I may be out on a limb here but I don't think there will be a suitable one. Not only is not-flammable while lower specific gravity than water pretty rare, so are materials that dissolve completely in water.

Is density really that important for testing a venturi pump? Density is more of a centrifugal pump problem.

Nope, your only real option if you need to replicate the density is to use something like a white mineral oil.
These will be about the right density and they have flash points up in the range of 200C (depends on viscosity).
This can be modeled well, testing at different density and viscosity is not commonly done.

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P.E. Metallurgy, consulting work welcomed

The "something" would need to be added in at least 50% by volume I would have said and there is no magic power to do this.

If its designed to use diesel why not use diesel? I can't see why not at present.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Ucuhuba butter oil SG 0.870 is your only hope.
But I don't know how flamable ucuhuba butter is.
Probably expensive too.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

Thank you for the responses.

In my initial research, I pretty much only found alcohols that had a low enough specific gravity that could be added to lower water's specific gravity to match diesel's.

The density of the fluid involved does indeed matter in the efficiency of a jet pump. One factor of jet pump performance are loss coefficients based on Reynold's Number.

I have been testing with diesel, but a I want something that is easier to work with (no fumes, smell, less corrosive, can be used with more pumps, and won't give me a diesel rash haha).

It sounds like mineral oil with a similar viscosity is the best option as of now.

I think what you're looking for is a fluid that has a similar viscosity to diesel, not necessarily the same density.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

Diesel is a mineral oil. If you want something less adulterated, try kerosene or lamp oil as an alternative.

Here is a silicone oil at 5cSt.

https://www.clearcoproducts.com/pure-silicone-low-...

They have lower viscosities as well, you can mix and match to equal diesel at 3-4cSt

At near boiling, you can get the SG down some from your reference density but not near as much as you want of course.

Was going to suggest silicone oils also. But get a quote on the stuff before you start down that road...

Those silicone oils are expensive. I was looking for an oil with a gravity over 1.024 to displace salt water out of a dead ended head pressure line to a seal. It was cheaper to pull the boat out of the water and drain the seal housing.

We used to use a pure PAO base oil, viscosity about 3 and SG about 1.2 (10lb/gal).
And real expensive.

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P.E. Metallurgy, consulting work welcomed