Measurement Computing WaveView - Manual (Page 16)

WaveView, pg. 14 03-05-02 WaveView
Units -
This column allows you to change the voltage scale setting of each analog channel displayed
when the Scope option is selected. When a cell is selected, a selection box gives you a choice between volts
(V) or millivolts (mV). You can also enter user units and mX+b scales from this point. Making a selection
sets the choice into the individual cell or block of cells. This option has no effect on the Digital Input
channel. A discussion of mX +b follows.
Customize Engineering Units Dialog Box
From the Customize Engineering Units dialog box, you can enter values for m and b components of the
equation that will be applied to the data. There is also an entry field that allows you to enter a label for the
new units that may result from the mX+b calculation.
An example of mX + b equation use follows.
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)