question: how do we measure the percentage of water in something?
comment: Weigh the item. Heavily dry it, and weigh it again. This gives you how much mass was removed by the process, and then they are compared to find your percentage.
passages: Section::::Uses.:Food safety.
  
  Water activity is used in many cases as a critical control point for Hazard Analysis and Critical Control Points (HACCP) programs. Samples of the food product are periodically taken from the production area and tested to ensure water activity values are within a specified range for food quality and safety. Measurements can be made in as little as five minutes, and are made regularly in most major food production facilities.
   Permeation can also be measured through intermittent contact. This method involves taking a sample of the test chemical and placing it on the surface of the material whose permeability is being observed while adding or removing specific amounts of the test chemical. After a known amount of time, the material is analyzed to find the concentration of the test chemical present throughout its structure. Along with the amount of time the chemical was on the material and the analysis of the test material, one can determine the cumulative permeation of the test chemical.
   1 fluid ounce of water weighs approximately 1 ounce so a recipe calling for a UK pint (20 fl oz) of water can be substituted with 20 oz of water.
  
  More accurate weight equivalents become important in the large volumes used in commercial food production. To an accuracy of five significant digits, they are:
  
  Even a home cook can use greater precision at times. Water at may be volumetrically measured then weighed to determine an unknown measuring-utensil volume without the need for a water-density adjustment.
  
  Section::::United States measures.
   Watercut meter
  
  A water cut meter measures the water content (cut) of crude oil and hydrocarbons as they flow through a pipeline. While the title "Water cut" has been traditionally used, the current API naming is OWD or On-Line Water Determination. The API and ISO committees have not yet come out with an international standard for these devices but there are however standards in place for fiscal automatic sampling of crude oil namely API 8.2 and ISO 3171.
   Environmental laboratories typically report concentrations for anion and cation parameters using units of mass/volume, usually mg/L. In order to convert the mass concentration to an equivalent concentration the following mathematical relationship is used:
  
  For example, a water with a calcium concentration of 120 mg/L would have the following calcium equivalent concentration:
  
  Section::::Software programs in common use.
  
  BULLET::::- HyCA Commercial software available from KWR waterware
  
  BULLET::::- AqQA Commercial software available from RockWare, Inc.
  
  BULLET::::- AquaChem Commercial software from Schlumberger Water Services
  
  BULLET::::- ChemPoint Pro Commercial software from Starpoint software, Inc.
  
  BULLET::::- The Geochemist's Workbench Commercial software from Aqueous Solutions LLC
   National surveys of commercial buildings by the U.S. DOE Energy Information Administration and EPA Energy Star provided proxy metrics data on the mean and median values of water use per square foot of buildings floor space. The median values are useful when placing water use in a CII establishment among its peers because the sample distributions typically have a pronounced right tail skew. Mean (or average) values are useful to planners who need to estimate total category use based on results from a sample of establishments.
   Salt concentrations of outgoing water (either from one reservoir into the other or by subsurface drainage) are computed on the basis of salt balances, using different leaching or salt mixing efficiencies to be given with the input data. The effects of different leaching efficiencies can be simulated varying their input value.
   Another important derived quantity is the amount of substance concentration (also called "amount concentration", or "substance concentration" in clinical chemistry; which is defined as the amount of a specific substance in a sample of a solution (or some other mixture), divided by the volume of the sample. 
  
  The SI unit of this quantity is the mole (of the substance) per liter (of the solution). Thus, for example, the amount concentration of sodium chloride in ocean water is typically about 0.599 mol/L.
   As a unit of culinary measure, one teaspoon in the United States is tablespoon, that is, exactly 4.92892159375 mL; it is exactly 1 US fluid drams, US fl oz, US cup, and US liquid gallon and or 0.30078125 cubic inches. 
  
  For nutritional labeling and medicine in the US, the teaspoon is defined the same as a metric teaspoon - precisely 5 milliliters (mL)
  
  Section::::Culinary measure.:Dry ingredients.
   where "p" is the partial vapor pressure of water in the solution, and "p*" is the partial vapor pressure of pure water at the same temperature.
  
  Alternate definition:
  
  where "l" is the activity coefficient of water and "x" is the mole fraction of water in the aqueous fraction.
  
  Relationship to relative humidity:
  
  The relative humidity of air in equilibrium with a sample is called the Equilibrium Relative Humidity (ERH).
  
  Estimated mold-free shelf life in days at 21° C:
  
  Section::::Uses.
  
  Water activity is an important consideration for food product design and food safety.
  
  Section::::Uses.:Food product design.
   On-line sodium measurement in ultrapure water most commonly uses a glass membrane sodium ion-selective electrode and a reference electrode in an analyzer measuring a small continuously flowing side-stream sample. The voltage measured between the electrodes is proportional to the logarithm of the sodium ion activity or concentration, according to the Nernst equation. Because of the logarithmic response, low concentrations in sub-parts per billion ranges can be measured routinely. To prevent interference from hydrogen ion, the sample pH is raised by the continuous addition of a pure amine before measurement. Calibration at low concentrations is often done with automated analyzers to save time and to eliminate variables of manual calibration.
   The water content of most compounds can be determined with a knowledge of its formula. An unknown sample can be determined through thermogravimetric analysis (TGA) where the sample is heated strongly, and the accurate weight of a sample is plotted against the temperature. The amount of water driven off is then divided by the molar mass of water to obtain the number of molecules of water bound to the salt.
  
