Railway Preservation News

GW Air Flow Meter
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Author:  RCman2626 [ Sun Jul 26, 2020 11:24 pm ]
Post subject:  GW Air Flow Meter

Have a 26L equipped locomotive with a Graham White 796-600 Series Air Flow Method (AFM) Indicator. The meter mysteriously stopped working. I have no way to calibrate or test the unit to check for internal failure. My few questions are...which volume of air does it get it's reading from? I know it obviously monitors movement in the BP, but does it use MR air to do so? What internal moving parts might be suspect to failure? Never had one of these quit on me before!

Author:  jvanniew [ Mon Jul 27, 2020 9:21 am ]
Post subject:  Re: GW Air Flow Meter

Typically it measures a differential using a choke or A-19 on the MR feed to port 30 on the 26C.

Author:  hadder [ Mon Jul 27, 2020 12:01 pm ]
Post subject:  Re: GW Air Flow Meter

We only have one of these, and have not had any trouble with it other than performing our calibration checks every 92 days.

If it is not working, either a blocked line connection or an internal problem. If blowing out the line doesn't work, consider removing it entirely. They are not required in most operations and really aren't that valuable on short passenger trains.


Author:  E.B. Levin [ Mon Jul 27, 2020 5:18 pm ]
Post subject:  Re: GW Air Flow Meter

As stated in the other replies the AFM can be piped into an A19 block that is flanged connected into the MR line going into the automatic pipe bracket or if the MR pipe has a 19/64 choke applied it can be piped from the MR line below the choke to the middle port on the back face of the 26C automatic pipe bracket.

The AMF is required to be tested under FRA 229.29 at 92 day intervals. The test is performed by using a dummy coupling with a calibrated choke in the train line. The test coupling is available from Strato Inc.

Typically the AFM fails internally and we send back to GW/WABTEC for repair. Some causes of erratic or erroneous operation are leaking O rings in the gauge base and leaking 1/4 pipe connections to the gauge. Typically if the 19/64 MR choke is blocked it affects the entire AB system with slow recharge. I can only recall two times in 30 years I had a blocked MR choke, once when a slug of debris came up through the bottom and once from malfeasance during a 3 year air cleaning. Both times resulted in extremely slow BP recovery times.


Author:  RCman2626 [ Tue Jul 28, 2020 11:35 pm ]
Post subject:  Re: GW Air Flow Meter

Thanks guys. Removed the unit and found that one of the orifices was partially blocked and the small gears inside the gauge had some gunk in it. Was able to free it up very carefully. But one of the rubber o rings inside was also cracked and broke when I attempted to remove it (see pic).

Eric, during your 92 day inspection, what reading should you get on the AFM gauge with the calibration dummy orifice hooked to the BP? I was going make my own tester with an old dummy coupling with a 7/32 hole drilled in it.

C768C217-E93F-4552-B434-149FBB59EE1F.jpeg [ 121.6 KiB | Viewed 664 times ]

Author:  E.B. Levin [ Thu Jul 30, 2020 8:40 pm ]
Post subject:  Re: GW Air Flow Meter

We test with a Strato Inc. calibrated choke dummy coupling. With a 90 pound BP and MR pressure at 130 and rising the AFM should indicate 60 CFM.

All you ever wanted to know about the history of the AFM can be found here:

https://railroads.dot.gov/sites/fra.dot ... OR%20T.PDF


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