Fluid Components International MT86_MT86HT Manual Troubleshooting - Manual (Page 3)

Doc. No. 003162 Rev. F 5 - 3 Models MT86, MT86HT
CHAPTER 5 - TROUBLESHOOTING FLUID COMPONENTS INTL
Verify Faulty Flow Elements
Caution: Inform all personnel involved with monitoring the flow media that the flowmeter output readings
may change. The change of output could affect the plant peripheral equipment and or alarms.
To identify a bad flow element rotate the Head-Select switch, SW2, through its numbered positions. Pause at each
position to monitor a red Light Emitting Diode (LED), CR1. CR1 is located on the MT86 Control Board. If a
problem exists with a sensing point, the LED will flash. The LED may also flash if there is no sensing point that
corresponds to the switch position. If the Head-Select switch is pointing to a valid sensing point and the LED is
flashing the sensing point is bad.
Verify Resistance Readings
Use Figure 5-1 and Table 5-2 to determine if the flow elements are wired incorrectly or have failed. Turn the power
OFF to the flow transmitter. Unplug the problem flow element at the Input Board. Measure the resistances
described below by touching the DMM test leads to the terminal screws. (Remember to reconnect the flow element
when the troubleshooting is finished.)
Note: If the system process does not allow the flowmeter power to be disconnected or the flow elements to be
unplugged then proceed to the section, In-Depth Troubleshooting - Voltage Measurements.
All resistances in Table 5-2 are based on a temperature of 32°F ( 0°C). Resistances across the ACT and the REF
RTDs for an MT86 are approximately 1000 ohms, resistances for an MT86HT are approximately 100 ohms. The
resistances will continue to increase for higher temperatures at the sensing points. Resistance values will vary with
temperature.
Table 5-2. Terminal Plug Resistances
The resistance of the active RTD will be greater than the resistance of the reference RTD whenever the heater is on
and the flow rate is below the high-limit flow. Also if the flowmeter has been on for some time, the resistance of the
active RTD will be greater than the reference until it cools down.
The flow element cable has a shield that ONLY connects to the flow transmitter side of the cable. There is no shield
connected to the flow element.
If one sensing point appears to be an open circuit and the other sensing point appears to be twice the resistance, a
wiring problem probably exists.
Voltage and current checks can be made on the flow elements. Before checking the flow element voltages and
current, other voltages need to be verified first. See the following paragraphs for the proper sequence of checks to
make.
If the flow element is bad, replace it with a spare flow element. Return the bad flow element to FCI for repair and, or
replacement. See Appendix C for the Customer Service return procedure.
*For the MT86HT flow element divide by 10.
Figure 5-1. Terminal Plug
Pin
Number
Approximate
Resistances
2 to 3 1000 Ohms*
2 to 4 1000 Ohms*
2 to 5 1000 Ohms*
2 to 6 1000 Ohms*
8 to 7 220 Ohms