UnitMath FAQ ( Miscellaneous )

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• What controls the output units when I don't use "as"?
• Can I make my own set of default units?
• Can I do currency conversions with UnitMath?
• How do I get the most from UnitMath?

What controls the output units when I don't use "as"? TOP

Can I make my own set of default units? TOP

1. Switch the current defaults to the set that is closest to what you are looking for.
   
2. Select all the defaults you want to change. To make multiple selections hold down the ctrl key.
   
   
3. In the top of the "Defaults" window, open the "Options" menu and select "Paste selected defaults". All the selected defaults will be pasted into the UnitMath window and will look like the following:
   UMDefault_area: sq m;
   UMDefault_length: m;
   UMDefault_volume: liter;
   
   
4. Modify the values to meet your needs and then select and evaluate them all.
   UMDefault_area: sq km;
   UMDefault_length: km;
   UMDefault_volume: cu m;
   
   
5. When you have the defaults the way you want them open the "Defaults" window and select "Save default units set as..." from the options menu. In the next window enter the name for this set of defaults and press the "OK" button.

Can I do currency conversions with UnitMath? TOP

Define Exchange Rates

franc: $/6.0235;
pound_sterling: $/0.6109;
yen: $/117.98;
dm: $/1.7959;

Make conversions

$100 as franc ~ 602.35 franc
$100 as pound_sterling ~ 61.09 pound_sterling
$100 as yen ~ 11,798. yen
$100 as dm ~ 179.59 dm

100 franc as yen ~ ( 1,958.56 to 1,958.76 ) yen
100 dm as franc ~ 335.4 franc
500 yen as dm ~ 7.61 dm

Use Currencies in calculations.

$5.26/pound as dm/kg ~ 20.8 dm/kg
$( 5.26 pm 0 )/pound as yen/kg ~ ( 1,368.13 pm 0.06 ) yen/kg
$280/ ounce_troy * lb as dm ~ 7,333. dm

How do I get the most from UnitMath? TOP

• Use Raw Data in Your Calculations
  
  Using raw data prevents unit and precision errors. Every time you manipulate data you add another chance for an error, so use raw numbers, units, and precision. UnitMath will easily and correctly handle all the unit conversions and precision calculations.
  
  For example if you want to find the end area of a pipe, you measure the diameter not the radius, so use the measured diameter in your calculation.

  diameter: 6 inch pm 1/16 inch;
  radius: diameter / 2;
  area: pi sq radius as sq cm ~ ( 179. to 182. to 186. ) sq cm

  This is particularly important with precision; for example 1.0 feet and 6.0 inches do not have the same precision.
  

  • ( 1.0 feet ) / 2 as inch ~ ( 5.7 to 6 to 6.3 ) inch
    That's a precision of pm 5%

  • 6.0 inches as inch ~ ( 5.95 to 6 to 6.05 ) inch.
    That's a precision of better than pm 1%

  Using the above values to find the volume of a sphere ( 4/3 pi r^3 ) we get three very different answers based on the data we use:

  • Based on the two significant digits in "1.0"

    4/3 pi cu ( 1/2 foot ) ~ 15. liter

  • Assuming 6.0 inches and doing the calculation correctly

    4/3 pi cu ( 6.0 inch ) ~ ( 14.5 to 15.2 ) liter

  • The correct answer.

    4/3 pi cu ( 1.0 foot/2 ) ~ ( 12.7 to 17.2 ) liter

• Use Variables

  Equations are much easier to understand and less likely to have errors if they use variables with descriptive names. For example, compare the same calculation with and without variables.

  With Variables:

  diameter: 48.6 inches;
  radius: diameter/2;
  height: 22.5 inches;
  volume: pi radius^2 * height;
  buckets: ( 4.5 to 5 ) gallons;
  volume as buckets ~ ( 36. to 38. to 41. ) buckets
  
  

  Without Variables:

  pi sq ( 48.6 inches / 2 ) * 22.5 inches as ( 4.5 to 5 ) gallons ~ ( 36. to 38. to 41. ) ( 4.5 to 5 ) gallons

  
  
• Get the answer in the units you need.
  
  If you want to know the number of buckets, then directly calculate the number of buckets. This is common sense, and easy to do with UnitMath. In the previous example I wanted the number of buckets not the number of gallons, liters or cubic feet ..., so I defined the size of a bucket and used the equation volume as buckets.
  
  
• Check the results precision
  
  Check that the precision of the result is good enough to answer your original question. Resist both under and over precision. To little precision and the answer will not be useful, and to much precision is a waste of time.
  
  For example:
  Given a 3/4 ton pickup, is it OK to fill the back with water?
  
  

  Very rough answer based on very rough data is not good enough to answer the question.

  volume:(4 ft pm ft )* ( 8 ft pm ft ) * (10 to 21) inch;
  volume * g/cc as 3/4 ton ~ ( 0.7 to 1.7 to 3.3 ) 3/4 ton
  
  

  Rough answer based on better data, good enough to say NO.

  volume:(4 ft pm 6 inch )* ( 8 ft pm 6 in ) * (14 to 21) inch; volume * g/cc as 3/4 ton ~ ( 1.3 to 1.9 to 2.8 ) 3/4 ton
  
  

  Very precise answer, but the time spent getting the accurate data was wasted.

  volume:(4 ft pm 1/16 inch )* ( 8 ft pm 1/16 inch ) * ( 16 inch pm 1/32 inch);
  wheel_wells: 2 * 35.0 gal;
  ( volume - wheel_wells ) * g/cc as 3/4 ton ~ ( 1.379 to 1.386 to 1.394 ) 3/4 ton

  
  
• Use your common sense
  
  Write equations and ask for results in units that you are familiar with. This allows you to have a feeling for the data. In these circumstances, your common sense will often tell you if the information is in the right range. When you need to share the results with others use UnitMath again to convert the units.
  
  Example:
  
  A thermometer gives your body temperature as 244.26 Kelvin. Is that reasonable?
  
  With the temperature in Kelvin the answer for most people is "How would I know". When the temperature is converted to Fahrenheit or (Centigrade) -20 degF (-28.89 degC) you can apply your everyday experience and know the reading is wrong.
  
  244.26 K as degF ~ -20.0 degF
  244.26 K as degC ~ -28.9 degC

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