Background Notes on Measures - 4
 Contents are: Flow Rates Spread Rates Concentration Line Density Viscosity Acceleration
Additional information can be found in the Dictionary of Units
FLOW RATES
 This is a measurement of the amount of a liquid or gas which passes a fixed point in a given time.A simple example is that of a river, where the flow can clearly be seen, and the fact that a certain amount of water must have passed during the time that the observer was looking is readily acceptable.The units in which the time is measured can be anything from milliseconds to years, but these two extremes would probably be of interest only to scientists. The calculators offered here range (in time units) only from seconds to days. The amount of the flow can be measured by either its mass or its volume.So the mass flow rate can be stated inpounds/hour, kg/second, tonnes/day etc.. or any other combination of units of mass/unit of time.The volume flow rate can be stated in cubic feet/hour, gallons(UK or US)/day, litres/second etc.. or any other combination of units of volume/unit of time.Each of these is handled by a separate calculator.The two different types of rate can only be connected if the density of the material is known.
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 Spread rates measure how a quantity of material is spread over an area. It is assumed that the spread is even.An obvious example is of seed, fertilizer or weed-killer being spread over the ground. Clearly it needs to be spread at the 'best' possible rate. Too much and seed will be wasted or treatment may be harmful, too little and there will be gaps. The same units can also be used to express crop yields.The amount of the material being spread can be measured by either its mass or its volume.So the mass spread rate can be stated inpounds/acre, kg/hectare, tonnes/hectare etc.. or any other combination of units of mass/unit of area.The volume spread rate can be stated in cubic feet/acre, gallons/sq.yard, litres/sq.metre etc.. or any other combination of units of volume/unit of area.Each of these is handled by a separate calculator.The two different types of rate can only be connected if the density of the material is known. RainfallThe amount of rain that has fallen is usually given, in millimetres or inches, as being the depth to which it would have risen (if it could not drain away) in a certain period of time. Thus, '17 mm of rain fell in 2 hours yesterday'.The calculators will change this depth into an equivalent mass (or volume) that would have covered a unit area. From this, the total mass (or volume) that must have fallen on a known area can easily be calculated.So, following on from the above statement, 17 mm of rain can be seen to be the same as170 cubic metres per hectareor67.7 tons(UK) per acreFor this purpose, in the 'mass calculator', rain is assumed to be pure water with a density of 1 kilogram per litre.
 The MASS spread rate gives the area density of a flat, surface of uniform thickness which is also known as the plate density

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CONCENTRATION
 Concentration measures the amount of one substance (the solute) contained within a unit measure of a mixture (the solution).For example, a concentration of 2.7 millilitres per cubic metre would mean that 2.7 mL of the solute are contained in every cubic metre of the solution. Note that, in the calculator, the fluid ounces [fl.oz] are (UK) or (US) to match the gallon.It can also be expressed as mass per unit mass. (mg/kg etc.)Another common form for concentration is to give it in parts per million [ppm]; parts per thousand or 'per mil' [‰]; or parts per hundred or 'per cent' [%]. The advantage of these three is that, since they contain no units, the relative 'strengths' of two mixtures can be seen at a glance.A concentration cannot equal or exceed 100% (or 1000 000 ppm) since that would mean that there was as much as or more of the substance being held than there was of the space to hold it! A concentration can also be given in terms of mass per unit volume or volume per unit mass. In both of these cases there is no general way of changing to ppm or % since it depends upon the densities of the ingredients. Though it worth noting that mass/unit volume is the definition of DENSITY.Two very practical and commonly used measures of small volumes are the drop and the teaspoon. These are ill-defined but, for the purposes of the calculator1 drop = 0.05 (or 1/20 th) of a millilitreand, if needed for practical work, take 1 teaspoon = 5 millilitres or 100 drops.In some specialist fields (such as chemistry and medicine) the unit of a mole is often used. Then a concentration is given in moles per litre or similar. A knowledge of the substances under consideration (and their molecular weights) is needed before it can be converted into some other measure. So, it is not included in this calculator.

