This blog post is on the strange side.  I was rummaging through some of the work I did back in 1999 and found a tare load correction proof.  I probably never published it as we were all waiting for the world to end at the turn of the next century.  When the world did not end, I forgot about it and moved on to other things. This proof was done using a 6,000.00 LBF Capacity Proving Ring S/N 7232 on July 9, 1999. The purpose of this blog is to share the correction and proof for anyone using Digital or Analog Morehouse Proving Rings.

1.                  Exercise the Proving Ring and take readings to determine the tare load and record the Proving Ring temperature.

23.5O C                    Ring temperature

3.                  Determine net deflection for tare load.

98.125 div.             Net Ring deflection to calculate tare load at 23.5

4.                  Compensate net Ring deflection of tare load for temperature.

98.125 div.  Net Ring deflection to calculate tare load at 23.5

-  .075 div.   Correction to 23 using the formula in Operation and Maintenance Instructions For Proving Rings manual.

98.1135 div.           Net Ring deflection to calculate tare weight at 23.5

98.1135 div.           Net Ring deflection to calculate tare load at 23

98.1135div. =  299.70 lbf interpolation from load table

98.1135 div. =  299.70 lbf force of tare load*

6.                  Exercise the Proving Ring and take readings for the applied load.

23.5O C                   Ring temperature

-        98.25 div.  Net Ring Deflection to calculate tare at 23.5

1006.8 div. Change in Net Ring deflection

9.                  Compensate net Ring deflection of the load for temperature.

1006.80 div.             Net Ring deflection including tare at 23.5

-     0.13 div.  Change in Net Ring deflection

1006.67 div.  Net Ring Deflection Including Tare Load

The following is an example that uses a formula to correct for tare loads.  The tare load correction formula corrects for tare and is applied to the value of the applied load from the fitted curve data provided with your calibration report.  The tare load correction formula is represented as follows:       TCF =  (2c * L * T) - a

Where:      c = The value of the c constant found on the ring calibration report

L = The force of the load being applied

T = The force of the tare being applied

a  = The value of the constant found on the ring calibration report

Calibration Report Data:   c = 0.2648421D-05

L = 3,000.00

T= 299.70 Lbf

a = 0.1673432D+00

TCF = (2*0.2648421D-05*3000*299.70)-0.1673432D+00

TCF =           4.595 div

3,000 LBF =   +1002.050 div. From table

1006.645 div. Correction for tare

TLC   =  1006.670 div.

TCM  =  -1006.645 div.

Difference =   0.025  div.

The difference is below the readability of the proving ring which is 0.05.  Therefore, using the Tare Load Correction Formula introduces an insignifacant amount of error and can safely be used to correct for the tare.

We have made excel spreadsheets for anyone using a proving ring as a calibration standard in our Morehouse UCM.  The excel sheet handles temperature, tare, and other corrections to set to the appropriate force values.

written by Henry Zumbrun

www.mhforce.com