Calibration and Storage of Task S/G Balances, Inder Chabra, 1963

1-28-02 Website Editor’s Note:

The root cause of shifts was due to the type of adhesive used on the strain gage elements, which enabled moisture to be absorbed in the gage. In later years better adhesives were used during balance manufacturing so this problem of moisture would not cause shifts. Since then (from the 1970s), hot boxes and ovens were not required to help reduce zero shifts.

However, 100 deg F hot boxes are helpful and should not be ruled out as a method of storage. Such a box reduces rust buildup when the balance is unused for very long periods of time. These type of hot boxes are heated with a standard light bulb so the balance does not become too hot but does get enough heat to prevent condensation.

To: Chief, Experimental Investigation Branch

From: Inder N. Chabra, Research Engineer

Date: May 14, 1963

Subject: Calibration and Storage of Task Strain Gage Force Balances

1.0 Introduction:

From time to time in the past, wind tunnels have reported that some strain gage bridges of task balances have indicated very large zero shifts during wind tunnel test runs, even though there were no indications of any zero shifts prior to start of the run. It has also been reported, occasionally, that the shunt resistance calibration settings for sensitivity as provided by callbration data have disagreed with sensitivity checked in wind tunnel by as much as 2.5%. In particular there has been some concern when output force was higher than the calibration dead load, by using the sensitivity setting provided with the calibration data. A recheck of balance and calibration in the calibration room would Indicate that the balance functions properly and that the calibration repeats within ±0.1%. This memorandum is issued to report the factors that have been found to lead to these discrepancies. Some changes in procedures are suggested.

2.0 Zero Shifts:

Zero shift problem has been found to be associated with balances that have not been bakea out properly after having been out of hot box over long periods of time, without any DC power on the gages. Once destabilized, due to the moisture in the air, the gages do not return to stability unless cured for approximately 12 to 16 hours at 200 deg F. Destabilized strain gage bridges do not drift at room temperatures, but some strain gage bridges have been observed to drift as much as 2.5% between temperatures of 110 deg F and 135 deg F. These same gages were observed to be fairly stable when 4.5 volt DC power was left on them. Indication of large zero shifts occured only above 110 deg F after the balance was out of hot box, without the battery power on.

3.0 Sensitivity Change:

Deviation between sensitivity checked in wind tunnel from sensitivity settings provided by the calibration data may be due to any one or a combination of several factors. Apart from mechanical interferences, misalignments, etc., by far the largest change in indicated sensitivity is due to change of ambient temperature. The sensitivity increases with increase in temperature, and Task Corporation data on their balances, indicates this increase is approximately in a range of 0.01% to 2.5%, being characteristic of the strain gage bridge and the temperature.

Increase in sensitivity in a range of 0% to 0.45% has been observed on different strain gage bridges, using a shunt resistance for monitoring sensitivity, when a balance was heated with a sun lamp in the calibration room.

Other factors, e.g. repeatibility of Sandberg Serrell loading gear, effects of different lead lengths, use of calibrate resistors other than those used in calibration data, for sensitivity conversions, and difference in linearities of “IMP” and “ASGAR” measuring systems, did not reveal any measurable changes in sensitivity.

4.0 Recommendations:

The following recommendations are made:

4.1 Bake out all balances at 200 deg F for a period of 12-16 hours before every calibration, and also prior to use after any prolonged storage at room temperature.

4.2 Store balances in hot box (140 deg F ±10 deg F) at all times after bake out, except when in use.

4.3 Leave 4.5 volt DC power on all strain gage bridges if the balance is to be out of hot box overnight or more than 4 hours.

4.4 Temperatures of balances should be monitored during calibration and during test runs, and data should be corrected for temperature effects, as necessary.

Inder N. Chabra
Research Engineer
Instrument Systems Development Branch

JAW
RKH
INC: bw