INTERACTIVE
INSTRUMENTS
Model 504 Extensometer.
The Model 504 Extensometer is a rugged instrument capable of measuring precise strain for long durations.
It can easily attach to flat or round specimens of various thicknesses making it a very versatile instrument.
Unlike most extensometers the model 504 is capable of over and under ranging without disturbing the
sensitive bridge sensor. Since the calibration adjustments are internal to the interface connector, the
calibration stays with the gage.
Extensometer Calibration
All extensometer come factory calibrated but it is wise to verify the calibration before relying on
results. Calibration of the extensometer should be performed periodically to assure proper
results. The best method of calibrating the gage is to mount it to a known material and trim the
settings to match a known material modulus. The following is a step-by-step procedure for
calibrating the extensometer on an Interactive Instruments 1K load frame.
1) Adjust the strain filter as low as possible (< 0.5 Hz) to reduce noise and stabilize the output.
2) Be sure the extensometer is plugged securely into the upper analog port connector on the
side of the load frame and is screwed securely in place.
3) Check that the strain channel range is configured properly for 4% in the controller's strain
configuration menu and the strain digital offset is set to zero.
4) Allow time for the electronics to warm up and stabilize for approx. 15 minutes.
5) Place the extensometer on a table without anything touching the spring clips or knife-edge.
This should put the gage at 0% strain. Adjust the trim pot on the connector labeled "O" to
zero the analog strain voltage. The controller strain reading (lower left corner) should read
zero.
6) Attach the gage to a steel specimen installed in the load frame. See Extensometer Mounting
instructions below. Once the specimen is installed place the load frame in load control with
less than 5 lbs of load.
7) Readjust the "O" trim pot on the extensometer to re-zero the gage.
8) Apply a sufficient load to the specimen but not enough to yield the material. Typically 150
to 250 lbs is sufficient for steel with a 0.01" cross-area. More may be needed for larger
cross-areas.
9) Adjust the extensometer span trim pot (labeled S) so the LCD matches the calculated strain
of the applied load of the known specimen. (∆Length = ∆Load / (Modulus •gage cross-
area)). Remember the gage range at 4% is 4.0% of 0.50" = 0.020". For example the change
in length of a steel specimen (Modulus: 30,000,000) with a gage cross-area of 0.01 sq in
with a change in load of 200 lb is calculated as follows: 200 lbs / (30,000,000 psi •0.01") =
0.00066" = 0.133%.
10) Apply 0 load and verify the gage reads close to zero. If not zero then verify the material is
not yielding. If so replace the specimen and reduce the load applied and continue with step
8.
11) Apply the same load in step 10 and verify the same strain. If different then check that the
extensometer is not slipping on the gage. If the gage is slipping, increase the tension on the
spring clips by sliding them closer to the knife-edges and continue with step 8.
Once calibrated the gage should not need calibration for months. Once a calibration is
performed between a gage and a load frame the calibration is valid for the pair. Placing the
extensometer on another load frame may effect the calibration due to variations in the excitation
voltage.
