Cetane and Octane correlate to API specific gravity of the fuel.  the lighter the fuel in terms of API gravity, the higher the  Cetane or Octane.  Custom Made Item Not Refunable or Returnable - some assembly required

How do you KNOW what your fuel is - do you trust the numbers on the pump ???  You shouldn't.  In fact Diesel fuel is not even required to be listed as to it's Cetane on the pump.

You can measure it approximately with this 

Diesel fuel can be anything from home heating oil to Jet A kerosene.  API gravity can range between .8 and .9 (that is POINT 8 and point 9) and still be considered Diesel fuel - for Gasoline it  is point 7 and 8 - though gasoline closer to point 8 is really bad gas for today's autombile  - more like stuff know generically as white gas or "Coleman" fuel.  The higher up to .7 is considered to be a more "premium" fuel.

You can see the API gravity of various item with this chart => http://www.galloup.com/downloads/ref/specific_gravity_chart.pdf and keep in mind the "temperature at which they are measured is what matters.  In that chart as to Gasoline notice "Octane" and Gasoline both Natural and Vehicle.

Gasoline is complex blended beast - but the rule of thumb will always hold true to the closer to the gravity of "Octane" - .701 - the higher the actual "octane rating" of the gasoline - and lower - like .74 .75 etc the lower value the fuel.  It must be kept also in mind that adding Alcohol to fuel lowers the BTU per gallon and makes for a poorer performing Gasoline also, while it does make for a cleaner burning fuel - as in a lowering of noxious emissions.

This type of unit will measure either / or - but not both - so you need to specify which fuel you want to measure or the Diesel version will be shipped which measures 8 to 9.

This is a highly accurate version of this type of tool as one can see from the comparison picture of the scale on which our version each "hash' is ONE graduation of measurement - whereas on the competitors product one hash is FIVE graduation s of measurement.

It is hard to get photos of the hydrometer floating in fuel so we took one of them side by side - and on the right is the one that ships with this - showing .831 API - and the other showing essentially the same measurement but is really hard to read because the next graduation mark is just showing  at the liquid level which could be .830 or .831 or even .829. - and -  this is floating in out of the pump diesel fuel.  That is about 43 cetane - at the pump

.83 is about 39 cetane straight - whereas ..81 is about 43 and ..80 is 45 - Cetane and the straight non-additive performance out of the pump is substantial.    The heavier API fuel - the .83 is going to smoke a lot more under heavy acceleration and the lighter fuel will not - it will burn cleaner and give more power.

That is why accuracy is important and higher Cetane is always associated with better performance, but it is not the only factor - but you need to know what you start with out of the pump and then the smart Diesel driver treats it up from there with additives

See http://www.chevronwithtechron.ca/products/documents/Diesel_Fuel_Tech_Rev...

and 

https://bigbay4bestbuys.com/cetane_booster

and 

http://www.afdc.energy.gov/fuels/fuel_comparison_chart.pdf

This is a fragile precision instrument made of Pyrex glass shaped and sealed to float in fuel.  It can easily be broken by rough handling.  The item is made of a Pyrex tube upper with a cork cushion in the top so the impact of the actual hydrometer itself will not hit against a hard glass to glass contact; and the lower is made of plastic so as to absorb soft impacts by the different materials.  The plastic will fade and cloud and discolor over time - and the glass upper of course will not so you can always read the scale through ti.  The lower tube is irrelevant as you do not need to read anything through it

To use this item, you open both valves, and turn the unit perpendicular to the ground - that is horizontal - and insert the suction tube in the fuel (or tank) and pump as necessary to fill the device, then close both valves and then tilt it back up vertical.  The hydrometer will float in the fuel.  If you pumped in too much fuel and the top of the hydrometer is contacting the cork bumper, open the drain side and let some fuel out until the hydrometer is free floating both upper and lower in the fuel.

There is a stand shown in the photo, however it not included and not needed for the item to operate properly.  You can make the stand from a 3/4 railing flange plate and a 3/4 NPT to 1 inch plate PVC pipe union available at most any hardware, plumbing or home improvement - a.k.a. Home Depot type store. 

Included are the items otherwise shown, and the valves are ball valves pre-assembled as is the upper tube; and a  particulate filter so you do not pump debris into the hydrometer chamber. Some further assembly is required to install the hydrometer into the unit to create a sealed chamber.  The lower tube is epoxied in by you and does not come out - thereafter - with 5 minute epoxy that is included.  ALSO  included is a Cetane / API Gravity equivalent and index chart which can be used against a temperature index also.  There is no Octane chart  included

There is no warranty and it is not returnable, but will arrive in several pieces and the hydrometer sealed and protected in a box and foam so it arrives intact

We recommend you so not rough handle this item and you store it if possible outside of you vehicle in foam so it does not get bumped and tossed around in drying..  If you break the hydrometer after assembly it is not easily repaired.

If you learn to check and  treat your fuel - especially diesel fuel you can buy cheaper diesel and after you treat it - you will get better performance, easier starting and way less smoke.at cheaper costs per mile

This item will pay for itself when you can get the most out of the fuel you buy and by being able to know when you are being sold bunk.

Specify if you want the gasoline version which measures .7 to .8 in the checkout notes or the Diesel version will ship that measure .8 to .9.