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Thickness Measurement of Automobile Fuel Tanks

Application: Measurement of individual layers and total thickness in multilayer plastic automobile fuel tanks.

Background: Most contemporary automobile fuel tanks are fabricated with a multilayer plastic construction, typically made of two structural layers of high density polyethylene (HDPE) surrounding a thin gas barrier layer made of ethylene vinyl alcohol (EVOH). The purpose of the barrier layer is to prevent the slow leakage of gasoline vapor through the polyethylene wall. The HDPE structural layers are typically in the thickness range 0.1 in. to 0.2 in. (2.5 to 5 mm), and the EVOH barrier layer is typically 0.004 in. to 0.012 in. (0.1 to 0.3 mm). Tank manufacturers need to measure both the thickness and the depth of the barrier layer.

Equipment: The Model 38DL PLUS ultrasonic thickness gage with the Multi-Measurement software option is recommended for simultaneous measurement of individual layers and total tank thickness in this application. With this software, the 38DL PLUS gage is capable of using separately programmed setups (including sound velocity, gain, and blanking settings) for each layer being measured to optimize performance. In this test, the gage is most often used with an M2017-RM (20 MHz) delay line transducer. Contact Olympus for gage setup assistance.

Typical Procedure: The waveforms below show measurements of structural and barrier layers in a typical automobile fuel tank using appropriate setups. The gage's frequency-based barrier measurement mode is used to read the barrier layer whenever its thickness is less than approximately 0.010 in. (0.25 mm). Barrier depth is measured with a conventional Mode 2 setup, and the thickness of the inner polyethylene layer is also measured in Mode 2. Thick barrier layers, greater than approximately 0.010 in. (0.25 mm) may also be measured in Mode 2. Note that because of low pass filtering effects in the outer polyethylene wall, the minimum measurable barrier thickness will usually be approximately 0.004 in. (0.100 mm).

The reflection ratio at the boundary between any two materials is determined by the relative acoustic impedances of those materials. Because virgin and regrind material have essentially identical acoustic impedances, it is not possible to separately measure regrind layers. Also, adhesive layers adjacent to barrier layers are generally too thin and/or too closely impedance matched to measure with ultrasonic techniques and cannot be resolved.

outer polyethylene layer

barrier layer (frequency domain measurement)

inner polyethylene layer

As with any ultrasonic thickness measurement, accuracy is dependent on proper sound velocity calibration. Velocity calibration must be performed for each material being measured, on samples of known thickness.


Olympus IMS


72DL PLUS高级超声测厚仪小巧便携、易于使用,可以提供准确的厚度测量值。这款创新型厚度测量工具可与频率高达125 MHz的单晶探头相兼容,非常适合测定多层漆料、涂料和塑料等超薄材料的厚度。它可同时显示最多6层的厚度。


用途广泛的38DL PLUS测厚仪既可与双晶探头一起使用,测量被腐蚀管道的厚度,也可与单晶探头一起使用,对薄材料或多层材料进行非常精确的厚度测量。
Magna-Mike 8600霍尔效应测厚仪使用磁性探头对塑料瓶等非铁性薄壁材料进行准确的厚度测量。
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