AA Meters - Rating Problem

To: "E  - All WRD Employees"<distribution@usgs.gov>
Subject: AA Meters - Rating Problem
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Date: Fri, 01 Nov 1996 14:14:21 -0600
From: "Dorothy E Greenwood, Secretary (Steno), Stennis Space Center, MS" <degreenw>

Memorandum
To:     	All WRD Employees
Through:	Chief, Hydrologic Instrumentation Facility
From:   	Acting Chief, Field Service and Supply Section, HIF
Subject:	Price-Type AA Current Meters--Standard Rating Problems

The purpose of this memorandum is to inform you that the HIF is currently unable to supply Price-Type AA current Meters because they are not meeting the standard rating and briefly describe the nature of the problems encountered and discuss the actions being taken to resolve the problems.

Difficulties arose when an order of meters arrived at the HIF in late 1995. Due to changes in personnel and upgrading of tooling equipment by the manufacturer, various parts of these meters were made slightly different than previous meters. This resulted in the meters performing differently, and having ratings that did not fall within the tolerance limits of the standard rating.

Personnel at the HIF and the OSW Lab have been striving to find the variations in the parts that are causing the rating changes, but it is a slow process. The old standby, the spin test, checks only the pivot bearing friction, which mostly affects low-speed performance. The new meters give good results on a spin test, and rate within tolerance at the slower speeds. The rating variations with the new meters are at speeds 2.2 feet per second and faster.

A known "good meter" was tested in the tow tank along with a meter that varies from the standard rating to find which parts were causing changes in performance. Parts that were suspected of causing performance variations were switched between the meters, and they were tested again. Variations that affect the rating performance have been found in the bucket wheels, the contact chambers, and the yokes.

	Bucket wheels--The new bucket wheels appearance seem precise and are made extremely close to the specifications; however, when installed on the known "good meter," they do not perform to the standard rating. We are working with the manufacturer to "tweak" the new design to produce bucket wheels that meet the standard rating, and are making good progress. 
	
	Contact chamber--The variations in the contact chamber that affect the rating performance seem to involve the finish on the bore of the upper bearing, or perhaps an accumulation of individual alignment tolerances when the contact chamber is installed to the yoke. We are still investigating the exact cause of this problem as the differences are subtle and hard to define.
	
	Yoke--The variations in the yoke appear to involve warpage of the upper and lower yoke arms between the time the yoke is cast and the time that it is machined. A check of the alignment using the yoke alignment tool shows that holes are perfectly in line; however, some of the yokes may be slightly warped so that, although the holes are in line, the bucket wheel assembly is not held at the proper angle to the water flow. Again, the differences are subtle and not easy to find.

We apologize for the delay and any inconvenience this may cause. It is important for us to ensure that these meters meet the USGS requirements, including the standard rating and parts interchangeability. This is also important because there are no assurances that commercially available "off-the-shelf" meters conform to the USGS requirements or meet the standard rating.

As much as some of you need AA meters and as much as you want the HIF to ship them, the HIF cannot and will not send you a meter that is not working properly and which is not meeting the standard rating. We ask for your support while we resolve these problems. As soon as we obtain meters that are acceptable, we will make another announcement by electronic mail and ship the meters out immediately.

If you have any questions, please call me on 1-800-382-0634, extension 3304, or Jim Jelinski on extension 1547. Your cooperation and patience is appreciated.

			/s/ Frank S. Henry