Compensating for Thermal Expansion

Compensating for Thermal Expansion

Working in environments that are not temperature-controlled can produce measurement errors created by the material’s temperature variations; when the object temperature varies from the standard 20˚C or 68˚F, the dimensions of any part change based on the material's physical properties. The Coefficient of Thermal Expansion (CTE) formula corrects most thermal errors. Formula: Material coefficient  x Temperature deviation from 68°F x Length in inches.

Example: 240-inch piece of Aluminum @ 80°:  Aluminum’s Coefficient  0.0000130 X deviation from 68°F  12 X Length in inches  240 = 0.03744 = 240.03744
This temperature change is particularly significant when verifying distance measurements during a measurement task. The longer the object, the greater the Temperature affects the part.

Below is an example of the impact of Temperature on Aluminum and Steel over given distances:

Aluminum Part

Temp. in F

1 inch/mm

100 inch/mm

240 inch/mm

1000 inch/mm

65

0.999961

99.9961

239.99064

999.961

68

1.0000

100.0000

240.0000

1000.000

70

1.000026

100.0026

240.00624

1000.026

75

1.000091

100.0091

240.02184

1000.091

80

1.000156

100.0156

240.03744

1000.156

85

1.000221

100.0221

240.05304

1000.221

90

1.000286

100.0286

240.06864

1000.286

Steel Part

65

0.9999799

99.99799

239.995176

999.979

68

1.0000

100.0000

240.0000

1000.000

70

1.0000134

100.00134

240.003216

1000.013

75

1.0000469

100.00469

240.011256

1000.047

80

1.0000804

100.00804

240.019296

1000.080

85

1.0001139

100.01139

240.027336

1000.114

90

1.0001474

100.01474

240.035376

1000.147


It is easy to see that a small temperature change in material can equate to large distance errors between distant points. Compensating for Temperature is critical when comparing points to nominal values.

Applying Material Temperature Compensation for Measurement

In Verisurf, you may apply Material temperature compensation (Scale) anytime during measurement. When active, the Material Temperature is displayed in the Device Manager window (shown):


Device Temp is only active for Devices with internal Temperature Monitoring. Environmental Settings are accessed through the tool button on the Device Manager toolbar when the Device is equipped with a Temperature Sensor. 









 Important Note for Laser Trackers: The initialization process uses ambient air temperature and barometric pressure entries. They only correct the laser's interferometer measurements to a standard 68° F or 20° C. This is particularly important to note when measuring long distances between points.

A typical misconception is that entering this Temperature accounts for Part Temperature. Still, its only use is to ensure accurate Laser Tracker measurements.

Applying temperature settings to the measurement device applies a correction scale factor to all subsequent measurements for the currently entered material Temperature.

The type of measurement task determines whether you need to consider the part temperature. Suppose the task requires you to verify distances or dimensions on a part that is NOT 68° F or 20° C. To achieve accurate measurements, you must “Enable Material Compensation” in the Temperature Settings dialog. 

Important Note: Changes in the Temperature Settings dialog only affect new measurements; all previous measurements remain unchanged. We recommend applying compensation for Material Temperature at the beginning of the measurement task (before measurements).

Use Material Compensation – No Temperature Sensor Connected

It is essential to have an accurate material thermometer and one or more temperature probe samples for Material Temperature compensation.

1.     In the Device Manager dialog, choose the Temperature Settings button for the active Device. Verisurf displays the Temperature Settings dialog shown on the right.

2.       Select the Material tab and choose Enable Material Compensation.

3.       Enter the Reference temperature if different than  20°C or 68°F (not typical).
You can toggle the display units using the radio control marked Celsius/Fahrenheit.
4.       Using your temperature probe, measure the object temperature in three locations and average the Temperature value.

5.    Enter your average temperature in the  Material Temp field.

6.    Select the Material Type, then choose OK to activate Material Temperature Compensation.

Important Note – Material Compensation remains enabled until manually disabled.













Use Material Compensation – Temperature Sensor Connected

1.    Enable Material Compensation and verify the Reference Temperature, typically 68 F or 20 C, though your Material may vary (not typical).

2.    When an Object Sensor is available from the connected Device, choose it in the Device Sensor Selection drop–down menu.

3.    Select the Update Material Temp button to read the Object Temperature (be sure the probe is secured to the object).

4.    Choose the Material Type and select OK to activate Material Temperature Compensation.






















 


Monitor Object Temperature Change

1.       Choose Enable Drift Alarms if you have selected an Object Sensor and want Verisurf to warn you if the part temperature drifts outside a preset limit.


2.       Choose the Settings button to open the Temperature Alarm Settings dialog, customize your Drift Alarm settings, and select OK.
















Verify Scale of a Device

Determining whether a Device measuring distance (Scale) can accurately be accomplished using a Certified Scale Bar. These Scale Bars come in a wide variety of Materials. To perform this check, the Scale Bar must be of the same material as the object and acclimated to the same temperature as the measured object.

After applying the Material Compensation, the Scale Bar may be measured using Point measurement and the distance between the points compared to the calibrated distance of the Certified Scale Bar.

Note – it is common practice when using a Laser Tracker to verify Scale Bar distances after the Alignment is calculated due to the ability to  apply “Scale” in Auto Alignments.

















Above: Articulating Arm and Laser Tracker Scale Bars

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