Let’s Get Specific (Gravity, That Is): What Are Hydrometers?

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Hydrometers are scientific instruments with many applications that people can utilize. Hydrometers were designed to measure the specific gravity of various liquid solutions in their purest form. But what exactly does this mean, and what are the applications for a scientific instrument like this? In this article, we aim to break down exactly what hydrometers are, what they are used for, and the different types of hydrometers you’d be likely to come across. 


What Is a Hydrometer? 

At their core, hydrometers are essentially thin, hollow glass tubes marked with a scale. This tube runs for an undetermined length until it meets with a wide chamber at its base. This chamber, when inserted in various liquids, will sink. It also allows the hydrometer to float in whatever liquid it’s placed in, and based upon the point the surface of the liquid reaches on the scale when the hydrometer begins to float, a user can then read the scale marked on the thin tube. This measurement is then used to determine the liquid’s specific gravity (otherwise known as the relative density). 

Another tool that borrows the hydrometer’s name is the digital or battery hydrometer. These tools are also known as battery hydrometer testers, and they measure the electrolyte in batteries and can tell a user how much the battery is charged. This is useful when working in all kinds of industries, and the handheld version of these tools is reasonably practical for folks from all fields. Think of all the times your car just won’t start and you can’t figure out what’s wrong – this handy tool will let you know in a second if it’s your battery. 

But, it gets even more confusing, as there are digital hydrometers that you can use to test all kinds of different liquids as well. So, if you were wondering whether there were two kinds of digital hydrometers you could be confused by, you’re welcome!

Hydrometers measuring density of gasoline in a tube

What’s the Importance of a Liquid’s Relative Density?

Since all liquids have a relative density, knowing that density will allow you to test out a liquid in various ways using a hydrometer. For example, there are many different kinds of specialized hydrometers that you can use to determine different things about different types of liquids. For example, if you wanted to know how much alcohol there was in a specific type of liquor, you could use a tool known as an alcoholometer. Developed in 1824, this tool measures a batch of alcohol before and after fermentation. The post-fermentation reading is then subtracted from the pre-fermentation reading to determine the batch’s alcohol content. Hydrometers specific to different kinds of alcohol have also been developed, including beer hydrometers and wine hydrometers. One modern tool like this is known as the Tilt hydrometer, which actively measures the alcohol content and density of your beer throughout the entire brewing process.

Many other kinds of specialized hydrometers allow you to measure liquids in various ways. Here are some of them:

  • Lactometer
  • Saccharometer (other sugar measurements have also been calculated – maple syrup hydrometer, anyone?)
  • Thermohydrometer
  • Acidometer
  • Urinometer
  • Salinometer

Hydrometer vs Refractometer vs Hygrometer

When it comes to measuring liquids, refractometers are another scientific instrument that is widely used. However, they function slightly differently than hydrometers, as they measure the refractive index of liquids to determine the concentration of solids within them. Since there are several similar applications between hydrometers and refractometers, the two are easy to conflate, but the measuring process is very different. The real-world uses for the two are also quite different. While refractometers help measure small samples of liquids and are therefore highly portable and easy to transport, hydrometers are usually best used for individual liquid batches – but we’ll get more into practical applications later. 

Hygrometers are a tool that, while sounding like an evil twin of hydrometers, are quite useful in their own right. These measure the humidity level in the air, or more precisely, the moisture content in the air. It is essential to distinguish these two things, as humidity and moisture level aren’t always quite the same. Hygrometers could be described as an instrument that measures wetness and, therefore, can sometimes be used to measure the moisture level of the air in greenhouses. Mistakenly, some people call these tools “plant hydrometers,” or “soil hydrometers.” While there are tools you can utilize to measure the moisture content of the soil, these are called neither hydrometers nor hygrometers. However, this misapproximation is made quite frequently by various retailers. 

How Exactly Does One Use a Hydrometer?

While we gave a brief overview of the measuring process used with a hydrometer earlier, we can provide a more in-depth explanation. While it’s believed that hydrometers in some form or another have existed since the days of Archimedes, they aren’t necessarily self-explanatory. 

To use a hydrometer, you’ll need the unit and some type of container to hold it in. This container should be able to hold the entire length of the hydrometer but shouldn’t leave very much room on the sides of it, as you want your unit to be upright to take an accurate measurement.

Pour whatever liquid you test into this container, then insert your hydrometer. You might want to swirl the liquid slightly to dislodge any air bubbles. If your liquid is carbonated, then you will want to test a flat version of that liquid to get an accurate reading. The presence of bubbles will skew your reading. 

Finally, measure the lowest point of your liquid’s surface once things have settled down. This will give you your reading. 

Digital hydrometers work in various ways, but these days you can usually just throw them right into whatever liquid you’re trying to measure. The Tilt hydrometer that we mentioned earlier can be thrown right into a batch of beer and will send you informational updates to your smartphone via Bluetooth or app connectivity. These advancements allow digital hydrometers to give you in-depth information. You’ll never need to worry about human error again, as when you are trying to read a traditional hydrometer, human error can play a significant role. 

Man testing sample of beer with hydrometer in brewery

How Do I Know My Hydrometer Works Properly? 

Hydrometers aren’t very complicated when compared with the vast amount of scientific instruments there are in the world, but we understand how first, second, or even third-time users might be a bit confused when trying to utilize them. After all, with so few parts, how can you tell if things aren’t working correctly? 

Well, to put things simply, if your hydrometer is broken or faulty, you would generally know it. Just watch for any breakage in the glass of the tube, chamber, or container you’re using to hold your liquid. You might also want to look out for any small cracks or fractures in the glass if you ever find yourself measuring a heated liquid; the last thing you’ll want is a spill. 

Besides the hydrometer being damaged in any way (which as we’ve just said, you’d likely have realized on your own), you might be running into some maintenance issues. It’s a good idea when using a hydrometer to keep things clean at all times. All surfaces that come in contact with liquid or are generally handled should be wiped down thoroughly after each use. It’s never a bad idea to make use of an isopropyl alcohol solution to discourage any bacterial growth. If you don’t like the smell of alcohol, then white vinegar will also do the trick. 

Another thing that you will want to stay aware of during the measurement process is the overall temperature of both the liquid that you are measuring and its container. Temperature can also influence a liquid’s density, so make sure that your liquid and the container holding it remain at a consistent temperature throughout the measuring process. Otherwise, it will throw off your reading and be useless to you. 

Are You Ready to Use Your Hydrometer?

We hope that this article proved helpful to you. Hydrometers are useful tools that can benefit both amateur beer brewers and professional soda manufacturers alike. They are also just a fun scientific tool that any passionate science student should be able to access. Now that you know what they are, what they look like, and how they work, you can start measuring fluids and liquids around you to see what you can find out. Good luck!

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