Measurement Computing WaveBook rev.5.3 - Manual (Page 97)

WaveBook/512A, /516, /516A , /516E 897895 WaveBook Operation Reference 4-27
Engineering Units Conversion Using mx + b
Most of our data acquisition products allow the user to convert a raw signal input (for example, one that is
in volts) to a value that is in engineering units (for example, pressure in psi). The products accomplish this
by allowing the user to enter scale and offset numbers for each input channel, using the software associated
with the product. Then the software uses these numbers to convert the raw signals into engineering units
using the following “mx + b” equation:
Engineering Units = m(Raw Signal) + b (1)
The user must, however, determine the proper values of scale (m) and offset (b) for the application in
question. To do the calculation, the user needs to identify two known values: (1) the raw signal values, and
(2) the engineering units that correspond to the raw signal values. After this, the scale and offset
parameters can be calculated by solving two equations for the two unknowns. This method is made clear
by the following example.
Example
An engineer has a pressure transducer that produces a voltage output of 10.5 volts when the measured
pressure is 3200 psi. The same transducer produces an output of 0.5 volt when the pressure is 0 psi.
Knowing these facts, m and b are calculated as follows.
A - Write a pair of equations, representing the two known points:
3200 = m(10.5) + b (2)
0 = m(0.5) + b (3)
B - Solve for m by first subtracting each element in equation (3) from equation (2):
3200 - 0 = m(10.5 – 0.5) + (b - b) (4)
Simplifying gives you: 3200 = m(10) (5)
This means: m = 320 (6)
C - Substitute the value for m into equation (3) to determine the value for b:
0 = 320 (0.5) + b (7)
So : b = - 160 (8)
Now it is possible to rewrite the general equation (1) using the specific values for m and b that we just
determined:
Engineering Units = 320(Raw Signal) - 160 (9)
The user can then enter the values of m and b into the appropriate location using the facilities provided by
compatible data acquisition software, for example: WaveView, DaqView, Personal DaqView, LogView,
and TempView. The software uses equation (9) to calculate signal values in engineering units from that
point on.