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# film exposur time calculation

## film exposur time calculation

(OP)
We are having a external partner to do some radiographies for us and I am searching for a way to check if he is using the correct exposure time. The thickness is 0.5, 2, 5 for Al, cu and ss and Dustance= 540cm A=10,Kv=100 and the film is D4.
I have found the formula for time exposure but I dont really understand how to calculate Q. Is there any relevant programme or website calculator I can use?
The only one I have found is this one http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/TechCalibrations/exposurecalculations.htm but still seems not to be working.
Any ideas?
Replies continue below

### RE: film exposur time calculation

Ndtmetal,

In my opinion there are too many variables to calculate.  In addition to material thickness, SFD, mA, and KV you listed in your post, you would also need developer temperature and time in tank, and the desired density.  If manual processing, you need the age of the developer and fixer.

The applet you referenced lets you change mA, time or material type keeping KV and all developer parametes constant.  That is not the situation you have.

JR97

### RE: film exposur time calculation

(OP)
So what do you suggest??

### RE: film exposur time calculation

(OP)
Just tomake something clear.
I dont care about the development of the film time, but for the exposure to radiation time.

### RE: film exposur time calculation

Ndtmetal,

The longer the film is in the developer, the darker the film gets at a cost in latitude.  Increasing development time or temperature will allow a decrease in exposure time; to a certain extent.  Excessive time in the developer will increase fog to the point the film is useless.

Shorter development time must be compensated for by increasing exposure time.

Bottom line is all the variables are interrelated.  There is no single formula to calculate how to make a radiograph.  The radiographer must know his equipment, his processor, and the part being radiographed to make an acceptable radiograph.  The usual course of action is to take a guess at parameters, develop it, and adjust parameters based either on experience or a calculator like the one you found.

JR97

### RE: film exposur time calculation

I agree with JR97.  Common shadey practice is to underexpose & overdevelop, or overexpose & underdevelop.  But if you have the densities, and you can see the holes in the pennies that you want to see, then let your vendor work out the technique.

### RE: film exposur time calculation

Hello Ndtmetal

why worry about exposure-time er devellopement-time?
If you get a good and correct reading at the quality indicators (which should be used) the film is made according to the correct standards.
Why is it you want to look at the exposure time?
If you think it is too short, have the distance changed.
make it longer.
With the parameters you specify  you should get good pictures.
You use D4 but you donot give the the format. With the short focus you can only go to 10 x 24 cm or 24 x 24 cm

Harry

### RE: film exposur time calculation

Ndtmetal,
When dealing with a x-ray tube, each one will have it's own characteristic curve. This will play in with all the other factor for proper exposure like SFD, focal spot size, r factor of film, and density of material. The temp of the developer will change the time required in the developer. The hotter the developer the shorter the time needed to develop the film. As others have stated it is not uncommon for techs to over/under develop the film (also called sight developing) if the x-ray was shot at an incorrect time. This is done to "save the film" and is a poor practice. When film is developed like this it affects the longevity of the film. Most major codes require that the film be retained for 5 years. Film that is improperly developed will not last this long. I know I can get the appropriate wire to show on a film that was under shot by 25% and over developed by 25%. It have poor quality and not last long, although it will be code film.

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