How do I read a hydrometer?

[bristleworm]

I read this as 1.0258
And is this close enough?

 

[EnterName]

You read from below the water surface at the lowest point of the water.

(Source: https://www.beerlab.co.za/blogs/new...al-guide-to-using-your-hydrometer-effectively)

(Source: https://www.essentialdistilling.com/resources/how-to-use-a-hydrometer)
​

This applies to all hydrometers, so don't worry that these are for brewing and distilling

 

[bristleworm]

Ya I screwed up and got a plastic 500ml cylinder and you can't read through it.
Gotta get a glass one.

 

[KrisReef]

If you read it from the top, and use that method consistently you can keep your salt density stable, even though I bet a beer you wouldn't get the real number even with a glass cylendar to read "properly." In other words, your measurement is close enough, especially if you incorrectly read the density consistently from the top.

Always drink responsibly, and don't sweat the meniscus. Your reef will never know.

 

[winxp_man]

Ya I screwed up and got a plastic 500ml cylinder and you can't read through it.
Gotta get a glass one.

It’s what I used. And my tanks are all good. As long as the meniscus is visible and at the let’s say 1.025 even if the true look through value is at 1.0252 it will not make a diff. The bigger thing to understand is time. You need to use a temp adjustment calculator. The hydrometer is calibrated for 77*. Well the tropic marin one is.

 

[bristleworm]

Ha, I've attempted a few keg stands in my day!

Now the next question, so many questions,
Why is that reading so far off on my refractometet?
I calibrated it with calibration fluid and I trust the TM hydromerer more. But it's way off.

 

[BeanAnimal]

I have a glass 1000ml graduated cylinder. I float the hydrometer and take a photo.

 

[Gone Reefin’]

Ha, I've attempted a few keg stands in my day!

Now the next question, so many questions,
Why is that reading so far off on my refractometet?
I calibrated it with calibration fluid and I trust the TM hydromerer more. But it's way off.

I’m with you. I have multiple refractometers (not sure why) that all read in line. However, my hydrometer ALWAYS reads 0.001-0.002 lower than them. Even my ICP agrees with the refractometers, however I understand this to be a calculated value so not much weight there. All the talk about the gold standard hydrometers, maybe everything else is wrong…

 

[BeanAnimal]

Check both against Randy’s DIY standards.

 

[EnterName]

Check whether all your devices are actually calibrated the same way. Each device has two calibration points (this includes hydrometers and refractometers):

1. T₁ = The temperature at which you're supposed to take the sample for the measurement.
2. T₂ = The temperature of the freshwater you're comparing the density against. After all, you're measuring the relative density, meaning relative to the density of freshwater, which also changes with temperature. So T₂ is another calibration point the manufacturer has to decide on.

For "specific gravity" measurements, these two temperatures are usually equal:

T₁ = T₂​

But it doesn't have to be this way. My hydrometer, for example, has T₁ = 25°C/77°F, so it can be used with reef tank water directly, and T₂ = 4°C, where water approaches a density of 1 kg/L. That way I can read the actual density (not the relative density) directly off my hydrometer.

The international version of the Tropic Marin hydrometer (the one you get in the US) is calibrated with

T₁ = T₂ = 25°C = 77°F​

while your refractometer might be calibrated with

T₁ = T₂ = 20°C = 68°F​

This is often shown as "d20/20" in a corner when you look through the refractometer. In that case, the measured specific gravity will differ slightly. On a d20/20 device, taking a sample at 20°C/68°F and reading 1.025 means a salinity of 32.9, while on a d25/25 device, taking a sample at 25°C/77°F and reading 1.025 means a salinity of 33.19.

So the way you convert your specific gravity readings to salinity differs slightly depending on the calibration. That might explain the difference in readings between your devices.

Resources for the nerds:

• Millero & Poisson, 1981 - International one-atmosphere equation of state of seawater. Deep-Sea Research Part A, 28(6), 625–629.
• Bigg, 1967 - Density of water in SI units over the range 0–40 °C. British Journal of Applied Physics, 18(4), 521–525.
• Lewis, 1980 - The Practical Salinity Scale 1978 and its antecedents. IEEE Journal of Oceanic Engineering, 5(1), 3–8.

