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Technical Discussions
Neil Breslow
Teacher, - -
NDT Supply.com. Inc., USA, Joined May 2009, 3

Neil Breslow

Teacher, - -
NDT Supply.com. Inc.,
USA,
Joined May 2009
3
02:07 Oct-12-1998
ultrasonic thickness gauging at cryogenic temperatures

What is the effect of cold on common ut thickness (dual) probes?
Does the -1% per 100 deg. F. velocity correction work in reverse?
What couplant works best?


    
 
 Reply 
 
Paul Meyer
R & D,
GE Inspection Technologies, USA, Joined Nov 1998, 47

Paul Meyer

R & D,
GE Inspection Technologies,
USA,
Joined Nov 1998
47
02:38 Oct-12-1998
Re: ultrasonic thickness gauging at cryogenic temperatures
Niel,
At cryogenic temperatures several significant changes occur in the probe. PZT's will increase in frequency. Their dielectric constant, which determines the electrical impedance, will decrease by as much as 50%. I also beleive that the coupling constant, which is a measure of the amount of electrical energy converted to acoustic energy, also decreases. This means that the performance of the piezoelectric element will seriously degrade form its performance at ambient conditions. Additionally, the acoustic impedance of the damping material will increase changing the pulse shape. This assumes that the adhesives used in the assembly of the probe remain entact. Overall performance will probably be poor at best.
Most "standard" probes are designed to operate in a temperature range specified by the manufacturer. I suggest you not use the probe beyond this range. If you need to perform testing at cryogenic temperatures I believe you will need a probe designed specifically for this purpose.
Is the application a long-term test? Can a probe be fixtured in place before the temperature is reduced? Or do you need to move the probe while at cryogenic temperatures? Answers to these questions, and others, will determine the probe design.
I would be interested in hearing from someone who has ultrasonic experience at very low temperatures.
Paul

: What is the effect of cold on common ut thickness (dual) probes?
: Does the -1% per 100 deg. F. velocity correction work in reverse?
: What couplant works best?




    
 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1261

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1261
01:48 Oct-13-1998
Re: ultrasonic thickness gauging at cryogenic temperatures
Paul Meyer gave an excellent accounting of the complications that result within the probe at very low temperatures.

If the probe you have is suited to the application and you can get a couple of multiples your correction for dV/dT should be small.
For compression mode the dV/dT is on the order of 0.6metres/second/degree C.

If you try using the time to the first backwall without due regard for the plastic delay line in a dual element probe your error could be significant.
dV/dT in plastics (e.g.Perspex) is on the order of 3m/s/degreeC

Ed


: What is the effect of cold on common ut thickness (dual) probes?
: Does the -1% per 100 deg. F. velocity correction work in reverse?
: What couplant works best?




    
 
 Reply 
 
Tom Nelligan
Engineering,
retired, USA, Joined Nov 1998, 390

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
07:05 Oct-13-1998
Re: ultrasonic thickness gauging at cryogenic temperatures

: What is the effect of cold on common ut thickness (dual) probes?
: Does the -1% per 100 deg. F. velocity correction work in reverse?
: What couplant works best?

A couple notes to add to the responses already posted by Paul Meyer and Ed Ginzel--

Commercial dual-element transducers that are designed for high temperature corrosion survey work will generally tolerate brief contact with very cold surfaces as well, potentially down to minus a couple hundred degrees. The exact temperature limit will depend on the transducer design as well as the surface contact time. You should contact the transducer manufacturer for their recommendations. Keep contact time with the surface brief. In both high and low temperature situations, transducer failure is typically due to internal bond failure when the interior of the transducer reaches a certain temperature limit; although delay line materials used in such transducers are typically good thermal insulators, they're not able to insulate against continuous contact beyond some specified temperature.

Delay line zero offset will indeed change as the transducer cools, so you should check zero calibration frequently.

Coupling is definitely a challenge. A low-temperature grease or oil that remains semi-liquid should be OK down to its freezing point, but I haven't seen a common couplant that will work at liquid nitrogen temperatures. What is the surface temperature in your application? Another approach that has been used in research applications is to bond a metal waveguide to the test piece and warm up the far end sufficiently to couple with a liquid or grease, but I know that usually isn't practical in plant inspection situations.

--Tom Nelligan
Senior Applications Engineer
Panametrics, Inc.


    
 
 Reply 
 
Anthony J. Egan
Consultant,
USA, Joined Nov 2001, 5

Anthony J. Egan

Consultant,
USA,
Joined Nov 2001
5
08:44 Oct-15-1998
Re: ultrasonic thickness gauging at cryogenic temperatures
: What is the effect of cold on common ut thickness (dual) probes?
: Does the -1% per 100 deg. F. velocity correction work in reverse?
: What couplant works best?

You might want to try a cellulose gum mixed with antifreeze and water and a wetting agent. (Ratio depends on Temp) and also be aware of the fact that antifeeze has severe corrosion effects on some materials.
In regards to which transducers to use, several manufacturers can supply dual transducers with removable / replaceable delays. I have found that these work quite well at 0 degrees and below. Use the antifreeze mix between the transducer and the delay.



    
 
 Reply 
 

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