Recommended Starting Torque for Shaft Encoders

To: "A  - Division Chief and Staff",
        "B  - Branch Chiefs and Offices",
        "FO - State, District, Subdistrict and other Field Offices",
        "PO - Project Offices"
cc: " , WRD Archive File, Reston, VA ",
        "Annette L Ledford, Clerk Typist, Reston, VA "
Subject: OSW Technical Memorandum No. 96.12
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Date: Wed, 25 Sep 1996 13:04:33 -0400
From: "Annette L Ledford, Clerk Typist, Reston, VA "

In Reply Refer To:                                 September 18, 1996
Mail Stop 415


SUBJECT:  Recommended Starting Torque for Shaft Encoders

In the past, gaging stations typically used a stilling well with a
12-inch diameter float attached to an A-35 recorder. The flotation 
force provided by the large-diameter float was needed to minimize 
errors caused by friction in the mechanism that moved the pen on the 
recorder. Today, data are typically recorded electronically, and 
shaft encoders are available with very little friction. On the other 
hand, it is frequently desirable or necessary to use a much smaller 
float. As the diameter of the float decreases, the driving force it 
exerts on the shaft encoder decreases rapidly. For example, the 
driving force developed by a float that is displaced 0.01 feet is 
given below for four commonly used float diameters.

| Driving force and torque developed when a float of the indicated   |    
| size is displaced by 0.01 feet                                     |    
| Float diameter, inches          | 12.00  | 10.00  | 2.50  | 1.500  |
| Driving force, ounces           |  7.84  |  5.45  | 0.34  | 0.123  |
| Driving torque, inch-ounce      | 15.00  | 10.40  | 0.65  | 0.230  |
| (wheel circumference = 12 inch) |        |        |       |        |

The above table shows that unless the starting torque on a shaft 
encoder connected to a 1.5-inch float with a 12-inch wheel is less 
than 0.23 inch-ounce, a water displacement of 0.01 feet will not 
activate movement in the encoder and the recorded value can be in 
error by more than 0.01 feet.

Overcoming the starting torque of the shaft encoder is only one of 
many potential sources of error when recording water levels and it is 
Water Resources Division policy to attempt to meet an accuracy of at 
least 0.01 feet at lower stages (Office of Surface Water Technical 
Memorandum No. 96.05).  Broad categories of sources of error include 
the means by which the river stage is brought into communication with 
the stage-sensing component, the means by which the response of the 
sensing component is converted into a form suitable for recording, 
the means by which the data are recorded and uploaded, and the means 
by which the recorded and uploaded values are verified and calibrated 
against direct field observations of river stages.  The shaft encoder 
is one means of converting the position of a float tape to an 
electronic signal.  The accuracy of the recorded stage depends on the 
thermal expansion of the tape and the unbalanced force caused by 
differing lengths of tape on each side of the wheel, for example, in 
addition to the starting torque of the shaft encoder.  In order to 
allow for other sources of error and still meet the desired accuracy 
criterion, it is recommended that the starting torques of shaft 
encoders be no more than one-third the values shown in the above 

To allow interchange of shaft encoders among sites and because of the 
trend toward using smaller floats, it is recommended that Districts 
use shaft encoders with starting torques that meet the criterion for 
a 1.5-inch float (less than 0.08 inch-ounce).

                                       Thomas H. Yorke
                                       Chief, Office of Surface Water

WRD Distribution:  A, B, FO, PO