Spectrum Controls 1746sc-NI8u - Manual (Page 51)

Chapter 4: Channel Configuration, Data, and Status 39
The proportional counts are scaled to fit the defined temperature,
voltage, or current range. The input signal range is proportional to your
selected input and scaled into a (-32,768 to 32,767) range.
Using Scaled-for-PID and Proportional Counts
The universal module provides eight options for displaying input channel
data. These are 0.1°F, 0.1°C, 1°F, 1°C, 0.01 mV, 0.1 mV, Scaled-for-PID,
and Proportional Counts. The first six options represent real Engineering
Units provided/displayed by the 1746sc-NI8u, and do not require
explanation. The Scaled-for-PID and Proportional Counts selections
provide the highest NI8u display resolution, but also require you to
manually convert the channel data to real Engineering Units.
The equations below show how to convert from Scaled-for-PID to
Engineering Units, Engineering Units to Scaled-for-PID, Proportional
Counts to Engineering Units, and Engineering Units to Proportional
Counts. To perform the conversions, you must know the defined
temperature or millivolt range for the channel’s input type. Refer to the
Channel Data Word Format table on the following page. The lowest
possible value for an input type is S
LOW
, and the highest possible value is
S
HIGH
.
It is important to note that the Scaled for PID and proportional counts
format do not linearize inputs that are not linear. The module assumes that
current and voltage inputs are linear prior to insertion into the universal
module’s input stage. Thermocouple inputs are cold junction compensated,
and are linearized in their temperature conversion process through the
NIST ITS-90 tables. RTDs are converted from their resistance value to
degrees according to their associated IEC or JISC standards.
Scaling Examples Scaling Examples Scaling Examples Scaling Examples Scaling Examples
Scaled-for-PID to Engineering Units
Equation: Engr Units Equivalent = S
LOW
+ [(S
HIGH
-S
LOW
) x (Scaled-for-PID value displayed/16384)]
Assume type J input type, scaled-for-PID display type, channel data = 3421.
Want to calculate °C equivalent.
From Channel Data Word Format table, S
LOW
= -210°C and S
HIGH
= 760°C.
Solution: Engr Units Equivalent = -210°C + [(760°C-(-210°C)) x (3421/16384)] = -7.46°C.
Engineering Units to Scaled-for-PID
Equation: Scaled-for-PID Equivalent = 16384 x [(Engineering Units desired -S
LOW
)/(S
HIGH
-S
LOW
)]
Assume type J input type, scaled-for-PID display type, desired channel
temp. = 344°C.
Want to calculate Scaled-for-PID equivalent.
From Channel Data Word Format table, S
LOW
= -210°C and S
HIGH
= 760°C.
Solution: Scaled-for-PID Equivalent = 16384 x [(344°C - (-210°C))/(760°C - (-210°C))] = 9357