Varec 8200, Инструкция по установке и эксплуатации, Страница: 22 / 38

Installation
14 Installation and Operations Manual
2.10 Initial Calibration
Transmitters are calibrated at the factory for full scale range operation. The scale may be
selected by the user to any range between 45% and 100%. Refer to Chapter 3, Configuration on
page 17 for calibration procedures. Calculate the loop resistance of the installed
instrumentation for full-scale operation before installing the wiring. Then check it after the
wiring has been installed.
2.11 Loop Resistance
It is necessary to compute the total circuit resistance to determine the correct gauge of field
signal wire to use. The maximum allowable total loop resistance (the sum of the wire line and
receiver load resistance) may be calculated with the aid of the chart in Figure 2-10 on page 15.
The following two equations can be used to calculate maximum loop resistance (line and load):
4-20 mA Current Output
MAX RESISTANCE = 50(DC POWER SUPPLY VOLTAGE -15)
10-50 mA Current Output
MAX RESISTANCE = 20(DC POWER SUPPLY VOLTAGE -15)
Note If the transmitter uses an internal power supply, the value of the DC POWER SUPPLY
VOLTAGE entered into the equation is 30 VDC.
Varec recommends that 22 gauge or larger size wiring be used. (The larger the gauge number
the smaller the wire diameter.) Typical telephone field wiring is 21 gauge or larger. Mechanical
field conditions affect the wire choice as well as cost.
As an example, determine the minimum gauge of wire that is required to connect a transmitter
(30 VDC, 4-20 mA range) located 3000 feet from the receiving device. The receiving device load
is 100 ohms.
From Figure 2-10 on page 15, it is seen that the maximum loop resistance is 600 ohms.
According to wire data charts, 22 gauge wire has a resistance of 15.14 ohms per 1000 feet.
Twice the wire run is 6000 feet, the total loop distance. The wire resistance is 91 ohms. The total
loop resistance (receiving device plus loop resistance) is 191 ohms, well under the 600 ohm
maximum.