Sera Hydrometer

[hairiffin]

What mean "N" and "A" at sera Hydrometer?

 

[Reefing Madness]

 

[JulianHuntToronto]

What mean "N" and "A" at sera Hydrometer?

On a sera hydrometer, N stands for "Normal" or fresh water, and A stands for "Absolut," which is a term for pure or distilled water. These markings are used to calibrate the hydrometer, ensuring it reads the correct density for a given liquid.

 

[JulianHuntToronto]

What mean "N" and "A" at sera Hydrometer?

On a sera hydrometer, N stands for "Normal" or fresh water, and A stands for "Absolut," which is a term for pure or distilled water. These markings are used to calibrate the hydrometer, ensuring it reads the correct density for a given liquid.

 

[JulianHuntToronto]

On a sera hydrometer, N stands for "Normal" or fresh water, and A stands for "Absolut," which is a term for pure or distilled water. These markings are used to calibrate the hydrometer, ensuring it reads the correct density for a given liquid.

Understanding the markings

• N (Normal):
  This marking indicates the density of regular, freshwater. On many hydrometers, fresh water is represented by the number
  1.000
  on the specific gravity scale.
  
  
• A (Absolut):
  This marking refers to the density of distilled or pure water, which serves as a reference point for calibration. On some hydrometers, it might represent a different type of liquid, like brine.
  

How to use them for calibration

1. Fill the hydrometer with distilled water and make sure it is level.
   
   
2. Place the hydrometer into the sample and wait for it to settle.
   
   
3. Check the reading on the scale.
   
   
4. If the reading is not
   1.000
   , you may need to adjust the hydrometer's calibration.
   
   
5. If the reading is too high, you can use a file to shave off some glass from the bottom of the hydrometer until it reads
   1.000
   .
   
   
6. Once the hydrometer is calibrated, you can use it to measure the density of other liquids.

 

[EnterName]

Side note because this might cause confusion otherwise:

Short Version first:

I think the Sera Hydrometer uses a different underlying calibration standard than most devices for the hobby. Don't be confused when you use another device and get different readings. Both devices are working perfectly fine, but the numbers have different interpretations. Each device has an underlying conversion table so your readings can be translated to salinity in ppt. And this conversion table is a bit different for the Sera (and some other hydrometers like JBL, AquaMedic DensiMeter, Tropic Marin hydrometer sold in Europe) than it is for most refractometers or US hydrometers.


Long/Detailed Version:

I think the Sera hydrometer is similar to the AquaMedic DensiMeter, JBL Hydrometer, and Tropic Marin hydrometer sold in Europe.

Most devices (especially in the US) measure the density of the saltwater relative to the density of freshwater at 20°C (≈ 0.998207g/mL). Pretty much all refractometers for the hobby work this way. Looking through a hydrometer you often see a text "d20/20" which specifies this. Sometimes it's also written on the neck of a hydrometer.

But the listed hydrometers above measure density relative to freshwater at roughly 4°C (≈ 0.999975g/mL) which is pretty dang close to the actual density so they are often called "Densimeters" as well.
This means the readings on the listed hydrometers will differ from refractometers for the hobby or US hydrometers.

For example:
1.025 on a d25/4 calibrated device is roughly 37.2ppt where 25°C being the temperature of your sample to measure and 4°C the reference freshwater temperature the density is measured relative to.

1.025 on a d20/20 calibrated to device is roughly 32.9ppt where 20°C being the sample temperature and also the reference freshwater temperature.

You can double check using this calculator: https://www.capitalcityaquatics.com/salinity

As you can see the difference is 4.3ppt. Small but at least visible. This can cause confusion as to why two devices show different readings.

I had a small discussion about this topic with Randy Holmes-Farley and he said he wouldn't recommend 25/4 devices for most people. While I personally prefer 25/4 because it's a scientific standard due to measuring the actual density, I have to agree with him. Most people are confused when two of their measuring devices don't show the same readings. This doesn't mean you need to throw the Sera hydrometer away. Just be aware that if you see articles online talking about specific gravity/relative density "1.025" means something different for them, than it does for you.