  Section::::Other solvents of crystallization.
   In countries with often-unsafe/unclean tap water supplies, such as much of India, the TDS of drinking water is often checked by technicians to gauge how effectively their RO/Water Filtration devices are working. While TDS readings will not give an answer as to the amount of microorganisms present in a sample of water, they can get a good idea as to the efficiency of the filter by how much TDS is present.
  
  Section::::Water classification.
  
  Water can be classified by the level of total dissolved solids (TDS) in the water:
  
  BULLET::::- Fresh water: TDS = 500 ppm
   Sometimes multiples of teaspoons and tablespoons are included. There may also be scales for the approximate weight for particular substances, such as flour and sugar.
  
  Section::::For dry measure.
  
  Many dry ingredients, such as granulated sugar, are not very compressible, so volume measures are consistent. Others, notably flour, are more variable. For example, 1 cup of all-purpose flour sifted into a cup and leveled weighs about 100 grams (3½ oz), whereas 1 cup of all-purpose flour scooped from its container and leveled weighs about 140 grams (5 oz).
   Milligrams per litre are often used in medicine and in medical prescriptions. For example, a description of a solution that involves two substances, where one of the substances involves adding water, would state: "10 mg/L water and "substance"" (i.e. 10 mg of substance for every L of water). Blood sugar concentration is sometimes measured in milligrams per decilitre (mg/dL).
   When quantifying water use in the CII sector, the total volume of consumption (over any given time interval) at any given CII facility is “normalized” by the scale of water-using activity, which corresponds to the rate at which water is used (also referred to as water use intensity or WUI). Two scaling factors (or "proxy" measures of scale), the number of employees and the square footage of buildings (or facilities) are commonly used in the CII sector because the employment and floor space data are generally available. Examples of the corresponding proxy metrics are: water use per employee in manufacturing plants, or water use per square foot in office buildings. However, more appropriate are "functional" metrics such as water use per occupied room in hotels, or water use per meal served in restaurants, since they are linked more directly to water use. 
   The depth of the water table, calculated from the water balances, is assumed to be the same for the whole area. If this assumption is not acceptable, the area must be divided into separate units.br
   Whatever pore size is used in the definition, the resulting salinity value of a given sample of natural water will not vary by more than a few percent (%). Physical oceanographers working in the abyssal ocean, however, are often concerned with precision and intercomparability of measurements by different researchers, at different times, to almost five significant digits. A bottled seawater product known as IAPSO Standard Seawater is used by oceanographers to standardize their measurements with enough precision to meet this requirement.
  
  Section::::Definitions.:Composition.
   Measures are classified as either dry measures or fluid measures. Some of the fluid and dry measures have similar names, but the actual measured volume is quite different. A recipe will generally specify which measurement is required. U.S. recipes are commonly in terms of fluid measures, even for dry ingredients. Most of these units derive from earlier English units, as applied to the U.S. gallon. Typically they follow a pattern of binary submultiples, where each larger measure consists of two units of the next-smallest measure. An exception is with the commonly used teaspoon as one-third of a tablespoon.
   Typically grammage is measured in paper mill on-line by a quality control system and verified by laboratory measurement.
  
  Section::::Basis weight.
   Section::::Methodologies.:Membrane filtration.
  
  Most modern laboratories use a refinement of total plate count in which serial dilutions of the sample are vacuum filtered through purpose made membrane filters and these filters are themselves laid on nutrient medium within sealed plates. The methodology is otherwise similar to conventional total plate counts. Membranes have a printed millimetre grid printed on and can be reliably used to count the number of colonies under a binocular microscope.
  
  Section::::Methodologies.:Pour plate method.
   BULLET::::- The precipitable water can also be calculated by integration of radiosonde data (relative humidity, pressure and temperature) over the whole atmosphere.
  
  BULLET::::- Data can be viewed on a Lifted-K index. The numbers represent inches of water as mentioned above for a geographical location.
  
  BULLET::::- Recently, methods using the Global Positioning System have been developed.
   Lowering the water activity of a food product should not be seen as a kill step. Studies in powdered milk show that viable cells can exist at much lower water activity values, but that they never grow. Over time, bacterial levels decline.
  
  Section::::Measurement.
  
  Water activity values are obtained by either a resistive electrolytic, a capacitance or a dew point hygrometer.
  
  Section::::Measurement.:Resistive electrolytic hygrometers.
   BULLET::::- Hydraulic conductivity The rate of flow of water through a unit cross sectional area of an aquifer, at a unit hydraulic gradient. In US units the rate of flow is in gallons per day per square foot of cross sectional area; in SI units hydraulic conductivity is usually quoted in m per day per m. Units are frequently shortened to metres per day or equivalent.
  
  BULLET::::- Specific storage or storativity: a measure of the amount of water a confined aquifer will give up for a certain change in head;
   Salt concentrations of outgoing water (either from one reservoir into the other or by subsurface drainage) are computed on the basis of salt balances, using different "leaching or salt mixing efficiencies" to be given with the input data. The effects of different leaching efficiencies can be simulated by varying their input value.
  