Does anyone know if the red butyl phthalate oil for U gauges is still available?  I have had some people request it recently and was told that only the clear/amber stuff was available and since the red colored stuff is far more visible, it would be nice to find a source for it.  (Note, Voltarc lists it on their website, but may not actually have it.)  Alternatively, does anyone know what low vapor pressure compound was used to give it the red color?

I don't know where to get the red oil, but if you do find it you can add my name to the list of people who'd like some. :) Incidentally, what is the correct amount of fluid you should have in the butyl gauge? I think I may have too much in mine because sometimes when I'm filling to higher pressures the fluid shoots up to the top.

I will definitely let you know if and when I locate a source for it.  As for your question, it is dependent upon your specific U-gauge.  You must have enough fluid in it in order to reach the highest graduation on your scale (say, 25mm, for example) but when doing so, the entire U at the bottom, plus a little safety margin on the other side MUST also be fully filled with oil.  If you do not have enough oil in it and go to fill to a higher portion of your scale the oil will flow over to that side and uncover the bottom of the U part of the tube and allow gas (air or otherwise) to bubble past the U and up into the measurement column which will render the pressure measurement invalid.  The U can be thought of as a trap of sorts that keeps the two vertical tube columns separated (by the oil) so that a pressure applied to one side can be read on the other, so keeping an adequate amount in it is crucial to its operation through your scale height, but the actual volume you need will be determined by your specific gauge and since many of us make our own, there isn't an arbitrary amount.

Oh wow. Yep. That's exactly the problem. Here I was thinking that I might have too much fluid in my U gauge when actually I didn't have enough! And now that you've explained it, it seems so obvious to me. :) I feel a little silly that I didn't figure it out. In any event, thank you! I do have some butyl oil so I'm going to add more to my U gauge this week. I'm curious though...you mentioned that many folks make their own oil for the U gauge. What would you use for that? Maybe you could color it with something to make your own red oil.

Oh no, I didn't mean they make their own oil....I meant that they made their own gauge.  (bending the U tube and fitting the stopcock, etc......back when we all made our own manifolds, this was part of the process.....and since the heights of such a gauge glass could vary with personal preference or intended scale range desired, this is why an arbitrary fixed volume of needed oil for all can't be known.)