There seems to be quite a bit of confusion around using temperature units in Mathcad. When we describe the implementation and the proper way to use temperature units, the question of “Why on earth would you do it like that?” often arises. Allow us to explain…
Simple units conversion
In most cases, unit conversions are simple: Converting a quantity, say length, from one unit (e.g. inches) to another (e.g. meters) involves multiplying the quantity by a factor (namely 0.0254). This is very convenient because the following calculations can be done without any ambiguity:
Why am I even writing about this? This stuff is trivial.
A paradox with temperature units
Well, when it comes to temperature, things get a bit tricky. Consider a set of similar equations involving temperatures in degrees Kelvin and Celsius. Define
The result of subtracting the two will depend on interpretation of what these quantities mean. For example, if T1 represents the air temperature of 0°C, then the expression T:=T1-T2 would indicate the result of temperature change by 10°C, yielding air temperature of -10°C. Expressing this temperature in different units will yield 263.15K, 14°F, etc.
However, if we look at the expression ΔT:=T1-T2, and interpret it to mean the temperature difference of air (at the temperature of 0°C) and a heating element (at temperature of 10°C), then the temperature differential would be -10°C. Expressing this temperature differential quantity in other units will yield -10K, -18°F, etc.
Something is not adding up here! We assigned the same values to T1 and T2, and got different results for the difference T1-T2 when expressed in K, °F, etc., because we stored the first result as an absolute temperature value and the second result as a temperature differential.
Perhaps another example will clarify the matter. Consider the case of a hot summer day. At noon, the temperature is T1:=100°F. The forecast predicts the temperature will drop by 30°F by sunset, so intuitively, we might set T2:=30°F. Now, also intuitively, we might define T:=T1-T2 and expect to get 70°F for the temperature at sunset. When we implement this calculation in Mathcad using absolute temperature values for both T1 and T2, we don’t exactly get 70°F:
But, if we define T2 as a temperature difference, then our sunset temperature is what we intuitively expected:
The apparent paradox lies in the fact that there are two different conversions between degrees Kelvin and degrees Celsius/Fahrenheit depending whether we are referring to the absolute temperature of an object or a substance or a temperature difference.
When converting from °C to K, if we interpret a quantity as the absolute temperature of an object or a substance, we use conversion K = °C + 273.15. However, if the quantity represents a temperature difference, the conversion factor is 1. Thus in K, the above equations from the very first example would look like:
The second example would look like:
Is there a problem?
Before I go into how to resolve the ambiguity in temperature conversions, is this a problem? Usually not if doing calculations by hand or using only degrees Kelvin. However, if one wants to use a calculation tool to do such calculations and automatic conversions, the tool needs to know what the author had in mind. This is useful to automate the process of conversion enabling the author to display results in desired units, or to catch the possible errors.
When communicating with engineers in different countries, it’s important to enable conversions to a unit system that is most commonly used. This makes documents easier to read. The units should always be explicit, i.e. never say temperature is 10, as this is guaranteed to cause errors.
Resolving the ambiguity
If we look closely at the examples above, the hints to interpret T1, T2 and the result were subtle. In one case, I used T to name the result, implying it is a temperature of an object or piece of matter. And in the other I used ΔT to hint that the result was a temperature difference. That’s too subtle and it would not work in situations like T+(T1-T2).
Algebra of temperatures and temperature differences
One approach comes from an observation that it (almost – see below) never makes sense to add two temperatures together. Using T for temperature and ΔT for temperature difference, we can derive the following simple algebra:
T1 + T2 yields an error
T1 – T2 yields ΔT
ΔT1 + T2 yields T
ΔT1 – T2 yields an error
T1 +/- ΔT2 yields T
ΔT1 +/- ΔT2 yields ΔT
Such system would still require declaring some of the quantities as either a temperature or temperature difference, but would help catch mistakes. It also has a drawback in not allowing temperatures to be added in computing an intermediate result such as average temperature.
A new unit for temperature difference
An alternative approach takes directly from the difference in converting from degrees Celsius to Kelvin when talking about a temperature and a temperature difference. Specifically, one can define two units:
°C – for the temperature in degrees Celsius, and
Δ°C – for the temperature difference in degrees Celsius
Thus we can define T2 as either:
Note that, for degrees Kelvin, this distinction is not necessary. With this system we can disambiguate the example above:
Assuming that T2 is a temperature difference,
And assuming T2 is a temperature (not a temperature difference),
This solves the problem, but requires the engineer to be careful in specifying different units for temperatures and temperature differences – something that we don’t normally worry about. It is mathematically correct, but it is also tricky…
Visit our dedicated page focused on engineering unit conversion.