Model Equation
Q: What is the model equation ?
A: The model equation, f, expresses the measurement procedure and the combination of the result through a mathematical expression. The expression describes the dependency of the output quantity on the input quantities. In most cases we deal with an analytical expression. The model equation may also be a group of such expressions which include correktions and/or correctionfactors for any systematic influences, which are sufficiently documented to allow for such correction. In doing so one may arrive at a more complicated relationship, which need not be expressed in terms of just one function.
Denote the output quantity with Y, and input quantities wiht X followed by a suitable index. Then in the general case, the model equation is of the form:
Example of model equation:
Measuring electrical power.
Consider an electrical resistor. The resistance is dependant on temperature.
Assume that there exists a linear temperature koefficient, 
, which describes
the influence of temperature on resistance. A voltage, V is applied over
resistor. The resistor has a refernce value of Ro at
the fixed temperature to.
The electrical power, P may then be written:
Using known physical relations a model equation has in this manner been established describing the proces of measuring electrical power.
Note that other procedures or other equipment to measure the power, P, would lead to different model equations.
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