Why I Use a Graduated Cylinder for Measuring Specific Gravity
…And what to use if you don’t have one
Slender is More Accurate
I use a graduated cylinder for measuring specific gravity. A slender container is going to have smaller increments than a wide container, giving higher accuracy. You could compare this concept to using a scale with 1g increments vs 5g increments. The smaller measurement is more accurate.
At home, my graduated cylinder has a capacity of 500mL with 5mL increments. This has been a pretty good size for me. I think 250mL would work well too and would be smaller, if storage space is an issue.
At the studio where I work, we have a graduated cylinder that measures 1000mL in 10mL increments. It’s quite large and could be awkward to wash so I don’t recommend that size unless you have a nice big wash basin and a place to store it. Luckily, at my work studio there are large sinks with sprayers.
If you get a graduated cylinder, you’ll need some sort of cleaning device if you can’t fit your hand in, or you don’t have the luxury of a water sprayer. The sponge on a stick that you might use when throwing narrow pots would work well.
Some people use a syringe to measure specific gravity. A syringe could have 1mL or even smaller increments which would also be highly accurate. The one time I tried a syringe, it wasn’t giving an accurate reading compared to the graduated cylinder, but I know it works well for people. It’s possible that my syringe had too thin of an opening.
If you want to get really fancy, you can buy what’s called a Specific Gravity or Density cup. This is a small, stainless steel container that has been calibrated to hold exactly 100mL of liquid.
There is a small hole in the lid. So you fill the container up with glaze and screw the lid on. The hole allows air to escape and any excess of 100mL will also come out the hole in the top. What remains in the container is exactly 100mL and then you can weigh and divide by 100 to find the specific gravity. These are probably the most accurate devices you can use, but they are also quite expensive.
Calibrating your Container
Before you start measuring your specific gravity, you should calibrate your container. Don’t assume it’s accurate, especially if it’s made of plastic and cost you less than $10.
What we can always rely on is water. Did you know that 1mL of water weighs 1g? This is very valuable information that allows us to calibrate our measuring containers, or even create a measuring container from what we have around. All we have to do is weigh some water in grams and we will have the volume in millilitres.
Put your empty container on the scale. Write down the weight of the empty container somewhere. I write the weight directly on the container with a Sharpie so it’s always there. This comes in handy if I forget to tare or zero my scale before weighing a glaze.
After noting the weight of your empty container, tare your scale so it reads zero with the container on it. Now fill the container up to the top line with water. Since water weighs 1g/mL, the gram reading on your scale should match the mL reading on your container.
So if the water comes up the 500mL line, it should weigh 500g. If it doesn’t, you might want to make some new lines on your container, or adjust the numbers accordingly.
If it weighs 495g at the 500mL mark, you could note that on the container, as a reminder.
Or, you could add more water until it weighs 500g and then make a new 500mL mark on the container.
My graduated cylinder that I ordered on Amazon is off by 10mL. The first 50g (=50mL) comes to the 60mL line, probably due to some curvature in the bottom of the cylinder. Then, as I continue to add more water, the increments are accurate the rest of the way up, so 100g comes to the 110mL line and so on. The rest of the cylinder is out by 10mL. I drew new lines on my cylinder to remind myself. Now that I know that, I just adjust the mLs each time I measure.
If you don’t have a graduated cylinder you can get one on Amazon for around $5. Another place to look is wine making stores. In the meantime, you can make your own measuring container out of an existing container you have lying around.
Make your own measuring container
Grab any container where you can see the liquid level through the walls of the container. Ideally clear plastic, or something translucent and as narrow as possible.
Add any volume of water to the container and the reading in grams will equal the volume in millilitres. Draw or carve a line into the outside of the container and write the volume on the side. Then, when measuring your specific gravity, you can fill the container up to that line and know how many mLs you have.
You can weigh 100g and make a 100mL mark and then weigh 200g and make a 200mL mark, and so on.
Again, a slender container will be more accurate than a wide container but if you only have a wide container, then measuring larger volumes will be more accurate than smaller volumes.
The reason for this is, say you’re trying to measure 100mL in a homemade measuring device and because you drew the 100mL line by hand, your measurement could easily be off by 10mL. You think you’re at the 100mL line but you actually have 110 mL. 10 mL is 10% of 100mL.
But, if you’re at the 500mL line and you actually have 510 mL, you’re much better off because 10 mL is only 0.5% of 500mL. Less margin of error.
So, if you’re making your own measuring container, weighing higher volumes will always give you the most accurate Specific Gravity reading.
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If you haven’t see it yet, I created a free, full colour, step by step guide to measuring specific gravity for repeatable glaze results. It includes a one page summary that you can print out and take into your studio. I hope you find it useful!
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