Speed of sound in liquids
Reference values, and more usefully, how to use the sound speed your meter reports as the fastest diagnostic you have.
| Water, 0 °C | 1403 m/s |
|---|---|
| Water, 10 °C | 1447 m/s |
| Water, 20 °C | 1482 m/s |
| Water, 30 °C | 1509 m/s |
| Water, 40 °C | 1529 m/s |
| Water, 50 °C | 1543 m/s |
| Water, 60 °C | 1551 m/s |
| Water, 70 °C | 1555 m/s |
| Water, 80 °C | 1555 m/s |
| Ethylene glycol 30%, 20 °C | ~1560 m/s |
| Ethylene glycol 50%, 20 °C | ~1620 m/s |
| Propylene glycol 30%, 20 °C | ~1580 m/s |
| Sea water, 20 °C | ~1522 m/s |
| Light fuel oil, 20 °C | ~1250 m/s |
| Diesel, 20 °C | ~1250 m/s |
| Lubricating oil, 20 °C | ~1440 m/s |
| Methanol, 20 °C | ~1103 m/s |
| Ethanol, 20 °C | ~1160 m/s |
| Glycerine, 20 °C | ~1920 m/s |
Note the shape of the water curve: sound speed rises with temperature to around 74 °C, then falls. It is the only common liquid that behaves this way, and it means a temperature error near the peak has very little effect on the reading while the same error at 10 °C matters considerably.
Using measured sound speed as a diagnostic
Both meters report the sound speed they are actually measuring. Comparing that to the expected value for your fluid at your temperature is the single fastest way to catch a setup error, and it takes ten seconds.
- Measured value close to expected — pipe data is probably right. Proceed.
- Measured value significantly high or low — the wall thickness entry is wrong nine times out of ten. The meter derives fluid sound speed from the total transit time minus the time it believes the signal spent in the wall. Get the wall wrong and the remainder is wrong.
- Measured value wildly out, weak signal — check for a lining you have not entered, heavy internal scale, or entrained air.
- Value drifting during the reading — usually temperature, occasionally air passing.
Glycol is where people get caught
A 30% ethylene glycol mix runs around 1560 m/s against water's 1482. Enter it as water and the meter's geometry assumptions are off by roughly 5%, which propagates into the flow figure. Worse, glycol sound speed varies with both concentration and temperature, so a mix that somebody topped up with plain water three winters ago is not the concentration on the label. Measure it with a refractometer.