What scale has no negative values?

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A kelvin (K), is the base unit of temperature in the International System of Units (SI).

1 kelvin = -272.15 Celsius

1 kelvin = -457.87 Fahrenheit

Unlike the Fahrenheit and Celsius scales, the Kelvin scale does not have degrees. Each increment is just 1 kelvin. The Kelvin scale is referred to as an absolute temperature scale, and 0 kelvin is set at absolute zero, the temperature at which almost all molecular motion stops. Other notable temperatures on the kelvin scale are 273.15 K, the temperature at which water freezes, and 373.15 K, the temperature at which water boils.

Each unit in the Kelvin scale is the same size as a degree in the Celsius scale. Since 0 K is at absolute zero, there are no negative values in the Kelvin scale. This property of the Kelvin scale is one reason that it is widely used in scientific contexts. It is easier to compare temperature values when there are no negative numbers on the scale.

Converting kelvins to Celsius and Fahrenheit

Since kelvins and degrees Celsius share the same unit size, it is relatively easy to convert between a value in kelvins and a value in °C. To convert from a kelvin to a °C, just subtract 273.15. To convert from °C to kelvins, add 273.15.

Example

Convert 25°C to kelvins:

25°C + 273.15 = 298.15 K

Converting between kelvins and Fahrenheit involves converting either unit to °C first. To convert from kelvins to °F, we first convert to °C by subtracting 273.15. Then we multiply by

Examples

1. Convert 295.3722 K to °F:

(295.3722 K - 273.15) × + 32 = 72°F

2. Convert 212°F to kelvins:

(212°F - 32) ÷ + 273.15 = 373.15 K

By definition, ratio scales include an absolute zero point. If it's possible to have negative quantities than it's not an absolute zero.

For example, in the physical realm, you can have 0 or 3 apples but you can't physically have -1 apples. Because the concept you are measuring is the existence of apples it's possible to use a ratio scale.

But if the thing you were measuring is the abstract concept of possession of apples, you can have -1 apples. But this means it's no longer on the ratio scale but is instead on the interval scale. Since it's possible to owe 10 apples or 10M apples, it's no longer possible to define an absolute zero. Any choice of zero becomes arbitrary.

As another example, the Kelvin temperature system is a ratio scale and 0°K is representative of an absolute physical inexistence of kinetic energy. While -273.15°C and -459.67°F are equal to 0°K, °C/°F are considered interval scales. That's because they are actually representing the concept of existence of energy in relationship to some criteria, like the freezing temperature of water. This again makes zero an arbitrary point which means it's impossible to define an absolute zero.

Without an absolute zero, ratios of values have no meaning.

In Stevens, S.S., 1946. On the theory of scales of measurement. he says:

If, in addition, a constant can be added (or a new zero point chosen), it is proof positive that we are not concerned with a ratio scale.

More importantly, though, Steven's framework is a tool for thinking, not an absolute truth. You can read about how it fails in real data in Velleman, P.F. and Wilkinson, L., 1993. Nominal, ordinal, interval, and ratio typologies are misleading. The American Statistician, 47(1), pp.65-72.:

Many of the discussions of scale types, and virtually all of the mathematical results, treat them as absolute categories. Data are expected to fit into one or another of the categories. A failure to attain one level of measurement is taken as a demotion to the next level. However, real data do not follow the requirements of many scale types.

British scientist William Thomson (Lord Kelvin) invented the Kelvin scale on theorising that if the temperature was low enough, gas molecules would stop moving and their volume would become zero. He defined this temperature of -273.15 °C (or 0 K) as 'absolute zero' and used it as the lowest point for his scale. Thus, there are no negative numbers on the Kelvin scale.

The volume of gases decreases with a decrease in temperature. Scientists theorised that at constant pressure, an ideal gas would reach zero volume at the absolute zero of temperature. In reality, gases condense to a liquid or a solid before reaching absolute zero. To convert Celsius (°C) to Kelvin (K) we add 273.15. For example, 0°C = 273.15 K, or 25°C = 298.15 K