Creating Shunt Calibration Values for Balances

Perform this task if there are no useful balance calibration shunt values.

Following procedure assumes balance connected to facility taper pin block.

Important Conditions:

• No load on balance (bare balance preferred)
• Balanced allowed to warmup at least 1 hour
• No touching on balance
• Use a precision/accurate voltmeter such as HP3458A
• Use a stable DC power supply such as HP6612B
  SDS balance channel can be used for excitation, but don't connect signal leads
• Do not connect any other signal measurements such as SDS balance amplifier signal input (it will alter measured signal).

Test Equipment:

• Voltmeter: HP3458A (note ECN and calibration date)
• DC Power Supply: HP6612B (note ECN)

  Set HP3458A to 100 PLC. Do an autocal on HP3458A.

Procedure:

Allow enough time for voltmeter to settle on a measured signal. DVM typically displays results in millivolts.

1. Measure excitation at taper pin block on +/- SENSE connection.
2. Measure "No Load Zero" on +/- signal at taper pin block with no shunt applied.
3. Apply shunt resistor across -Exc and -Sig at taper pin block.
4. Measure "Shunt Applied" on +/- signal at taper pin block.
5. Subtract "No Load Zero" value from "Shunt Applied" value.
   This is the shunt calibration value in mV (multiply by 1000 to express in uV).
6. Divide shunt calibration value by measured excitation, multiply result by 1000.
   This is the normalized shunt calibration value in uV per volt.

Reason for expressing shunt calibration value normalized in uV per volt is data reduction matrix is usually formatted in uV per volt.

Record balance model, serial number, ECN, and applicable test. Shunt resistors should always be stored with balance.

Example below using measurements from Mk41A (#32560):

Gage	Exc, Volts	No Load Zero, mV	Shunt Applied, mV	Shunt - No Load, mV	Normalized shunt calibration (shunt-no load)/exc, uV/Volt
N1	5.003623	-0.10966	4.64485	4.75451	950.213
N2	5.003975	-0.07794	4.67555	4.75349	949.943
AX	4.971598	-1.40245	3.65071	5.05316	1016.406
S1	5.006260	+0.01141	4.76788	4.75647	950.104
S2	5.008186	-0.12399	4.66300	4.78699	955.833
RM	4.976900	+0.09250	5.16750	5.07500	1019.711

Values on last column are the zero corrected calibration shunt values to be used in Balance Constant Conversion Form, Line F. These numbers were normalized as typical of units from Ames Balance Calibration Lab.

Responsible Official:
    Don Nickison, 650-604-1748, Donald.J.Nickison@nasa.gov
Webmaster:
   Michael Wright, 650-604-6262, michael.f.wright@nasa.gov

Last modified on 7.2.08

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