13:36 Oct-23-2009 aldo Engineering Italy, Joined Jan 2009 39
viscosity of penetrant
Hy everybody.... I have readed on siste www.ndt-ed.org the follow sentence: " the viscosity has little effect on the ability of a penetrant material to enter a defect but it does have an effect on the speed at which the penetrant fills a defect.". On quiz recenly for second livel MPT there was a question on viscosity:
"the speed with wich a penetrant penetrate a surface flaw is influenced to the greatest extent by wich of the following properties? :
b) surface tension and weat ability
d) capillary action"
On base of the sentence on internet site i replied the "C" option but the exact option was "D"...
Who is right?
N.B. : the quiz are ASNT......
I would say C, viscosity. Capilary action is just a term that has to with "the possitive force that causes movement of certain liquids along narrow or tight passages." Viscosity is a veriable that can change, and changing the viscosity changes the rate of capilary action.
I will agree with the test. Answer 'D' is the most correct. For any given penetrant, capillary action is the most important factor in penetration rate. Changing viscosity is a second-order variable: if the viscosity of a given penetrant changes, the rate of penetration will change; but the major influence on rate of penetration is still the capillary action.
The viscosity of the penetrant can change due to tempurature. The manufacturer also controls the viscosity of the penetrant. In colder tempuratures, the penetrant gets thicker, resulting in the capilary action moving slower. This will mean a need for an increased dwell time.
I think 2 concepts will be cleared: 1 is SP( STATIC STATE PENETRATION) IS NOTHING TO DO WITH viscosity and this is the nature bility of liquid penetrant;but KP(Danamic penetrant) is relative with viscosity,and it will affect the penetrant speed rate that mean dwell time will be long and another will be affect the developing time or mask the defect. for such status,you should do contrast test between block and testing specimen to assure the dwell time and developing time.
As in the ASNT Guide ' Viscosity affects the rate of penetration ' . Higher the Viscosity slower the Penetration & Lower the Viscosity faster the penetration, But if a liquid has to penetrate it has to have low contact angle & good wettability which are responsible for the cappillary action of a liquid. A liquid with high contact angle & low viscosity may not even penetrate a flaw. So 'D' would be right choice
Bit worrying this, as a 15 year level III (admittedly not in PT) i would have said 'answer is wrong, you were right, its C' and wished they could have a higher standard of test papers. But it seems we can't even get a consensus here is the group....
Hi all and tanks for your words and your opinio... While i was reading your opinions i readed on same above inetrnet site the expression of the fill time. In this formula i have not viscosity term but i have only the capilarity term (wite angle and energy surface). So on base of this i think that the viscosity i have not influence on speed of penetrant to penetrate into flaw...Is correct my opinion? and is possibile to have relation between the fill time and the speed ?
tanks and best regards....
I think vicosity is a major influence in wetting ability and the contact angle.But minor in ability to enter a defect,that could be capillary action.Some highly viscous fluids, such as molasses, have very good penetrating ability, while some low viscosity liquids, such as pure water, have very poor penetrating ability. However, from an application viewpoint, viscosity affects the speed with which a penetrant enters a discontinuity.Therefore both viscosity and capillary action can determine to apply the time to penetrate or Dwell Time.
I think this multiple choice question should not even be on an exam, whether Level I, II or III. Capillary action is an event that happens. Density and viscosity are properties of the penetrant. Wetability is a property of the solid surface. Surface tension is related to the solid surface and to cohesive or adhesive properties of the penetrant. I'm not sure that the exact mechanism is really all that well understood and I don't think NDT personnel need to be experts in this. we do need to understand its practical aspects. One thing we can tell from using test blocks is that penetration time may have to be increased at very low ambient temperatures when the viscosity of the penetrant (or any liquid) is higher. This establishes a relationship between viscosity and the speed with which a penetrant enters a surface flaw.
The key to the question is the word "speed". The answer should be viscosity. Low viscosity equals a greater rate of speed in which the penetrant penetrates the defect. And obviously, higher viscosity means decreased speed. Caplliary action is just the ability of the pentrant to be effective penetrating into tight flaws. It doesn't have much to do with the speed in order to accomplish it.
viscosity is property of liquid and control the rate of flow but capillary action is the ability of liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces such as gravity.It occours because of intermolecular attractive force between the liquid and solid surrounding sufrace.Both Answers C & D are very close and both have affect on rate of penetration.but the closest answer will be (D) in my opinion.which seems to me more acurate.