Grantville Gazette, Volume 72

by Bjorn Hasseler

  On early thermometers, the scale was drawn on paper that in turn was mounted on a wood board. Scales have also been engraved on metal, glass, or ivory back plates, or directly onto the thermometer tube (etched with hydrofluoric acid).

  Even a calibrated thermometer needs to be recalibrated from time to time. For example, the residual strain in a glass thermometer eases slowly, causing the glass to shrink. Most of the change occurs in the first year (Bentley 2:98).

  Recalibration involves carrying an "inspector thermometer" (precisely calibrated in the laboratory) to each weather station. The station thermometer and the inspector thermometer are exposed to an ice bath (the single calibration point is good enough for a liquid expansion thermometer, see Ripple) and the station thermometer's correction table updated.

  In twentieth-century practice, inspector thermometers are mercury-based. It may have a narrow bore, so the change in length of the column is greater for a different temperature change. The downside is that the inspector thermometer must either be longer than the norm, or have a restricted range (say 30oC) (Srivaslava 103).

  Accuracy. Spirit thermometers typically have an accuracy of 1-2 degrees Celsius in the meteoroogical temperature range (Facts).

  In 2014, for NWS land stations, current and maximum temperature must be measured with 1oF accuracy in the range -20 to 115oF, and 2oF in the extreme ranges -40 to 20oF and 115-140oF. Minimum temperature accuracy is 1oF for -20 to 110oF, and 2oF for -80 to -20 (NWSRS 7). The data is nonetheless reported to the nearest 0.1oF (9).

  Interestingly, this performance standard is inconsistent with the WMO recommendation that in the central range the maximum error be less than 0.4oF; NWS comments, "in practice, it may not be economical to provide thermometers that meet this performance goal."

  The accuracy with which temperature is measured can be increased by using a panel of several thermometers. If the thermometers are equally inaccurate and there is no systematic bias, the average of four thermometers will be twice as accurate as just one. (The standard error is proportional to the individual standard deviation and inversely proportional to the square root of the sample size.)

  Maximum and Minimum Thermometers. These indicate the extreme value since they were last reset by the observer.

  EB11/Thermometry (836) describes three different kinds of maximum thermometers: the Rutherford (1790) type, in which the mercury in a horizontal tube pushes a steel (originally, glass) index and leaves it behind when the temperature drops; that of Negretti and Zambra, with a constriction in the horizontal tube pass the bulb (the mercury expands past the constriction but the "column" breaks there when it contracts); and that of Phillips (1832) and Walferdin (1855), where the horizontal "column" is divided by a bubble of air that acts as an index. Note that the physician's thermometer is really a maximum thermometer of the constriction type. The Rutherford type was "little used" by 1911; the problem was that the mercury tended to seep past the index (Middleton 152).

  Rutherford also invented the favored minimum thermometer; again, a horizontal tube, but the liquid is amyl alcohol (originally, ordinary alcohol) and the index is made of porcelain (or glass).

  There was also Six' combination minimum/maximum thermometer (1782), a U-tube with a bulb at both ends. There is mercury in the middle and spirit in the legs, but one bulb also contains spirit and the other a mixture of air and alcoholic vapor. The mercury merely serves as an indicator, the "thermometric fluid" being the spirit, and unfortunately the alcohol can wet the glass and pass by the mercury. (Middleton 161).

  Thermographs. These provide continuous records of temperature, and thus reduce the utility of minimum and maximum thermometers. Note, however, that they tend to be less accurate than thermometers. In essence, they couple a thermometer to a readout mechanism.

  If the internal thermometer is of the liquid-in-glass type, the liquid must be mercury rather than alcohol, as the latter is too sluggish. The dominant design used a photographic readout; light shining around the mercury column onto photographed paper moved by clockwork (193). The temperature record was thus a negative image (black except where the paper was shadowed by the mercury), and the device evolved, taking advantage of improvements in light sources and paper. There were ingenious alternatives of uncertain practicability; one design balanced the thermometer horizontally on a knife edge; the temperature change shifted the center of gravity, and the tilt was recorded. It is uncertain how this would fare in a strong wind.

