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Checking the Ambient Air, Penetrant and Component Temperature
by Stan Weatherly*
| Here is a quick and effective way of checking temperature accurately that stretches way beyond penetrant testing into other NDT methods. Many material properties are temperature dependent (for example, the sound velocities, magnetization, viscosity and surface tension). Perhaps the availability and cost of laser-based thermometers will have a positive effect on NDT, just as the electronic gaussmeter did for magnetic particle testing 20 years ago.
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Introduction
During my tenure of auditing various facilities, I have received many different replies whenever I ask how one determines the ambient air, penetrant and component temperatures. One answer I received that I will never forget is, "It's California, it never gets that cold here." Due to these types of responses, I spent some time researching various temperature measuring devices and how they can be used to ensure that a facility is compliant with ASTM E 1417 (1999). To perform these temperature checks, technology provides us with some useful and wonderful products. However, sometimes we fail to hear about these products until someone mentions them to us or we're informed by reading, seeing an advertisement or having to answer an audit question. In an effort to further nondestructive testing knowledge, I would like to share my research with you.
Component, Penetrant and Ambient Temperature Specification
ASTM E 1417 states: "The component, penetrant, and ambient temperatures shall all be in the range from 40 to 125 °F (4 to 52 °C) unless otherwise specified" (ASTM International, 1999). The writers of the ASTM E 1417 specification realized that the air, penetrant or component could all be different temperatures. The penetrant or component could be cooler or warmer than the ambient air temperature. ASTM E 1417 is written to be used all over the world and in a multitude of environments; therefore these checks must be performed regardless of a facility's location.
To perform these temperature checks, technology provides us with some useful and wonderful products.
The basic question, then, is how do you perform these checks? A simple enough question, but does your facility actually perform them? And does the temperature measuring device truly measure the temperature of the object, whether the inspector is checking a component, a solution or air temperature? The first response provided by most test facilities when asked how they perform the above check is an explanation of how they check ambient air temperature. The conventional methods they use will likely vary, from the usage of bimetal dial thermometers and glass bulb thermometers, to electronic digital thermometers and resistance temperature devices. All are excellent instruments when used in the right environment. A technician may point to a common wall thermometer for the ambient air temperature. This is probably a typical bimetal dial thermometer (Figure 1). The technician may direct the auditor to a glass bulb thermometer, preferably filled with red alcohol or some other organic material versus the older hazardous silver mercury style. Or perhaps they have a digital resistance temperature device with a probe that can indicate ambient air temperature. All of these can quickly provide the inspector with the ability to determine the ambient air temperature.
Technicians have many options when it comes to checking the penetrant temperature. The first option is a dial thermometer with a sensing stem for dipping into a solution. Another option is the same handheld glass thermometer that is used to determine the ambient air temperature (Figure 2). Due to the response time of these thermometers, however, it is imperative that ample time is provided for the thermometer to respond. The technician could also use a quick response digital thermometer or a resistance temperature device with a probe attachment. Different probes are required for air versus liquids when using resistance temperature devices. All of the above temperature devices work well for checking penetrant temperature; just remember that the temperature should be checked below the surface.
Finally the most dreaded question - how do you measure the component temperature? Most often auditors will receive answers such as "we go by feel." It is assumed that if the ambient temperature is within range and the part feels normal to the touch, then the component is within range. This is not a good assumption. It is impossible to tell by touch if a material is between 277 and 283 K (40 and 50 °F), which can often indicate when a dwell time should be doubled or the testing should not be performed. Holding the end of a digital or stem thermometer against a part or sandwiching it between components does not provide a true reading. Dial or digital stem thermometers do not work well for surface temperature checks. A more viable answer that a facility can give is that they use a resistance temperature device. These can be purchased with multiple probes (air, immersion or surface); a facility must use a surface probe to correctly perform this test with a resistance temperature device. But quite often due to the cost or a lack of knowledge, a facility has not purchased these multiple probes. This leaves the technician to guess the component temperature. So what is an efficient way to perform this final check?
This is where information along with technology can be an asset to the nondestructive testing (NDT) world. Infrared thermometers are not new by any means, but few NDT technicians are aware of them. Infrared thermometers use a laser beam, a filter, a photon detector and a microprocessor to pick up an object's infrared radiation (Figure 3). Liquids, metals and people all give off infrared radiation and, combined with electronics, this can be converted directly to a digital temperature reading. Infrared thermometers can provide an accurate reading of the component's temperature. They give instant readings, require no or little clean-up, and come in very small sizes that can be easily stored and secured. NIST certified infrared thermometers are a highly efficient method to check component temperature and can be purchased inexpensively.
Summary and Conclusion
This article has described some of the devices that can be used to perform temperature checks before the testing process. The ambient air temperature can be cost effectively measured with a bimetal dial or glass bulb thermometer. The temperature of the penetrant must be checked below the surface by using one of many options which include: a dial thermometer with a sensing (dipping) stem; a fluid expansion glass thermometer (precautions must be taken); a quick response digital thermometer; or a resistance temperature device with an attachment probe. An infrared thermometer is a resourceful tool to measure the temperature of a component. There are resistance temperature devices that can perform all three tests with the correct probes, but these units, with separate probes for air, immersion and surface readings, can be quite expensive. Because there are a variety of instruments that are available to check the required temperatures, financial resources should be considered when weighing the options of which instrument will work best for your nondestructive testing department.
I hope this article has aided in identifying the tools that can effectively measure the component, penetrant and ambient air temperature, and ultimately help to keep your facility compliant with the ASTM E 1417 specification.
References
ASTM International, ASTM E 1417: Standard Practice for Liquid Penetrant Examination, West Conshohocken, Pennsylvania, 1999.
* The Boeing Company, 2401 E. Wardlow Road M/C C54-023, Long Beach, CA 90807; (562) 982-7073; e-mail stanley.l.weatherly@boeing.com.
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