 

[bristleworm]

Check whether all your devices are actually calibrated the same way. Each device has two calibration points (this includes hydrometers and refractometers):

1. T₁ = The temperature at which you're supposed to take the sample for the measurement.
2. T₂ = The temperature of the freshwater you're comparing the density against. After all, you're measuring the relative density, meaning relative to the density of freshwater, which also changes with temperature. So T₂ is another calibration point the manufacturer has to decide on.

For "specific gravity" measurements, these two temperatures are usually equal:

T₁ = T₂​

But it doesn't have to be this way. My hydrometer, for example, has T₁ = 25°C/77°F, so it can be used with reef tank water directly, and T₂ = 4°C, where water approaches a density of 1 kg/L. That way I can read the actual density (not the relative density) directly off my hydrometer.

The international version of the Tropic Marin hydrometer (the one you get in the US) is calibrated with

T₁ = T₂ = 25°C = 77°F​

while your refractometer might be calibrated with

T₁ = T₂ = 20°C = 68°F​

This is often shown as "d20/20" in a corner when you look through the refractometer. In that case, the measured specific gravity will differ slightly. On a d20/20 device, taking a sample at 20°C/68°F and reading 1.025 means a salinity of 32.9, while on a d25/25 device, taking a sample at 25°C/77°F and reading 1.025 means a salinity of 33.19.

So the way you convert your specific gravity readings to salinity differs slightly depending on the calibration. That might explain the difference in readings between your devices.

Resources for the nerds:

• Millero & Poisson, 1981 - International one-atmosphere equation of state of seawater. Deep-Sea Research Part A, 28(6), 625–629.
• Bigg, 1967 - Density of water in SI units over the range 0–40 °C. British Journal of Applied Physics, 18(4), 521–525.
• Lewis, 1980 - The Practical Salinity Scale 1978 and its antecedents. IEEE Journal of Oceanic Engineering, 5(1), 3–8.

I think you nailed it. My refractometer is 20/20.

My calibration solution lists both the 25c and 20c values and i calibrated to the 25c value.

 

[Randy Holmes-Farley Verified Community Expert]

As noted above, you read the bottom of the meniscus:

Chemistry and the Aquarium: Specific Gravity: Oh How Complicated! ? Advanced Aquarist | Aquarist Magazine and Blog

from it:

Figure 4. A Tropic Marin hydrometer showing the meniscus rising to about 1.0260, but the actual reading is about 1.0265.

 

[Gone Reefin’]

Check whether all your devices are actually calibrated the same way. Each device has two calibration points (this includes hydrometers and refractometers):

1. T₁ = The temperature at which you're supposed to take the sample for the measurement.
2. T₂ = The temperature of the freshwater you're comparing the density against. After all, you're measuring the relative density, meaning relative to the density of freshwater, which also changes with temperature. So T₂ is another calibration point the manufacturer has to decide on.

For "specific gravity" measurements, these two temperatures are usually equal:

T₁ = T₂​

But it doesn't have to be this way. My hydrometer, for example, has T₁ = 25°C/77°F, so it can be used with reef tank water directly, and T₂ = 4°C, where water approaches a density of 1 kg/L. That way I can read the actual density (not the relative density) directly off my hydrometer.

The international version of the Tropic Marin hydrometer (the one you get in the US) is calibrated with

T₁ = T₂ = 25°C = 77°F​

while your refractometer might be calibrated with

T₁ = T₂ = 20°C = 68°F​

This is often shown as "d20/20" in a corner when you look through the refractometer. In that case, the measured specific gravity will differ slightly. On a d20/20 device, taking a sample at 20°C/68°F and reading 1.025 means a salinity of 32.9, while on a d25/25 device, taking a sample at 25°C/77°F and reading 1.025 means a salinity of 33.19.

So the way you convert your specific gravity readings to salinity differs slightly depending on the calibration. That might explain the difference in readings between your devices.

Resources for the nerds:

• Millero & Poisson, 1981 - International one-atmosphere equation of state of seawater. Deep-Sea Research Part A, 28(6), 625–629.
• Bigg, 1967 - Density of water in SI units over the range 0–40 °C. British Journal of Applied Physics, 18(4), 521–525.
• Lewis, 1980 - The Practical Salinity Scale 1978 and its antecedents. IEEE Journal of Oceanic Engineering, 5(1), 3–8.

Really good stuff here. Going to check on this!