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LINE DENSITY
 Line density is a measure of the mass per unit length of a substance when it is in a drawn-out form, like a string or wire. It is assumed that it is uniform in size and consistency throughout its length.The SI preferred unit is kilograms per metre [kg/m]. But this unit is inconvenient in many contexts and much use is made of the SI prefixes, as well as a few units which are outside the SI system.There are two principal places in which this type of measurement is used.One place is in those industries which use wire, rope, cables and things of that nature, where the mass per length is of importance in establishing loads and stresses that will be encountered in handling them.Another (and probably more familiar) place in which line density is used is the textile industrty. In the textile industry, line density is used to measure the 'thickness' of yarn. As might be expected in such a long-established industry, several different measures have been used over the years. In wool especially, each area had its own system, as can been seen by the names listed in the Yarn calculator.There were also two different ways of doing it. One is the yarn number which is mass per unit length, the same as line density. The other is the yarn count which is a measure of the length needed to produce a certain mass. These two are also known, respectively, as direct and indirect systems.Unfortunately, the counts vary considerably in the way the length is established (like so many 'hanks') and the weight to be made (such as ounce, pound, 24 ounces). The Yarn calculator shows how each count is defined.The tex is now the internationally agreed unit of line density for yarns. It gives the yarn number as the number of grams of the yarn which are needed to measure a distance of 1 kilometre.

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VISCOSITY
 The viscosity of a substance (fluid) is a measure of its resistance to flow.The higher the value of the viscosity then the greater is the resistance.So, a substance with a low viscosity will flow more freely than another substance with a higher viscosity. RememberLower number = Faster flowWhich seems to be opposite to what we 'naturally' expect.The viscosity of any substance is dependent upon temperature. Generally, as the temperature goes up the viscosity goes down (though there are exceptions). We use this very commonly in things like warming up our syrup (or whatever) to lower its viscosity so that it spreads (or flows) better.Usually when the word 'viscosity' is used on its own it is DYNAMIC VISCOSITY which is meant. The SI unit is the pascal second [Pa s]1 Pa s = 1 newton second/square metreAnother unit often used is the 'poise' where10 poises = 1 Pa s KINEMATIC VISCOSITY of a substance is a measure of its dynamic viscosity per unity density of that substance.The SI unit is sq.metres/second but, as that is such a large unit, many prefer the 'stoke' where10,000 stokes = 1 sq.metre/secondNotice that units for Kinematic Viscosity are Area/unit time, and these same units are used to measure Thermal DiffusivityFLUIDITYThe measure of the fluidity of a substance is given by the reciprocal of its dynamic viscosity.So now, after standing viscosity 'on its head' as it were, we do have the situation with fluidity thatHigher number = Faster flow
 There are some other units of Kinematic viscosity with names like Saybolt, Redwood and Engler which, although they are obsolete, are still to be found in the literature. Unfortunately, they do not lend themselves to being converted easily into the sort of units given in these calculators and so they have been excluded. If there is a need to convert them, then a search on the Web, using the appropriate word, will find several tables to serve that purpose.

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ACCELERATION
 Acceleration is a measure of the rate at which a velocity is changing. It may be positive (for increasing velocity) or negative (for decreasing velocity).When it is negative it is often described as deceleration or retardation.The SI unit of acceleration is metres/second² and that is the one most often seen. All (except one) of the other units shown are obsolete, but are still to be found, of course, in older texts. The exception is the rate of acceleration of a body in free fall. That is, when a body is falling towards the Earth with only the force of gravity acting upon it, and there is no air resistance (as in a vacuum). This unit is designated as 'g'. Its exact value depends upon whereabouts on the Earth the action is taking place. But the variation is slight and may be ignored, unless a high order of precision is required. The exact value of g in the SI system is 9.806 65 metres/second²

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