Systems of Measurement in Veterinary Pharmacology

Chapter 6

Dr. Dipa Brahmbhatt VMD MpHDr. Dipa Brahmbhatt VMD MpHdbrahmbhatt@vettechinstitute.edudbrahmbhatt@vettechinstitute.edu

Systems of Measurement

• Household system: lacks standardization; not accurate for measuring medicine

• Metric system: developed in late 18th century to standardize measures and weights for European countries– Units based on factors of 10

– Prefixes denote increases or decreases in size of unit

• Apothecary system: system of liquid units of measure used chiefly by pharmacists

Household System

• Uses household measures when an approximate dose is acceptable

• Uses a system of weights and measures based on 1 pound containing 16 fluid ounces

• Lacks standardization, but is calibrated in units e.g. teaspoon and cup

• The drop, tablespoon, and teaspoon are the only household measures still used in pharmacology

• Disinfectants: cups, quarts and gallon

Apothecary System

• System of liquid measure used by pharmacists; also called the common system

• Prescription vials• Derived from the British apothecary

system of measures• Units in the apothecary system:

– Volume: Minim = liquid volume of a drop of water from a standard medicine dropper• 60 minims = 1 fluid dram• 8 fluid dram = 1 ounce

– Weight: Grain = basic unit of weight measurement• 1 grain of wheat = 60 mg.• 480 grains = 1 oz = 1 oz , 12 oz = 1 lb

Metric System

• Units are based on factors of 10• Fundamental units are meter (length), liter (volume),

and gram (weight)• 1 mL = 1 cc = 1 g

Volume of 1 cubic centimeter of water = 1 mL water = weighs 1 gram

• Prefixes commonly used: – Micro- = one millionth of unit = 0.000001 (10-6)– Milli- = one thousandth of unit = 0.001 (10-3)– Centi- = one hundredth of unit = 0.01 (10-2)– Kilo- = one thousand units = 1,000 (103)

Converting Within Systems

• Convert from one unit to another within the same system

• Use of conversion factors : # used with * or / to change a measurement from 1 unit to its equivalent in another unit of measure

• Converting from larger to smaller units – multiply

• Converting from smaller to larger units – divide

Converting Within the Metric System

• Use dimensional analysis (unit calculation)• Must know metric equivalents called

conversion factors• Conversion factors are used to change between

units and always have a value of one• Cancel units to achieve answer in desired unit

of measure• Desired unit of measure should be on top of

the conversion factor• Always validate answer

Shortcut Method

• Move decimal point appropriate direction based on units

• Examples:– kg to g = move decimal point 3 places to the right

– g to kg = move decimal point 3 place to the left

– l to ml = move decimal point 3 places to the right

– ml to l = move decimal point 3 places to the left

From gram to miligram

Move largerUnit to smaller

Unit hence quantityIs larger

Remember…

• When converting from larger units to smaller units, the quantity gets larger

• When converting from smaller units to larger units, the quantity gets smaller

From mg to gram

Move smallerUnit to larger

Unit hence quantity

Is smaller

From kilogram to grams

From grams to kilograms

From liters to mililiters

From milliliters to liters

Converting within the Apothecary System

• Using the dimensional analysis method

• Determine the apothecary equivalents

• Create a conversion factor

• Determine what format to write the conversion factor in

• Set up the conversion in an equation

• Perform the calculation

From dram to ounces

64 dram to ______ Ounces?8 dr = 1 oz.64 dr = X

X = (64 dr / 8 dr) * 1 oz. = 8 oz.

Volume: Minim = liquid volume of a drop of water from a standard medicine dropper

60 minims = 1 fluid dram8 fluid dram = 1 ounce

Weight: Grain = basic unit of weight measurement1 grain of wheat = 60 mg.

480 grains = 1 oz = 1 oz , 12 oz = 1 lb

Converting within the Household System

• Determine the household equivalents

• Create a conversion factor

• Determine what format to write the conversion factor in

• Set up the conversions in an equation

• Perform the calculation

• Make sure the correct answer is determined by proving the work

From cups to quarts

8 cups = ____________ qt?

1 qt = 4 cups

X = 8 cups

X = (8 cups / 4 cups) / 1 qt = 2 qt

Conversions Between Metric and Household Systems

• Use the dimensional analysis method

• Determine the conversion factor between the two systems

• Determine what format to write the conversion factor

• Set up the conversions into an equation

• Perform the calculation

• Make sure you have the correct answer by proving the work

From pounds to kilogram

66 lbs = __________ kg.

1 kg = 2.2 lb

X = 66 lb

X = (66 lb / 2.2 lb) * 1 kg = 30 kg

Conversions between Metric and Apothecary Systems

• At times, you may need to make conversions between systems

• Need relationship between two systems to serve as a bridge

• Bridges are found in Table 6-6

• A conversion factor is a number used within either multiplication or division to change a measurement from one unit of measurement to it’s equivalent in another unit of measure

From grain to grams

240 gr = ____________g.

1 g = 15.4 gr

X = 240 gr

X = (240 gr / 15.4 gr) * 1 g = 31.2 g

Table 6-6

Temperature Conversions

• In the Fahrenheit system, water freezes at 32 degrees; water boils at 212 degrees

• In the Celsius system, water freezes at 0 degrees; water boils at 100 degrees

• Comparison: – 212 – 32 = 180– 100 – 0 = 100– 180 ÷ 100 = 1.8: ratio

between the differences– C = F – 32/1.8– F = 1.8C + 32

From Fahrenheit to Celsius

98.6 F = ___________ Celsius

Degree Celsius = (Degree Fahrenheit – 32)/1.8

Degree Celsius = (98.6 degree Fahrenheit – 32)/ 1.8 = 37 degree Celsius