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This might be a strange question, but I am currently thinking about a good phrase to use if I want to emphasize that the theoretical omissions I am making are not caused by neglect, laziness, or because they don't fit my results or something. Instead, I want to quickly get across that the main reason I am not discussing them is that it would increase the section without adding anything of value.

Is there some good phrase that gets this across?

To give a specific example: Let's say I am describing an instrument that heats up a sample until it goes into a phase transition. The instrument works the same way no matter what phase the sample is currently in, but it's a different type of transition in each case. For solids, it would be melting. For liquids, it would be vaporization. For gases, it would be the formation of a plasma. The underlying principles are always the same, but depending on the transition different energies are necessary, the changes take different amounts of time etc., and of course the terminology is completely different. So even though it's the same, and I could talk about it in a very general way, I can't because language fails me. I simply cannot phrase it elegantly, so I have to omit stuff that is not absolutely necessary.

I can't say "I'm leaving this out because modern languages are not advanced enough to discuss this phenomenon in a general way, even though it's actually a very simple phenomenon. And my professor wants this to be 150 pages or less and I'm running out of space." But this is kind of what my problem is.

Any suggestions?

You could explain one case in detail, then cover the other cases by listing what terms to substitute for the other cases. Especially if you're dealing with applying the same equation with different terminology, you could provide a table of which variable is named what in each case.

E.g.

Because the principles only differ in terminology, only the solid to liquid transition is covered in detail. The equivalent terminology for each transition is in table 3.

Find a good source of the appropriate discussion and include a reference. "This is discussed in detail in Smith, Jones and Brown, 2005."

The mathematicians have a good way of doing it: declare it trivial.

In your heater example, let's say that you demonstrated some results in a liquid. Then you start your section with:

In this section, we present results obtained from heating a liquid sample with a Widget2000. These results can be trivially extended to samples in a gas or solid phase. Since the liquid case is of particular interest for this thesis, other types of sample are not described in detail.

As long as your readers know very well that your statement is correct and intuitively agree that the other cases are trivial, without having to think about why they are equivalent, this is a good way to do it. If you have some doubt - e.g. because somebody has to have a certain level of background in your field to know that it is indeed trivial to demonstrate the equivalency - it is also advisable to add a citation of a source which discusses the equivalency.

Alternatively, if you think that your readers will disagree with the assessment of "trivial", but also don't have sources which show proof, you can write instead:

In this section, we present results obtained from heating a liquid sample with a Widget2000. Solid and gaseous samples show analogous behavior, but demonstrating a proof for that is outside of the scope of this thesis.

Or, if you want to play it safe

In this section, we present results obtained from heating a liquid sample with a Widget2000. We also assume that solid and gaseous samples show analogous behavior, which is a standard assumption in the field of heating samples by widgets [here you cite 2-3 popular papers which do the same handwaving as you do].

I know that this might sound strange, since there "trivial" also has a different, negative meaning in everyday language. In a scientific text, it means "it can be proven with somewhat tedious, but entirely straightforward work, without any gotchas". There is no negative valence attached to it in this sense, and it is used for exactly the purpose you want: to signify that you are streamlining your text and omitting the true but boring parts.

1. 
   

   Just ignore the gap. Honestly, if you are just using a calorimeter in the context of several other aspects (synthesis, conductivity measurements, etc.) than nobody needs you to go through every detail of instrument design. Consider someone using X-ray diffraction, even as a primary measurement doesn't need to go through the Laue equations and how to build a diffractometer. They just give specific info pertinent to the particular research (used a Rigaku machine with blabla settings).

   
   
   

   Of course if your project is ON the DSC, you need to discuss it.

   
   
   

   Maybe it is sort of halfway between the examples I listed. But still. I think this "omission" is bugging you more than it will others. Just write up your work and don't bother discussing the instrument so much. I bet nobody notices.

   
   



2. Publish a paper on the calorimeter. (Even in a teaching or instrument journal.) Than cite that.



3. 
   

   Practical advice A: don't let something like this slow you down! Write the whole thesis and leave a little "hole" where the calorimeter theory goes. This is psychological. Once the entire paper is almost done, you will have a lot of momentum built up. Plus you will have been thinking about it on the back burner. At that point, you can just plop something in there.

   
   
   

   After all we have word processors now. You can work on different sections out of order. I used this strategy on my own science writing and it really pushed things forward. Felt good because I saw all this progress happening. And then the holes looked small at the end and got filled in easily. Heck. I even did it with experimental results! Wrote the paper without having all the experiments done (cause I was sick of them). And then having something all done except that one missing piece motivated me to finish off the gap.

   
   



4. Practical advice (B): Write it all "long". And then edit it down later. Sure it's a little more work. But this is actually common in the work world that you will have documents with length constraints. Write it up in detail and then go through an editing process to shrink it.

One solution is to describe one of the cases in detail and say that the others are "similar" or "analogous".

However, from the example you give, I'm really not seeing the problem. Why can't you just say something along the lines of the following?

The device heats the sample to the supply the appropriate amount of energy to cause a phase transition (melting for solids, vaporization for liquids, plasma formation for gases). The time taken will depend on the amount of energy required.

That's about a third of the length of what you wrote. Obviously, you'll need to add more details than you included in the example in your question but I don't see why you can't cover those in a similar way. It doesn't matter that the different phase transitions have different names, because you can just talk about "the phase transition"; it doesn't matter that the phases have different smells because you can just talk about "the smell of the phase".