FLUID COMPONENTS INTL
CHAPTER 5 - TROUBLESHOOTING
Model ST98 Flow Meter
5 - 6
Doc. No. 06EN003291 Rev. A
The resistance of the Active and Reference sensor will depend on the temperature of the sensing element. Refer to
the "Temperature Versus Resistance" Table in Appendix A.
When measuring the resistance of the flow element through a long remote cable, the cable resistance must be
subtracted from the measurement. The residual resistance of the DVM and its leads should also be considered.
See Table 5-4 to calculate the resistance for copper wire. Each wire gauge size number increase represents a factor
of 1.26 resistance increase over the previous size. Moving three gauge sizes higher doubles the resistance. To
convert the table values to ohms per meter, multiply the value by 0.0394.
AWG Size
Ohms Per 1000 Feet
14
2.52
15
3.18
16
4.02
17
5.05
18
6.39
19
8.05
20
10.1
21
12.8
22
16.2
23
20.3
24
25.7
Check the Flow Element Voltages
Use the following voltage measurements if power cannot be easily removed from the instrument or if resistance
measurement fail to resolve the problem. Be sure the sensor heater current is set to 75 mA LO by pressing [O] on
the FC88 and selecting the heater current. Be sure to set the heater current back to where it was before beginning
this procedure. Make the voltage measurements found in Table 5-5 at terminal strip TS2 on the flow transmitter,
or on the Local Terminal Block.
Table 5-5. Approximate Flow Element Voltages AT 70° F
LUG OR PIN NUMBER
VOLTAGE*
(7) HTR EXC TO (8) HTR RTN
Approximately 6.79 VDC
(4) ACT SEN TO (6) ACT EXC
Approximately 0.00 VDC
(3) REF SEN TO (5) REF EXC
Approximately 0.00 VDC
(5) REF EXC TO (1) RTD GND
Approximately 2.20 VDC
(6) ACT EXC TO (1) RTD GND Approximately 2.21 - 2.82 VDC**
(4) ACT SEN TO (3) REF SEN
Approximately 0.24 VDC**
Cable resistance of the remote flow element will affect the TS2 voltage readings at the
electronics enclosure.
*Voltages varies with Temperature and Flow and the Sensor Heater Current Selection.
**Voltage will vary with the process flow rate.
LUG OR PIN NUMBER
RESISTANCE
(7) HTR EXC TO (8) HTR RTN
110 - 118 OHMS
(4) ACT SEN TO (2) GND SEN
1.1K OHM
(3) REF SEN TO (2) GND SEN
1.1K OHM
(3) REF SEN TO (4) ACT SEN
2.2K OHMS
(1) RTD GND TO (2) GND SEN
0 OHMS
(4) ACT SEN TO (6) ACT EXC
0 OHMS
(3) REF SEN TO (5) REF EXC
0 OHMS
SHIELD TO HTR RTN (8) (REMOTE INSTALLATIONS)
Table 5-3. Flow Element Resistance at TS2 or Local Terminal Block
Table 5-4. Resistance Versus Wire Size (AWG)
