— copy and Flash slideshow by Ralph Birnbaum
(This article includes a Flash slideshow demonstrating refractometer use.)

Here in the Midwest, we normally have many winter days when the temperature drops to single Fahrenheit digits. Occasionally, however, we join our friends to the icier North, and experience arctic temps that blast down from Canada like a runaway freight train at 10 or more degrees below zero F.

When it's colder than a well-digger's belt buckle, it sure is nice to know that the tester you used to check your antifreeze last fall doesn't have a horseshoe pit accuracy of +/-10 degrees. Close enough won't get it. It's the reason I traded my hydrometer for a refractometer.

 

Hydrometers are still a common sight in toolboxes. In fact, hydrometers account for roughly 75 percent of all engine coolant/antifreeze testers sold, with refractometers and test strips rounding out the field. Let's face it, hydrometers continue to be popular because they're cheap, and the plastic jobs with the floating balls or pellets are virtually indestructible. We found dozens of cheap plastic hydrometers on the internet for under 10 bucks...some WELL under 10 bucks.

Question is: Is a hydrometer accurate enough when 5 or 10 degrees of protection is the difference between liquid coolant and a cracked engine block or popped freeze plug?

Even a precision, laboratory hydrometer used by a trained technician in a controlled environment, using readings mathematically corrected for fluid temperature, is not the most accurate way to test antifreeze concentrations.

According to the American Society for Testing and Materials (ASTM) Method D1124, the best accuracy achievable with a hydrometer used under these specific conditions is plus or minus 8 degrees F. Out in a shop, that unacceptably large window only grows larger.


Hydrometers
A hydrometer is a test device made of a container (commonly a glass or clear plastic cylinder) used to draw and measure the specific gravity of a sample fluid. Inside the sample cylinder, there is a float that contains a carefully measured ballast weight.

 

No Leonard, it's not a turkey baster.

A flexible pickup tube is commonly attached to the bottom of the cylinder: a squeeze ball located at the opposite end is used to create suction that draws a fluid into the cylinder through the pickup tube. As the fluid rises in the cylinder, the float rises. Its final floating height depends on the specific gravity of the solution.

A measurement scale is displayed on the float. This scale indicates the specific gravity of the sample.

Disc- and ball-type hydrometers use a similar, although simplified approach. Instead of a single calibrated float, the sample tube contains different weight discs (or balls). The more discs or balls that float on a drawn sample, the higher the specific gravity of the liquid. A few have a single float that pivots upward as it is lifted by the buoyancy of the incoming sample.

Specific Gravity into Degrees of Temperature
Hydrometers convert a specific gravity measurement into a freeze point level.

 

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Hydrometers convert a specific gravity measurement into a freeze point level.

The problem with this approach is that specific gravity is extremely temperature dependent. If the specific gravity reading is not temperature-compensated, we will almost surely end up with inaccurate readings.

Colder coolant is denser than warm coolant, right? So two samples of the same coolant can have very different specific gravity values, depending on their temperature when tested. A sample fluid tested at 100 degrees will indicate a freeze point many degrees lower than the same sample tested at 150 degrees!

Sampling technique is also critical to hydrometer accuracy. Air bubbles in the coolant sample can “float the float” and cause inaccurate readings. The hydrometer also needs to be held vertical when taking a reading to keep the float centered in the cylinder, away from the glass cylinder wall where surface tension or friction can affect float height. This is more important with single-float style hydrometers because the float is prone to lean into the wall. In disc- and ball-type hydrometers, the floats are entirely submerged and thus virtually free from friction.


Refractometers

 
The refractometer is easy to use.

Temperature Compensation

A worthy refractometer has automatic temperature compensation (ATC), and offers the greatest accuracy.

Temperature compensation occurs when a small bi-metallic strip inside the refractometer adjusts its lens position to keep the readings accurate, regardless of temperature. Without ATC, refractometer readings can be off by many degrees.

Unlike the hydrometer, the temperature of the instrument, not of the liquid, is the critical factor with the refractometer. This is due to the fact that for refractive index measurements only a drop of the sample is required. The tiny mass of two coolant drops is so small that it immediately assumes the temperature of the instrument.
 

 

A refractometer with automatic temperature compensation automatically corrects all readings, freeing you from concerns about temperature. In fact, a good ATC refractometer  is commonly accurate to ±1 degree F for determining the freezing point of aqueous engine coolants, according to ASTM Method D3321, the standard practice for use of a refractometer.


Why Accuracy is Important
Most general purpose coolant is a 50/50 antifreeze/water mix, providing freeze protection to -34 degrees F. Increasing antifreeze content in the coolant mix to roughly 60 percent improves freeze point protection levels, but above 70 percent, freeze point protection levels worsen. In this case more is not better.

Too much antifreeze causes problems. Years ago, an OEM used a coolant mix that was 70+ percent antifreeze and it caused all sorts of problems, especially overheating and water pump failure.

Water pumps are designed to work with a specific viscosity of fluid. Increasing concentration increases the viscosity of the fluid, making pumps work harder, reducing their service life.

There is also a phenomenon known as cavitation corrosion associated with concentrations exceeding 70 percent. This is the formation of microscopic bubbles within the coolant/antifreeze. These bubbles and the silicate particles from the additives that contain them abrade metal surfaces, rapidly eroding cylinder heads, liners, head gaskets, water pump impellers, and even the radiator.


Badly eroded impeller.

Other common problems associated with too much (or too little) antifreeze in the coolant mix include scale formation, gelation, poor heat transfer and boil-over, freezing and cracking engine blocks, and solder bloom; problems defined by the Society of Automotive Engineers (SAE) for incorrect antifreeze concentrations.

If you want to ensure that the antifreeze concentration in your coolant mix is correct, an accurate measurement tool is a must.