  The other major type was that in which a bimetallic strip is deformed in response to temperature change. The strip moves a stylus that draws on paper carried by a rotating drum; the rotation is driven by a clock mechanism inside the drum and thus protected from the elements (201-4).

  I was surprised to discover that in general there wasn't much meteorological use of thermographs featuring electrical thermometers.

  Thermometer Exposure. It is not easy to expose the thermometer in such a way that it displays the true air temperature; the heat exchange between the thermometer and its surroundings is complex. The down-timers know that it is warmer in the sun than the shade, and in the mid-seventeenth century Medicean meteorological network it was initially standard for thermometers to be placed at the north and south windows of each station (Middleton 208).

  For its Cooperative Observer Program, the National Weather Service advises that a temperature sensor be mounted four to six feet off the ground, in a level open clearing and away from obstructions and paved surfaces.

  It is also customary for meteorological instruments to be housed in an elevated shelter ("Stevenson screen" or "Cotton Region Shelter") that shades the instruments while providing ventilation. One shelter design appears in Popular Science (May, 1935). Typically, the shelters have louvers that slope downward and outward, are painted white to reflect solar radiation, and, in the northern hemisphere, the door faces north.

  There have been a couple of thermometer designs intended to increase ventilation beyond that provided by passive air movement through louvers. One is the sling thermometer; a thermometer mounted on a frame pivotably connected to an axle that terminates in a handle. This evolved into the sling psychrometer, using two thermometers (one with a wet bulb), and used to measure humidity. The other is the aspirated thermometer; with forced convection from a suction fan. This, too, evolved into a psychrometer.

  Temperature readings can be perturbed, not only by solar radiation and precipitation, but also by the observer's own heat. Hence, readings must be taken expeditiously.

  As of 2014, NWS Cooperative Observer Stations were equipped with a spirit-based minimum thermometer and a mercury-based maximum thermometer, and/or certain models of thermistor-type electronic thermometers. After the maximum thermometer is read, the tube can be spun in its mount to force the mercury in the stem past the constriction, joining the mercury in the bulb, and then it indicates the current air temperature (NWCSOM A-24).

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  Humidity

  Humidity is the amount of water vapor in the air. The warmer the air, the more water vapor it can hold. Somewhat non-intuitively, increasing humidity decreases air density (because the water vapor molecules replace heavier air molecules). So humidity is relevant to airship operations.

  Absolute humidity is the exact water vapor content of the air, whereas relative humidity is the current content compared to the maximum possible at the current temperature and pressure. The "dew point is the temperature at which airborne water vapor will condense to form liquid dew" (Wikipedia). The higher the relative humidity, the closer the dew point is to the current air temperature. The difference between the two is called the "dew point depression."

  To measure dew point depression (from which we can calculate relative and absolute humidity if the pressure is known), we need both an ordinary (dry bulb) thermometer and a "wet bulb thermometer." The latter, which approximates the dew point, is a thermometer that "has its bulb wrapped in cloth—called a sock—that is kept wet with distilled water via wi
cking action" (Wikipedia). (Some inventors replaced the water of the classic wet bulb thermometer with a more volatile liquid; Daniell (1820) used ether.) The combination of the two matched thermometers is called a psychrometer, a type of hygrometer.

  The thermometers can be ventilated by whirling (sling psychrometer) or by a fan. The so-called psychrometer coefficient (which relates the vapor pressure to the dew point depression) is 0.0008 for a naturally ventilated psychrometer inside a Stevenson screen, and 0.000667 for a force-ventilated one (Harrison 117).

  Accuracy is typically equivalent to 5% relative humidity and response time to get a reliable reading is about a minute (122).

  Crude gravimetric absorption hygrometers were designed by Nicolaus Cusanus (1450), Leo Battista Alberti (1470), and Leonardo da Vinci (1490). In essence, this was a balance with a hygroscopic substance (cotton, wool, sponge) in one pan and a water-repelling substance (wax) in the other. Under dry conditions, the pans are at the same level, but if humidity increases, the cotton absorbs water and that pan sinks lower (Robens 556).

