Strain Gauge Calibration Program for the LTL
Table of Contents
Introduction …………………………………. 1-1
System Requirements …………………………. 2-1
Entering Gauge Info …………………………. 3-1
Saving Data to a File …………………………. 4-1
Current Check …………………………………. 5-1
Performing a Calibration Run ………………… 6-1
Liquid and Temperature Check ………………. 7-1
Setting-Up for a Calibration Run ……………… 8-1
Sample Calibration Run on LTL ……………… 9-1
This program was designed to allow for the calibration of both beam gauges and
bullet gauges. Each data reading consists of a pressure value and the corresponding
resistance at that pressure. This data is saved to a file, as well as being displayed on the
To boot the program type CALSG at the C:\ prompt. All the data is saved to a directory defined by the variable pathspec$. This variable can be changed in order to create a new directory in which to store calibration data. The variable pathspec$ can be
accessed in the main module of the program.(Once changed the program must be re-
2-1 System Requirements
This program was designed to run with the following devices.
? HP 3548A multimeter
? Keithley 706 Scanner
? Keithley Model 220 Programmable Current Source
3-1 Entering Gauge Info
Once the program is started, the user will be presented with the following screen.
What type of gauge(s) is(are) to be calibrated ?
Choices are :
At this point the user must select a gauge type. Once the gauge type is selected
the type of gauge and the area of the gauge is stored by the program.
Once the gauge type has been entered the user is prompted by the following
Number of gauges to calibrate?
At this point the user must enter the number of gauges to calibrate. At this
time(8/3/98), the system allows either two strain gauges or four bullet gauges to be tested
at one time.
After the user has entered the number of gauges to calibrate he/she is asked to
confirm the selection.
Following confirmation of the number and type of gauges, the program asks the
user if the calibration run will be a warm run(300K) or a cold run(4.2K).
4-1 Saving Data to a File
After the program has allowed to user to determine the type of gauges to be
calibrated, the program displays the following.
Do you want to save data to file
At this point the user enters a Y or y for yes or a N or n for no. If the user enters
yes then he/she will be allowed to enter a line of comments to be included with the file.
The file is saved in the directory specified by the variable pathspec$. This
variable can be found in the main module of the program.
If the user enters file name that already exists in the directory, the user will be
promptly notified with the following.
File already exists. Overwrite?
Answering Y or y overwrites the existing file. Entering an N or n causes the user
to be asked to enter a different filename.
5-1 Current Check
Keithley 220 Current Source Operating Parameters Once the filename of the run has been entered and accepted, the program will display the following.
This statement will be followed by a description of the current being supplied and
the voltage limit set on the current source. If the value for the current reads 0 mA, then the operator must check the wiring of the gauges. Make sure that all connections are
either connected to a strain gauge or properly jumpered.
Once done the operator should continue to the point where he/she can take a measurement. A measurement should then be taken in order to confirm that the system is
6-1 Performing a Calibration Run
Once the task of setting up the run has been completed, the operator is ready to
start taking data. Data is displayed to the user as depicted below.
The calibration procedure is as follows.
1. At the beginning of a calibration run, the user will see that two values are displayed
for the current load provided by the LTL. These values are labeled LOADUP and
LOADDOWN. The user need only be concerned with the LOADUP value as long as
the force applied is increasing. At the point in the calibration run that the load applied
begins to decrease, it is necessary for the user to concern him/herself with the
2. Once the desired value of force is achieved and the load has become fairly stable, the
user can press any key to take a reading.
3. The program then stops updating the current load, and begins the process of taking
voltage measurements on each gauge channel.
4. Once the user has completed a calibration run, he/she can press Q or q to end the
7-1 Liquid and Temperature Check
At any time during a cold calibration run the user can press T or t to check the
temperature in the dewar. This temperature is given in degrees Kelvin. The command
also updates the percentage of liquid remaining in the dewar.
If the temperature in the dewar rises above 4.55 degrees Kelvin then the program
will display an error message in the lower right hand corner of the screen. If the liquid
level is below 25%, a similar error message will appear.
Note: Two temperatures are displayed on the screen. Each value represents a
different thermometer in the dewar. Both value should read very close to each other. If
the values differ by more then a few milli-Kelvin, then the user should make sure to
check the thermometers before the next run.
8-1 Setting-Up for a Calibration run
This section has been included in order to outline the basic procedure used to
prepare for a strain gauge calibration run.
1. All push-pins on the LTL must either be connected to a strain gauge or be
appropriately jumpered. Make sure to note which channel each gauge is
2. The diagram below depicts the proper alignment for strain gauge wires to the
3. Boot the program(CALSG) and follow the appropriate steps until able to take
a reading. Take a reading to make sure that all channels are reading as
4. Secure base plate to LTL and turn on LTL.
5. Determine point of contact between test fixture and ram. Back the ram of the
fixture by at least .5 inches. Note this position and use it as the zero-point for
all data runs.
6. Proceed with calibration until its conclusion.
9-1 Sample Calibration Run on the LTL
1. Once gauges secure, the operator can begin the procedure to calibrate the
2. If a cold calibration is being performed than the user must begin to cool the
gauges to 4.2 K. Once the gauges are at 4.2K and the liquid level is above
25%, the user may proceed.
3. Before collecting data, the operator should train the gauges to 7 times to 17k.
This must be done both warm and cold.
4. At this time(8/10/98), the procedure is to take points, while the gauges are
being loaded, from 0-14,000 lbs at every thousand. The gauges are then
unloaded to zero, taking points in a similar fashion.
As points are recorded the user should try to maintain no more than a 20
lbs. shift in the load over the period it takes for the program to obtain data for
the gauges. Be certain that, when loading, the force is always increasing as a
reading is taken. In turn, when unloading, be sure that the readings are only
taken as the force applied is decreasing.
Note: The zero points at the beginning and end of a cycle do not
correspond with zero on the screen. Instead, the user should determine the
position at which the fixture comes in contact with the ram. This position can
be determined using the LVDT., To be sure that the gauge is not under any
pressure, make sure the ram is at least .5 inches above the point at which it
makes contact with the fixture.
5. Once the user has completed one cycle, he/she repeats the process twice more.
Gauges must be calibrated both warm and cold.