Coolant antifreeze tester - does coolant temperature matter?

So you can help settle an argument for me. I bought one of those cheap $4 coolant/antifreeze hydrometer testers. The instructions say to make sure your engine is cool before you draw in the liquid. I said that this is only for safety reasons and that it wouldn’t impact the accuracy of the reading since it is testing for specific gravity. My friend says that the coolant can’t be hot or it will throw off the reading. I have asked two auto parts store guys and got the same two (different) answers. Checking online, - same thing …split decision.

So what is the real story? If the coolant temperature impacted the readings,then you’d get totally different readings if you tested it in the middle of winter when it is 20 degrees compared to a 100 degree hot summer day. That’s why I think it doesn’t matter if the coolant is hot or not - yet like I said…plenty of people and websites say the coolant has to be cold.

The coolant should be cool. These devices use density to infer the amount of coolant. Warm water is less dense than cool water (things expand when they get warm) Have you seen the temperature indicators that have floating balls in them, same thing.

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Copied this from somewhere: “… The hydrometer makes use of Archimedes’ principle: a solid suspended in a fluid is buoyed by a force equal to the weight of the fluid displaced by the submerged part of the suspended solid. The lower the density of the fluid, the deeper a hydrometer of a given weight sinks; the stem is calibrated to give a numerical reading…”

Since it measures relative density, you should measure when the fluid is cold. For the purpose of mixing coolant, however, it does not matter. The difference is too negligible.


I agree with @kurtwm2010 - it matters, but I’m pretty sure it matters very little. The balls also get hot or cold, so it’s now the difference in the thermal expansion rate between the coolant and the balls that would throw off the reading. Compared to the large difference in density between different mixes of antifreeze, I bet the temperature effect is VERY small. If it was large, you’d think there’d be a correction table for folks in Anchorage checking in the winter, vs. Houston checking in the summer…

It might depend on the details of the hydrometer you have.

Apparently, "Specific gravity is a property that relates the density of materials to the density of water at the same temperature. Specific gravity has no units because it is the density of a substance divided by the density of water.
[ ]
A liquid’s specific gravity can be determined with a hydrometer. The depth to which the hydrometer sinks is inversely proportional to the liquid’s specific gravity. A hydrometer is a hollow, sealed, calibrated glass tube. The hydrometer is put in the liquid and allowed to sink. When the hydrometer stops bobbing up and down the liquid level against the hydrometer will indicate the specific gravity."

Yeah, it depends on how fast and to what degree (pun intended) the internals of the tester change with temperature. Here’s the raw data from Dow Chemical. If the internals of the tester respond slowly, testing hot would indeed throw the results way off.

Picture 1

EDIT: Data in lb/gal.

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You could wait 3 minutes for the coolant in the hydrometer to cool and then view the results.

This would be a really easy way for the OP to figure out if temperature matters for his hydrometer.
Pick a time after the engine has run to hot and hot coolant has gone into the reservoir. Park and take some out of the reservoir and test a portion. Then continue testing a portion every X number of minutes until it cools to ambient temperature. If there is a temperature effect over that range, it will show up.
Science! Gotta love it!

If the numbers in the table are specific gravity measurements, testing hot looks like it’s going to err on the side of suggesting too little ethylene glycol (EG) being present.
For example, a 50% solution at 180 degrees is 64.80.
At 60 to 80 degrees, that value corresponds to something between 20 and 30%. So a bad conclusion would be add more EG to be at 50%.

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Actually they’re density…lb/gal, but your conclusion is correct.


Actually lb/cu ft

You are correct!

I have been wondering the same thing. Does temperature matter?

Answer is quite surprising - It sorta does but not downwards.

I have taken a sample from my Alfa 146 just today since temperature have fallen to negative C’s. I was worried about the accuracy because sample had -2°C and the label clearly says it should be 21°C.

So I took it home where I have comfy 23°C and have put it on a window to warm up. Aaaaand result was no difference. None whatsoever.

So a poured the content of my tester back in to the car and began experimenting.

I wanted to know what result do I get with just water, which is a great testing medium, since depending on pressure and wind it freezes between 5 to -1°C.

I have used the tester on the hottest water my tap could provide (about 70°C) and tester sank to zero.
Than I poured the coldest water I can get here (about 10°C) and tester sank only to -7 on glycol side of the scale.

Using this result, I feel confident saying that the measuring mistake by taking hot coolant out of your car can offset your reading by some 10°C. That is not taking in to account that glycols could potentially give vastly different results.

When it comes to my mixture, I found It’s okay, since the tester refused to sink all the way to the scale, leaving me to guess. My guess is it will take solid -70°C since it didn’t sink at all, and I know it works because of the test.