  Condensation (on the outside of a vessel containing snow or ice) was weighed directly by Grand Duke Ferdinand in the 1660s (Bentley 181).

  Mechanical hygrometers may be constructed using a substance whose mechanical properties are altered by humidity. Such materials include hair, goldbeaters skin (also used for airship gas bags), and animal horn or antler (109). In 1614, Santorio Santorre (1561-OTL 1636) stretched a center-weighted cord between two fixed points; absorption of water vapor caused the cord to contract and lift the weight (Wiederhold 4). In 1664, Francesco Folli (1624-85) made similar use of a paper ribbon, but the weight was connected to the center of the ribbon by a cord running over a pulley, and the pulley was connected to a dial pointer. Later, ivory (de Luc 1773) and goose quills (Buissart and Retz, 1780) were used as humidity sensors (Zuidervaart).

  A hair tension hygrometer was proposed by de Saussure in 1783 (Wikipedia/Hygrometer); hair increased length by 2-2.5% for a 100% change in RH. The response is not linear (but still better than the sensors used previously) and depends on the type of hair. At subzero temperatures, response time and responsiveness are reduced. The hair length changes more when humidity increases than when it decreases. Hair is very sensitive to contamination (dust, finger oils, etc.). A hair hygrometer is usually calibrated with an aspirated psychrometer. But in a pinch, you can wet the hair bundle to reach 100% RH (JMA). Likewise, to get to 0% RH, find an up-timer with a blow dryer. Trowbridge (1896) showed that the RH error didn't exceed 3% if the true RH was 20-85%.

  In the late twentieth century, hair hygrometers were still in use. In these modern iterations, a bundle of human hair of different types is used, the hairs are carefully cleaned, and in some instances the scale is nonlinearly divided (Belfort; Ambient Weather).

  In a metal-paper coil hygrometer, the paper is impregnated with a hygroscopic (water-absorbing salt) and laminated to the metal. Its absorption of water changes the curvature of the coil in a manner analogous to how temperature changes the curvature of the strip in a bimetallic thermometer. Accuracy is perhaps 10% RH.

  Electrical hygrometers detect the change in electrical capacitance or resistance of a sensor element as a result of the change in humidity. Typically, capacitance changes are easier to detect. The capacitance-type hygrometer (developed in the 1930s, Wiederhold 5) features a thin film of polymer or metal oxide (the dielectric) deposited between the electrodes. The change in capacitance is about 0.2-0.5 picofarads for a 1% RH change (JMA). I am doubtful that the electrical hygrometer can be made in the NTL 1630s.

  Hygrometers are typically calibrated by sampling the humidity above a saturated salt solution (Potassium nitrate and chloride, magnesium nitrate, sodium and lithium chloride are all used.) within a sealed container at a controlled temperature (121). An older method was placing the hygrometer inside a container with a known mixture of dry air and saturated air, or in air saturated at one temperature and pressure and then increased in temperature or reduced in pressure (Middleton 1960, 116).

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  Precipitation

  Rain gauges date back at least to fourth-century BC India, where rain was collected in a bowl. A cylindrical shape facilitates the estimation of the volume of rainfall; such a shape was used in the Korean iron cheuguggi used from 1441 to 1907 (Strangeways). (I believe the volume was estimated by inserting a ruler and measuring the level of the rainwater.) A further improvement was made by Castelli (1639); he used a glass cylinder. The accuracy of the deduction of rain volume from level measurements of course is dependent on the goodness of the cylindrical figure, and the accuracy of the diameter and level gradations. Rain gauges of the recording type may use a float (connected to a pen), which moves with the water level in the gauge.

  Rather than reading the level, one may place the rain gauge on a balance of some kind and weigh the rain. The pan in turn can be connected to a pen for recordation, or, as in the Fischer Porter rain gauge, to a punch that puts a hole at a corresponding location on a "ticker tape" at intervals.

