When it comes time to choose the proper ultrasonic thickness gage for your needs, many factors must be considered. Many of the important points are obvious such as budget and your basic application. Other factors may not be, and careful consideration should be given to them so that you are happy with your short and long term selection from a vast group of quality manufacturers of these simple yet high technology devices.
Here are some things to consider when selecting your next ultrasonic thickness gage. Obviously, budget is always something to be conscious of. Handheld ultrasonic thickness gages, depending on the vendor and where they are made, can range from just a few hundred dollars to more than $4,000. There are also many factors involving the application. It is important to know beforehand what the instrument can and cannot do so that you and the supplier do not have a misconception of the instruments capabilities.
It is critical to list the materials and complete thickness range of each material along with desired accuracy, temperature at the time of the readings, surface finish (painted), if the surfaces are parallel, and if a fluid can be applied to the material. The fluid is usually a water soluble ultrasonic couplant required for high frequency sound to enter the part as the ultrasonic frequencies are in the millions of cycles per second and do not travel through air.
Other factors are more related to individual preferences and options such as the need to use echo to echo to ignore coatings and live waveform referred to as A-scan (more details below), an internal datalogger, temperature correction, oxide scale, high penetrations, use of different frequencies, diameters and types of probes such as a pencil probe vs. a dual probe. Other considerations should be given to the environment in which the product is used such as indoor vs. outdoor use for both IP rating for dust and moisture and display type and size for viewability including in direct sunlight.
You should also consider weight, size and battery type. Some units are very small and can fit in your shirt pocket and utilize the convenience of AA type alkaline batteries while others offer more than 24 hour battery life utilizing the latest in lithium Ion rechargeable batteries at the sacrifice of a larger unit. Also, careful consideration should be given to serviceability of the unit, where it is manufactured and the reputation of the vendors customer service and track record.
In addition to budget, here is a list of things to consider for these applications:
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Also, consider these questions:
In addition, consider the physical size of the unit (fits in shirt pocket vs. handheld); environmental concerns such as IP rating or explosion proof; display size; display color or monochrome; sunlight readability; and reputation of vendor.
Ultrasonic Thickness Gages
Ultrasonic testing and the use of thickness gages using ultrasound is a very well-known technique. The primary reason for using an ultrasonic thickness gage is you can nondestructively test the wall thickness of the material without destroying the part. Commonly the two primary uses are for detection of wall thinning due to corrosion or as a quality control tool for material thickness consistency. The applications range from measuring pipes, tubes, boilers, ship hulls-considered corrosion to plastic bottles, toys, coil steel, aircraft aluminum and coil steel-considered precision, to name a few.
There are many manufacturers of such products around the world. Handheld units can start as low as a few hundred dollars and can go up to several thousand dollars. Obviously, there will be a distinct tradeoff from the price you pay to the feature set that is included. Nowadays, software is very flexible and many manufacturers use the same basic electronics to satisfy budget and application needs but vary the internal software to satisfy more challenging needs. Several companies now offer the convenience to do field upgradability. What this means is a basic instrument of thickness only can unlock software to enable features such as echo to echo, datalogger and live waveform. This is a tremendous convenience and saves time and money as there is no downtime or extra shipping costs.
One common example of this is a technique referred to as echo to echo. Here is the basic application and solution. Many times parts such as a storage tank or steel ship hull are painted. When using an ultrasonic thickness gage without the echo to echo feature, the thickness of the paint will introduce an error of roughly 2.5 times the thickness of the paint. The reason for this is the sound speed used to measure the base steel is 2.5 times faster than paint. So a .010 (.25mm) thickness of paint will introduce a .025 or .625mm error. The echo to echo technique offered by many manufacturers measures the time between two successive backwall echoes in the base material thus taking the paint thickness out of the equation.
Some consideration should be given to the thickness of the base material as most vendors will state the base material should be at least .100 or 2.5mm in echo to echo mode. Also consider a more heavily damped transducer and ideally viewing the live waveform to verify the echoes are legitimate is recommended. Using a thickness only gage with just numbers and not a live waveform, particularly on thin materials can miss a doubling effect. This could show a reading of .120 when in fact the part is one half that or .060. By viewing the live waveform, a trained operator could see this error.
I like to compare this to fishing. On a boat you can have a depth finder. This would be equivalent to a gage with a thickness only display while a fish finder would be equal to a thickness gage with a live waveform. While a boater could determine if the changing depth of the water was just the water, it could be a rock, a whale or bait, the same is true of the thickness meter with the live waveform where an operator can look for things such as doubling, mode converted echo errors or even flaws and delamination if large enough. Some vendors call this top coat or through coat. In these cases, some companies have patents on their respective techniques.
Modern ultrasonic thickness gages have internal dataloggers, some with up to 32 GB of micro SD card memory. Other unique features include blue tooth and direct interface to Microsoft Excel. Readings can be given file names, user names, locations, prefix, notes and set up in files such as linear, 2D, 3D, boiler and with custom points. This allows for a great deal of flexibility when collecting and comparing readings over inspection outages.
Thats not how this one works. Youre referring to those pen shaped paint gauges that reveal more or less of the stick and on some of them the stick is color coded to make them easier to read.I have this paint depth gauge. I think its adequate for the most part but I dunno if Id... Well actually maybe I shouldnt say anything negative about it just yet, because it does seem to work. Check it out.It comes with a metal plate and several films that are used to calibrate it. Once its calibrated its able to measure each film and does it repeatedly spot on without fail.IME once its calibrated it stays calibrated meaning you dont have to do it everytime you use it. It stays accurate.Now lets go measure my paint.On my 02 Cadillac DTS, which prior to me owning it had seen quite a bit of neglect throughout the years I get a reading of 0.16mm [what is this? Microns?] on the trunklid.I cant even get a reading on the roof...Keep in mind that the roof was by far the most neglected part on the car, remember how it looked completely prepped for compound/polish. Beyond neglect, this paint was straight up abused.This is how it looks today. I didnt need anything to tell me this paint here is super thin and delicate, I knew it all along, thats why I didnt attempt to go any harder than it would allow me to and Im fine with where its at because any more wouldve lead to failure.Onto the hood... It gives me a reading of.27 mmNow check out the hood on the Kia Sorento, which Ive made it my goal to put off machine polishing until it shows visual signs of needing to be polished.. I get a reading of .70mmNow lets switch it to mils.Cadillac trunk: It goes from nothing to 6 mils on certain areas of the trunklid, but for the most part I get nothing.Cadillac hood: 11 milsKia hood: 27 milsIs it perfectly accurate? I dunno, but it definitely shows a difference.
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