, since the center output from the transformer is tied to ground. I actually haven't bought a transformer yet: I was just confused because I didn't see any of the listed at Jameco as 2x9, just 18. OldTimeNESter (talk) 14:03, 28 April 2017 (UTC)

That's a really confusing description, and I'm struggling to make sense of it. Akld guy (talk) 01:26, 29 April 2017 (UTC)

Actually it sounds very like the circuit in the pic, but the right hand centre tapped winding as two separate windings, joined and grounded.

add ground to the centre tap

If that's the case, and I'm not saying it's not, his transformer cannot be used because it apparently doesn't have a center tap (well, he hasn't mentioned one). Akld guy (talk) 02:34, 30 April 2017 (UTC)