Liquid Controls SP3000 - Manual (Page 46)

46
PRINCIPLES OF OPERATION
9.1 GENERAL
This chapter explains in detail how the instrument calculates the mass flow based on specific data input.
Each parameter is updated at a 2Hz rate. Therefore, all references to the machine’s “interval” in the
following descriptions indicates a period of 0.4 seconds.
9.2
TEMPERATURE CALCULATIONS
If an RTD is used as a temperature transducer, the computer runs a constant current through both the RTD
and the internal semi-precision resistor in series with it. (See section 1.52) The computer measures the
voltage drop across each individually and calculates a ratio; thus, determining the actual resistance of the
RTD. A look-up table is used to linearize and convert the ohmic reading to temperature in degrees. This
value is checked against keyed in alarm limits and the alarm relay is activated if necessary.
Any other type of temperature transducer must present a voltage or current to Channel 1 of the computer.
This value is checked against a keyed in temperature parameters to calculate the actual temperature.
9.3
PRESSURE CALCULATIONS
Any pressure transducer must present a voltage or current to Channel 2 of the computer. The raw
pressure value is digitized from Channel 2 and then scaled according to the two keyed in pressure
parameters to calculate the actual pressure. The value is checked against keyed in alarm limits and the
alarm relay is activated if necessary.
9.4
FLOW CALCULATIONS
The value provided by the digital input pulse accumulator is divided by the K-Factor to find the
instantaneous flow over the interval. The result is then filtered according to the keyed in flow filter
parameter to get uncompensated flow.
The following definitions and calculations are used to determine Volumetric, Mass and Heat Flow.
Definition of Terms:
BR:
Base Reference Temperature entered by user.
c: Thermal coefficient of expansion for the media (X 10
-6
).
DENSITY: Density as calculated from parameters and inputs.
DP: Differential pressure in inches of water calculated from flow input.
FLOW: Uncompensated flow calculated from input.
HZ: Input frequency in cycles per seconds.
K: The meter calibration factor in pulses per cubic foot or gallon.
K1: Factor used in Orifice/Pitot calculations.
kPa: The absolute pressure (Gage pressure + Barometric pressure).
MASS: Mass flow rate calculated using temperature, pressure and flow. Del Heat equation uses
temp 1 for Mass calculations.
PSIA: The absolute pressure (Gage pressure + Barometric pressure).
PSIG: Upstream gage pressure calculated from input.
SG: Specific Gravity as entered by user with respect to Standard conditions or the Base
Reference temperature.
SH: Specific Heat as entered by user or from Steam Tables.
SV: Specific Volume from Steam Tables.
TEMP: Temperature as calculated from input.
UCFLOW: Uncompensated flow as determined by calculations.
VOL: Volumetric flow as determined by calculations.
Z: Gas compressibility factor as entered by user.
%: Percentage as a decimal point of full scale of the raw input.