  With a standard rain gauge, if rainfall is heavy, you have to go out, measure the rain, and empty the bucket before it fills up. A single "tipping bucket" rain gauge was developed by Wren and Hooke in the late seventeenth century as part of a "weather-wiser" (a multiple element meteorograph!). The "tipping bucket" makes possible automatic operation; each time the bucket tips, the event is recorded in some way. Another self-emptying gauge design uses a siphon.

  The modern NWS non-recording precipitation gauge comprises a large (8") diameter overflow can with a small diameter measuring tube inside, and a funnel connecting the two. These are sized so that 2 inches of rain entering the funnel will occupy 20 linear inches in the measuring tube, making it practical to measure rainfall amounts to the nearest hundredth of an inch (A-6). The Fischer & Porter recording rain gauges used to have a mechanical weighing sensor and paper-tape recording assembly, but by 2014 all of the mechanical gauges were replaced with electronic ones (A-8). The NWS expects rain to be measured with an accuracy of 0.02 inches, and (melted) snow and sleet to 0.04 inches (11).

  The biggest source of error for rain gauges is the wind catching droplets and wafting them away before they fall into the receptacle (or even causing eddies that remove them after they are below the lip of the container). This was combated by giving the container a funnel top (trapping the droplets) and surrounding the gauge with a wind shield. The NWS recommends that precipitation gauges be placed in a location "where the gauge is shielded in all directions (i.e., a clearing in a grove), but "the distance of the gauge to the nearest obstruction should be at least equivalent to twice the height of the obstruction" (A-5).

  Snow is more difficult to measure than rain because (1) snow is more readily deflected by the wind and (2) snow compacts with time. Ideally, snow is melted by the gauge. It is not strictly true that ten inches of fresh snow is equivalent to one inch of rain. That is correct only if the temperature is 30oF, and all of the precipitation is snow (Schwartz).

  The NWS requirements for measurement of rain is 0.02" or 4% hourly accuracy, and 0.01" resolution. For snow, it is 0.5-1" accuracy, 1" resolution

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  In part 2, I will look at measurements of pressure and wind (which is the result of pressure gradients).

  Balticon Minicon After-Action Report

  by Walt Boyes

  This year, the Minicon was bigger and better than ever. This was because the Balticon concom asked Joy Ward, the co-editor of Eric Flint's Ring of Fire Press and a Gazette author, to produce four days of panels and activities instead of our normal two. Joy put together a terrific program, which had to be modified very quickly when it became apparent that Eric Flint was not going to be able to attend, at least physically. Rick Boatright, the 1632 Universe's head geek, set up with the Balticon media people a Skype link so that Eric could attend, albeit virtually, and give his opening speech, his Guest of Honor speech, and attend some panels, via Skype.

&
nbsp; This year, attendance at the Minicon was estimated at close to 100, with more than 20 authors in the series in attendance. The mass book signing brought fans from as far away as Amsterdam, and a lot of Eric Flint's Ring of Fire Press books were sold.

  Rick Boatright, Kevin Evans, and yours truly, did our normal two hours of Weird Tech, to show how and why technology will differ in the 1632 Universe, whether it wants to or not.

  Other panels included War in the 1632 Universe, several looks at technology and society, and of course, the famous "Snerking the Plots.”

  Eric participated in Snerking via Skype, and he explained how . . .

  Oh dear, I'm afraid that I can't tell you what he said. In order to know, you have to go to the Minicon and sit in the panel.

  We aren't sure where next year's Minicon will be, but word will be forthcoming shortly.

  ****

  Notes from The Buffer Zone:

  Wonder Woman

  by Kristine Kathryn Rusch

  For the first time in my life, I cried through a superhero movie. I should probably say, in fairness to me, that I'm a pretty easy crier, especially when faced with stories or something particularly heartwarming.

  When I was a typically moody tween, I even declared to friends that I would consider no movie good unless I cried during it.

  I now have different standards. In fact, on the nights when I want to escape reality, I often go for the shoot-em-up, Explosions R Us movies rather than anything that's heavy, intellectual, or tear-inducing.