MODIFICATION NOTE XX (for NWS REP)

DRAFT

+----------------+-----------------------------------------------------+
| SUBJECT:       | Bond FPU Chassis to Gauge Chassis, Installation     |
|                | Instructions                                        |
+================+=====================================================+
| PURPOSE:       | Reduce damage to load cell from surges              |
+----------------+-----------------------------------------------------+
| EQUIPMENT :    | Used with:                                          |
| AFFECTED       |                                                     |
|                | D111C                                               |
|                |                                                     |
|                | Replaces:                                           |
|                |                                                     |
|                | N/A                                                 |
+----------------+-----------------------------------------------------+
| PARTS REQUIRED | Locally supplied                                    |
| :              |                                                     |
+----------------+-----------------------------------------------------+
| SPECIAL TOOLS: | None                                                |
| REQUIRED       |                                                     |
+----------------+-----------------------------------------------------+
| MODIFICATION : | None                                                |
| PROCUREMENT    |                                                     |
+----------------+-----------------------------------------------------+
| EFFECTIVITY:   | All FPU sites                                       |
+----------------+-----------------------------------------------------+
| ESTIMATED      | 2 hours                                             |
| TIME: REQUIRED |                                                     |
+----------------+-----------------------------------------------------+
| EFFECT ON :    | None.                                               |
| OTHER          |                                                     |
| INSTRUCTIONS   |                                                     |
+----------------+-----------------------------------------------------+

**GENERAL:**

This Modification Note authorizes and instructs the NWS REPs to change
the physical configuration of all installed and to be installed Fischer
& Porter Upgrade (FPU) Units. This modification is necessary to provide
protection to the FPU load cell sensor and prevent damage caused by
surges or static charges. This modification should be carried out as
soon as practical, certainly before the next thunderstorm season, but
does not warrant emergency status priority.

![](media/image1.png){width="5.998611111111111in"
height="4.499305555555556in"}

As illustrated above, the load cell circuit floats within the body of
the F&P shell. Because the gauge and the FPU enclosure are typically
mounted with some separation, wind generated static charges and/or
lightning induced ground surges can generate large differential voltages
between the two devices. These large voltages will overcome and puncture
the delicate isolation barrier under the strain gauges in the load cell,
causing current leakage that renders the sensor inoperable.

Adding a bonding wire between the FPU enclosure and the F&P platform
that the load cell is mounted upon, will keep these two at the same
potential and eliminate the problem.

![](media/image2.png){width="5.998611111111111in"
height="4.499305555555556in"}

Installation:

-   ![](media/image3.jpeg){width="3.75in" height="2.8125in"}First check
    for low resistance (less than 1 ohm) between one of the PCB mounting
    screws on the face of the load cell, and the rear bolt as shown
    above. If more than one ohm, abrade the paint under the bolt head to
    get a good connection.

-   Then crimp a ring lug onto at least 10 AWG stranded wire and secure
    the lug under the bolt. The ring lug must be suitable for a ¼" stud.

-   Run the wire out the wire port, and over to the FPU enclosure.

-   ![](media/image4.jpeg){width="3.625in" height="2.71875in"}Cut to
    length, and strip ½" insulation from the end. Attach the bare end to
    the Gnd clamp under the center back of the enclosure.

-   Dress the wire as necessary to secure it in place.

This completes the modification.

**REPORTING INSTRUCTIONS:**

Report completed modification via CSSA, Inspection Report. Include the
following information in the report:

Equipment code of FPU in the EQUIPMENT block.

Check "Modified" in the Maintenance Performed section.

In the Remarks section, enter: Completed Modification XX, to bond FPU

chassis to gauge chassis.

A sample Report is provided as attachment A.

Mike Campbell

Chief, W/OS7

Attachment A Inspection Report Sample

Attachment A