Low viscosity, low surface tension liquid for volume measurement General Case
This question concerns the general problem of accurate volume measurement of a given object. Among other techniques, that of liquid displacement is sometimes used for this purpose.

In case of a solid material, the latter may be immersed in liquid, and its volume calculated by measuring the amount of fluid displaced in a graduated cylinder.

In case of a hollow object, the latter may be filled in with a liquid which amount is accurately determined, either while filling in, or by emptying the object in a graduated container a posteriori (or - maybe more accurately - by weighing).


In both cases, the choice of the liquid is of crucial importance for the accuracy of the measurement.
Specific Issue
I have a CO2 gas sensor with a complex hollow embodiment, also containing electronics, and since I wish to perform response time measurements, I would like to precisely know its inner volume. I came up with the idea of filling it with a liquid, as stated above. I do not have and cannot have accurate drawings of the inner sensor.
Questions
1. Is there other methods to perform such a volume measurement which may be preferable?
Indeed, I came up with this idea because I find acceptable to sacrifice a sensor to perform such volume measurements. Alternatively, I also thought of placing the sensor in a closed, accurately known volume, along with some amount of chemical reagents that would produce a given amount of CO2. Then, by dilution and measurement of the CO2 percentage inside the enclosure, I may know the volume occupied by the sensor. However, I believe that there are far too many sources of errors in such procedure for it to yield accurate results. I may be wrong, though, and other variations of the latter idea may work well...
2. Which liquid may be the best for such measurement?
This is the main issue here. I would tend to say that such a liquid should have both a low surface tension and a low viscosity. Additionally, low volatility would be a plus, since it would prevent the amount of poured liquid to decrease with time. I find methanol or ethanol to be good canditates despite their high-volatility. I cannot use acetone because of the risk of plastic dissolution and the resulting leakage of the sensor's embodiment. Would you know of a more suitable liquid, possibly readily available in the lab?
3. Can surfactant be added to ethanol or methanol? If so, do they further decrease surface tension? Which are they?
A quick search in the literature revealed that methanol or ethanol are themselves considered as water surfactant. Their mixture with other surfactants is often analysed in emulsions of oil, or other chemicals, but not alone.
Related topics:

Non-wetting fluids for precise volume measurements
Looking for a non-volatile liquid less viscous than water


Edit:

As pointed out by Poutnik, low volatility and low viscosity are mutually incompatible with each other. Indeed, low viscosity tends to be positively correlated with high volatility, both of them being the consequence of low intermolecular forces. More resources on the topic are available here (saved version). The requirement on low volatility was not mandatory though, as it was just to ensure that not too much of the liquid evaporates during the filling of the sensor...
Air would be a convenient fluid. Fill your sensor with air at a known (atmospheric) pressure - well, it is probably filled with air already.
Then connect it to a vacuum chamber tube of known volume with a pressure sensor, i.e., a tube with two valves and a pressure sensor. Attach a vacuum pump to one valve and evacuate the tube; attach the $CO_2$ sensor to the other valve and then open it and measure the new pressure in the closed system. All the air will have been supplied by the sensor volume, and the new system volume will be the sensor volume plus the (known) tube volume. Doing it this way is likely to be less messy and more accurate than using liquids, and it won't sacrifice a sensor - and you can check every sensor!
A problem with liquids in a blind experiment (you cannot see the internals) is that there could be blind alleys that would be hard to detect and hard to fill with liquid (or which would not empty easily).