Making the Grade
What is the meaning of those cryptic numbers used to identify engine oil?
- the range of freezing temperatures
- a range of viscosities
- its range of operating temperatures in degrees C, divided by 10
- the range of densities in grams per deciliter
On Thin Ice
A pilot of an airplane without floats would be glad to have a frozen lake to land on, if that engine (or engines) ever quit while out of reach of land. But if heat rises, then why does ice form first on the surface of lakes, ponds, rivers, and even oceans?
- It is because evaporative cooling releases about 540 times the heat of a one Centigrade degree warmer parcel of water, per gram (despite radiative heating from the sun).
- Due to the viscosity of water, mixing doesn’t occur to any significant degree, and the latent heat of cooling (the same excuse as in the previous choice) functions to reduce the surface temperature, first.
- Water actually contracts as it cools, becoming more dense (not less dense, as you would expect from having seen everything from icebergs to the ice cubes in your drink). However, that is only up until the point where it cools to four degrees Centigrade (about 39 degrees Fahrenheit). After that, it expands, becoming less dense, and rises to the surface. When it freezes, its density drops even more significantly (further assuring its higher status).
- If water were absolutely pure, it would form at the bottom first. However, the vertical profile of particulates causes the freezing point to be lowered. This is greatest at the bottom, and least at the surface. This is true for lakes, rivers, and even oceans.
Tides
The oceans have tides, but do any other bodies of water have tides as well?
- Only oceans have tides. Even the largest of the world’s lakes is too small to display tidal behavior.
- Larger lakes can have tides as well, although they are measured in inches or fractions of an inch.
- Tides are everywhere. All bodies of water have them in an absolute, theoretical sense (although for a small lake, they would not be measurable). So does the earth’s crust, and so does the atmosphere.
- Tides are everywhere indeed. But even small lakes can have them.
The Answers…
Making the Grade
Answer: There are several important properties of a motor oil, including viscosity, flash point, pour point, as well as the percentages of sulfated ash and zinc. One of the most important is viscosity. Scientifically speaking, it is the property of an oil to develop and maintain a certain amount of shearing stress dependent on flow, and then to offer continued resistance to flow. Thinner oils with too low a viscosity can shear and lose film strength at high temperatures. Oils with too high a viscosity may not pump to the proper parts at low temperatures and the film may tear at high rpm. The weights assigned to oils are numbers assigned by the Society of Automotive Engineers, and which correspond to “real” viscosity, as measured by several accepted techniques. The numbers indicate viscosities; higher numbers mean greater viscosity. These measurements are taken at specific temperatures. Oils that fall into a certain range are designated 5, 10, 20, 30, 40, 50, or 60 by the S.A.E. The W means the oil meets specifications for viscosity at 0 degrees Fahrenheit and is therefore suitable for winter use. Multi-grade oils have a dash between two numbers, one of which is a low value with the other being an upper range. The range of viscosities isn’t some schizophrenic property of the oil, but two different viscosities at two extremes of temperature (typically a low viscosity during winter and a high viscosity during the summer). With multi-viscosity oils, polymers are added to a light base such as 5W, 10W, or 20W, which prevent it from thinning as much as it warms up. When cold, the polymers are coiled up and allow the oil to flow as their low numbers would indicate. As the oil warms up, the polymers unwind into long chains, preventing the oil from thinning as much as it normally would. Thus, a 20W-50 oil is like a 20 weight oil that will not thin more than a 50 weight oil would, when hot. (Yes, it’s choice B.)
On Thin Ice
Answer: It sounds nutty, but water really does achieve its greatest density at 4°C. As water cools, it sinks to the bottom of the barrel (or pond, lake, etc.) -up until the point when the entire body of water is uniformly cooled to four degrees, that is. Once that happens, water that cools further becomes less dense, and it starts rising to the surface. As ice crystals form, they expand further by roughly 10%, to a much larger volume. Once the surface freezes over, water beneath will take a much longer time to cool enough to freeze as well, because the surface layer now acts as insulation for the water beneath. Incidentally, this cooling and then freezing of water doesn’t occur smoothly and without a price. When water molecules slow down enough to change from vapor to liquid, or further, to ice, the kinetic energy of their movement changes into (and releases) another form of energy: heat. At the freezing point, significantly more heat is lost per gram of water (dozens of times more than for each Centigrade degree change in temperature). This is called the latent heat of fusion. It is 80 calories per gram (which is about one seventh the latent heat of cooling from the gaseous to the liquid phase, of about 540 calories/gram.) The right answer is choice C.
Tides
Answer: The sun and moon exert a gravitational pull on the earth (which of course is mutual). However, there is actually a “gradient” of gravity, stronger on the facing side, and which can be thought of as causing a “bulge” of water being pulled away from the earth (and correspondingly on the other side, a second bulge caused by the sun and moon pulling the earth away from the oceans). In between are low tides as water “drains away” toward these high-tide areas. Common sense might convince you that even the largest lake is too small to have tides because the gravitational pull of the sun and moon would act on the entire body of the lake, more or less all at once. There wouldn’t be enough of a source of water (or a space toward which it could move) at points further along the globe to supply tidal flow manifesting this differential. Furthermore, even the land underneath the lake swells under the influence of a high tide, although it is imperceptible because there are no references for the casual observer which aren’t also themselves being pulled by the sun and moon. (Even though it is many orders of magnitude larger, the sun has only about half the moon’s pull because the latter is so much closer.) And yes, the solid earth has tides as well: as much as 18 inches! If you are a pilot, you probably already wondered if the atmosphere is also affected. Guess what? It is. However, because tidal effects are maximized at the equator and decrease rapidly toward the poles, don’t expect that you would be able to discern diurnal barometric changes unless you had a very sensitive instrument, and live near the equator. (Atmospheric tides are about 100 times stronger at the equator.) Atmospheric tides at the mid-latitudes are at the edge of our ability to detect them. At around 10 microbars (0.01 millibars), such small departures are almost nonexistent when compared to weather-related variations in the standard atmospheric pressure of 1013 millibars. Near the equator, an accurate barometer would record daily fluctuations on the order of about 1 millibar. Like oceanic tides, atmospheric tides have cycles, but they come in intervals of 12 solar hours (which is different than the sea’s tides, which are related to the moon’s position). However, all that said, there is a natural phenomenon causing horizontal movement that can be measured for smaller bodies of water. It is called a seiche (pronounced SIGHsh, or also SAYsh). A seiche is the free oscillation of the water in a closed or semi-enclosed basin at a frequency equal to its natural “resonance” period. Seiches are frequently observed in harbors, lakes, and bays, and almost any distinct basin of moderate size. They are usually caused by the passage of a pressure system over the basin, by earthquakes, or by the build-up and subsequent diminution of wind. Following its initiation, the water sloshes back and forth until the oscillation is damped out by friction. For small lakes however, these movements would not be considered “tidal” in nature. The answer is choice C.
