The observer shall operate the transmissometer in accordance with National Bureau of Standards Report No. 2588 (revised) and separate agency instructions. Where they differ, the separate instructions will take precedence, within the service concerned, over the National Bureau of Standards instructions.

These instructions are applicable at transmissometer stations not equipped with RVR computers. "Background Level" measurements shall be made whenever it is likely that the background level may be great enough to invalidate the RVV or RVR values determined from the equipment. These "Background Level" measurements are made as follows:

a.  Depress the BACKGROUND switch to the TEST position.

b.  Read the indicated value of the adjustment recorder trace.

c.  Release the switch button (to return it to NORMAL position).

d.  If the reading in "b" exceeds 1 percent of full-scale value, adjust the ZERO ADJUSTMENT control as necessary to reduce the reading to near zero.

In addition to the background measurement specified in paragraph 17-28, Transmissometer Background Measurements and Adjustments, daily and weekly checks of background level measurements shall be made.

17-30.   TRANSMISSOMETER READOUT CHECK

Concurrent readings of all readouts (e.g. in Tower, TRACON, and weather observer's office) shall be made once during each 8-hour shift. Simultaneous readings shall be made with the assistance of designated personnel at all locations and shall conform to the following standards:

a.  RVR computer readouts should display identical values at all positions and normally should agree within one reportable value with concurrent values derived from the recorder trace and appropriate tables.

b.  RVR meter readouts should display approximately identical values at all positions. Meter recorder comparisons are made with the system in a test condition, as follows, and involve the controls on the indicator panel:

(1)  Inform meter readout positions of the beginning of the test period.

(2)  Switch the ZERO control to TEST position.

(3)  Use the ZERO ADJUSTMENT control, if necessary, to zero the recorder.

(4)  Obtain meter readings from all positions in terms of the "±" departure of each meter needle from the "0" graduation of the meter scale in terms of the width of the needle at the circle separating the night equal to one needle width.

(5)  Reset ZERO switch to NORMAL.

(6)  Set the CALIBRATE switch to CALIBRATE.

(7)  Adjust the CALIBRATION ADJUSTMENT control for a recorder reading using the values in Figure 17-4, Transmissometer Conversion Tables, as follows:

(8)  Obtain meter readings from all positions in terms of the ± departure of each meter needle from the 1 7/16-mile DAY scale graduation mark on the meter face. The meter needles should not depart from the specified graduation mark on the meter face by more than four widths.

(9)  Adjust the CALIBRATION ADJUSTMENT control until the recorder record reading is 90.0 and reset the CALIBRATE switch to NORMAL.

(10)  Inform the meter readout positions of the end of the test.

(11)  Discontinue the use of meters that do not agree with calibrated recorder readings within the limits specified above. The observer shall notify the designated office of meters that fall outside these limits.

Figure 17-4. Transmissometer Conversion Tables

Almost all short-term fluctuations of RVV or RVR as displayed on the recorder and applicable meter or computer readout are real. The transmissometer is very sensitive to the varying light transmission characteristics of the atmosphere--variations which occur particularly under low visibility conditions. For this reason, caution should be used in rejecting visibility or visual range values as erroneous. A lamp which is failing can cause false transmission variations.

17-32.   TRANSMISSOMETER OUTAGE

The FAA is responsible for monitoring the performance and determining the operational status of all FAA RVR/RVV systems, for removing from and restoring to service such systems, and for advising all concerned of such actions. Whenever a malfunctioning system is suspected, the observer or designated tower person shall make arrangements for necessary electronic performance and visual checks on the system including, if feasible, a comparison of indicated values with conditions in the area of the transmissometer. If the malfunction is obvious or is verified by the checks, the system shall be taken out of service until repairs are made. Upon removal of the system from service, the following actions shall be initiated:

a.  Immediately notify all readout positions (either RVR or RVV) that data are no longer valid and shall not be used.

b.  Notify the designated electronics technician as quickly as possible.

c.  Enter a notation of the outage on the FAA maintenance form and show the date and time the system is taken out of service.

d.  Issue a Notice to Airmen (NOTAM).

17-33.   TRANSMISSOMETER RESTORED TO SERVICE

When the system has been restored to operational status and returned to service, all readout positions shall be notified that RVR or RVV data may now be used, and an appropriate notation will be made on the FAA maintenance form showing the date and time the system was placed back in service. The observer or designated tower person shall issue a NOTAM when the system returns to service.