Temperature Measuring System

The word "scale" is used with temperature measuring devices to identify a system of measurement. The scale must have definite "fixed point" or standards that always have the same value and may be reproduced easily.

Fixed reference points on temperature scales :
Certain standard temperatures are used that depend upon the physical conditions of a material. For example, the temperature at which water freezes was selected as one of the standard temperatures that can always be reproduced. This freezing temperature is influenced by extreme changes in pressure and only minutely by ordinary changes in atmospheric pressure. So, the freezing point of water at atmospheric pressure is the first of the standard temperatures which is used as a fixed reference point.

Another common temperature is the boiling point of water under normal conditions. Unlike freezing, the temperature at which water boils is greatly affected by atmospheric pressure. For this reason it is important to know what the pressure conditions are when boiling takes place.

The third fixed reference point is known as "absolute zero". This is the temperature at which scientists believe all movement of molecules ceases. Since their movement causes heat energy, it follows that if there were no movement there would no longer be any heat. Exprimenters have been able to produce temperatures within a few hundredths of a degree of this absolute zero.

Ambient and operating temperature :
The temperature of the surrounding air is knownts
in refrigeration work as "ambient temperature". This ambient temperature is usually expressed in degrees Fahrenheit, and is used whenever an "operating temperature" is required. The operating temperature is equal to the sum of the ambient temperature and the rise in temperature of the unit itself.

Suppose for instance that the temperature-rise as market on an electric motor spesification plate reads "55 degrees centigrated". Under normal operating conditions the motor can be expected to run hot to within 55 degree centigrade above the temperature of the surrounding air. If the ambient temperature is 30 degree centigrade the operating temperature is:
55 + 30 = 80 degrees centigrade
Temperatur Measuring Scales:
Changes of temperatur are measured on either the "Farhenheit" or "Centigrade"scale. These two basic scales are known as the "normal scales". There are two fixed points on both scales which indicate:
1. The temperature at which ice melt
2. The temperature at which pure water boils under a standard atmospheric pressure.
These fixed reference point may be duplicated exactly in any part of the world.
Measurements are made on either the Fahrenheit or the Centigrade
scales in units called "degrees". The degree is written and is follewed by a letter to show to what scale it applies. A reading like 25 degrees Farhenheit, means that the temperature is 25 degrees as measured on the Farhenheit scale. The letter "C" is used for the Centigrade scale. It is becoming standard practice to-omit the symbol and state simply "25 F".
The point at which ice mrlts is known as zero degrees on the Centigeade (C) scale and 32 degrees on the Fahrenheit (F) scale. The boiling point of pure water at standard pressure is 100 degrees centigrade and 22 degrees Fahrenheit. The locations of the freezing and the boiling points are shown in the illustration.

There are 180 Fahrenheit degrees between the freezing and boiling points of water, but only 100 Centigrade degrees. Thus, the diffrence between the melting point of ice and the boiling point of water ia 100 degrees in the Centigrade system and 180 degrees in Fahrenheit system. This means that each degree change in temperature on the Fahrenheit scale is equal ti five-ninths of a degree on the Centigrade scala.
Note from the drawing that the freezing point on the Centigrade scale starts at zero while the Fahrenheit scale begina with the number 32. These two numbers must be remembered when a value on one scale is changed to its equivalent value on the other scale.
Knowing that a degree on the Fahrenheit scale is smaller (5/9's)than a degree on the Centigrade scale, and that the freezing point of water is 0 degree Centigrade and 32 degrees Fahrenheit, it becomes a single matter to change readings from one scale to the other.

To change a Centigrade reading to Fahrenheit:
1. Multiply the reading by 9/5
2. Add 32 degrees

To change a Fahrenheit reading to Centigrade :
1. Subtract 32 degrees (or add a - 32 degrees)
2. Multiply by 5/9

Three examples are used to show how temperatures are converted (changed)from one normal scale to the other. The reason for saying "add a -32 degrees" will be shown when below zero readings are to be converted.

EXAMPLE 1 : Find the Fahrenheit temperature equal to 35 degrees Centigrade.
1. Multiply the reading by 9/5
35 degrees x 9/5 = 63 degrees
2. Add 32 degrees
63 degrees + 32 degrees = 95 degrees Fahrenheit. Ans.
EXAMPLE 2 : Find the Centigrade equivalent of 77 degrees Fahrenheit.
1. Subtract 32 degrees from the reading
77 - 32 = 45 degrees
2. Multiply the remainder by 5/9
45 x 5/9 = 25 degrees Centigrade. Ans

EXAMPLE 3 : Find the C temperature equal to -58 degrees Fahrenheit
1. Subtract 32 degrees from the reading.
Note: Because this is a minus reading, subtraction actually
increases the minus quantity because a minus 32 is added.
-58 + (-32) = - 90 degrees
2. Multiply by 5/9
-90 x5/9 = -50 degrees Centigrade Ans