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14
4.3 Conversion to "Digits"
A useful frequency related unit that is directly proportional to pressure is the "digit". These units
are related to vibrating wire frequency as:
The 8020-59 provides analog outputs that are directly proportional to digits.
Referring to the sample calibration in Figure 36 of Appendix E, a value in digits can be derived
from the voltage and current outputs as described below.
For the following "positive slope" transducers: 4000, 4100, 4150, 4200, 4210, 4300, 4350, 4360,
4400, 4420, 4425, 4450, 4650 and 4700; use Equation 2 for Analog Voltage Output (assumes
Vout is in volts (V)), or Equation 3 for Analog Current (4-20mA) Output (assumes Iout is in
milliamperes (mA).
Digits = (Vout * (Digits Span/5V)) + Minimum Limit
Digits = ((Iout - 4mA) * (Digits Span/16mA)) + Minimum Limit
For the following "negative slope" transducers: 4500, 4600, 4675, 4800 and 4900; use Equation
4 for Analog Voltage Output (assumes Vout is in Volts (V)), or Equation 5 for Analog Current
(4-20mA) Output (assumes Iout is in milliamperes (mA)).
Digits = Maximum Limit – (Vout * (Digits Span/5V))
Digits = Maximum Limit – ((Iout - 4mA) * (Digits Span/16mA))
Where:
Minimum Limit = Minimum limit parameter set in the channel specific setup screen of the
8020-59 configuration utility (default is zero)
Maximum Limit = maximum limit parameter set in the channel specific setup screen of the
8020-59 configuration utility (default is 25,000)
Digits Span = Maximum Limit - Minimum Limit
Digits = [Frequency (Hz)]² x 0.001
Equation 1 - Vibrating Wire Frequency to Digits Conversion
Equation 2 - Analog Voltage Output
Equation 3 - Analog Current Output
Equation 4 - Analog Voltage Output
Equation 5 - Analog Current Output
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