I have a ground source heat pump with methanol antifreeze in the ground loop; and a hydrometer which is supposed to read methanol or ethylene-glycol solutions (at the time, I couldn't find one with specific-gravity readings). I need to check the freezing point of the ground loop fluid. I don't have instructions for the hydrometer, and the scales have me baffled. Photos attached, °F and °C scales. Note that on the top (Methanol) °C end, the degrees are decreasing, starting at -40, but once it gets to -12 it jumps back and starts repeating starting at -34, with a smaller increment between values. Similar for the ethylene-glycol scale, except that the Methanol scale temperatures increase as you go down, while the Ethylene-Glycol scale temperatures decrease as you go down. Also note the point where the background of the scale changes from yellow to white, and that the readings on the white scale and the first yellow line are not as far apart as the ones above, and appear to be of varying widths. The varying widths would make sense because of non-linear characteristics, but the numbers are totally baffling. °C scale °F scale

  • It’s all about density and you have a good answer - accept it.
    – Solar Mike
    Commented Dec 28, 2022 at 9:42
  • You can also put a small sample in a freezer and measure the temperature at which it melts. Slower but accurate. Commented Dec 28, 2022 at 16:00
  • Can propylene glycol be used instead of ethylene glycol in these ground source heat pump loops? Propylene glycol is relatively non toxic at least to mammals. If there is ever a leak with a methanol or an ethylene glycol mixture, pets or local wild fauna could be poisoned. If propylene glycol -- water mixtures have higher viscosity or lower heat capacity than ethylene glycol, I can see it might not be a drop-in replacement, but it should be investigated. Commented Dec 29, 2022 at 12:57
  • @Jim Stewart Propylene glycol can be used in some situations, but it is not the best choice for colder climates because of viscosity issues. see geojerry.com/earthloopantifreeze.html Commented Dec 30, 2022 at 15:13

1 Answer 1


Methanol is less dense than water. The more methanol in solution with water, the greater the freezing point depression and the lower the specific gravity. The bulb sinks more in the less dense fluid, so lower temperatures are at the top of the hydrometer.

Ethylene glycol is more dense than water. The more glycol in solution with water, the greater the freezing point depression and the greater the specific gravity. The bulb floats higher in the denser fluid, so lower temperatures are at the bottom of the hydrometer.

  • Assuming the methanol scale is the one above the line in the middle, why does the scale change from +10 to -30 and start repeating, where the scale color changes from yellow to white? If the fluid is good to -23°C, where will it read -- the white -23, or the yellow -23? Commented Dec 28, 2022 at 17:34
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    @GaryAitken If you take your current uncertainty to Chemistry SE, you may get someone to explain the scale to your satisfaction.
    – MTA
    Commented Dec 28, 2022 at 23:52
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    @Jim Stewart I understand there are two different scales; what I don't understand is why there are 4 instances of the same value -- e.g. on the F scale, there are 4 instances of -10. So there are two -10F for Methanol, and two -10F for ethylene-glycol. It can't float at two different places for the same fluid, so why are there 4 values? Commented Dec 29, 2022 at 4:45
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    @GaryAitken Actually, yes it can. See the above reference to a eutectic system. For ethylene glycol, the freezing point of -10F occurs at both 40% glycol and 93% glycol, and these have different densities. The lowest freezing point is at 62% glycol, with higher freezing points at higher OR lower concentrations and densities. That's how eutectic systems work. See graph at gi.alaska.edu/alaska-science-forum/dont-fill-her-antifreeze I don't have data for methanol.
    – MTA
    Commented Dec 29, 2022 at 5:25
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    @MTA Thank you! That's what I needed. I wasn't able to get that picture from th wikipedia article, as I didn't fully understand it. Commented Dec 29, 2022 at 23:39

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