Appendix G : Unit Conversions
In a current loop, any length of cable and number of devices can be connected in
series provided that the following conditions are met:
• All devices have floating differential inputs (both leads of signal input are
ungrounded).
• The total loop resistance does not exceed 500 Ω.
G.1 Converting Current to Voltage
Recording devices with voltage inputs can be converted to current inputs by shunting
their input terminals with an appropriate resistor. The resistor value is calculated as
follows:
Resistor (Ω) = 50 x input voltage range (in volts) of recording device
Example: A chart recorder with an input range of 1V uses a 50 Ω (1%, 1W) resistor
across the recorder input.
G.2 Analog Outputs – Converting ppm to mA (or Vice Versa)
To convert mA (milliampere) into ppm (or vice versa), use the following formula:
ppm = 125 x (mA - 4); or mA = 4 + (ppm÷125)
Example: For a 4.8-mA current, the level of gases in oil is 100 ppm. Likewise, if the
display reads 250 ppm, then the current value is 6.0 mA.
To Next Page
Loading...
