KROHNE BM 102 EN User Manual
Page 18
18
BM 102
Load impedance R
loop
Loop resistance, R
loop
R
HART
+ R
cable
+ R
ammeter
Min. R
loop
0
ohms
Max. R
loop
750
ohms
R
HART
resistance for HART
®
communication
250 ohms (recommended)
Line A = minimum voltage at the BM 102 terminals
Line B = voltage drop caused by a 250 ohm loop resistance
Example for calculating the required power supply:
The voltage drop is tested at 22 mA.
U
power min. 22
= 22 mA x load impedance + Uinput min. 22
U
power min. 22
= 22 mA x 250 ohms + 10 V = 5.5 V + 10 V = 15.5 V
In order to cover the whole current range, the voltage drop must also be tested at 4 mA:
U
power min. 4
= 4 mA x load impedance + Uinput min. 4
U
power min. 4
= 4 mA x 250 ohms + 18 V = 1 V + 18 V = 19 V
At a load impedance of 250 ohms a power supply voltage of 19 V is sufficient to energize the
device current range of 4 to 20 mA.
2.2.2 Hazardous-duty
version
The certified device may only be used with other ’Ex’ certified equipment. Refer to the BM 102
MICROFLEX KEMA 00 ATEX 1101X Supplementary Installation and Operating Instructions
supplementary instructions for further information on installing gauges approved for use in
hazardous locations. This document is available from your local KROHNE Sales office or on the
“Download Centre” on KROHNE’s website http://www.krohne.com/.