Re: Using supplies from home, how should I find the viscosity of ketchup?

Hi Katy,

Viscosity is a property that is commonly measured in chemistry labs, so 
this might be considered a chemistry project.  All fluids resist changes 
in shape or form.  The measurement of the resistance to change in shape is 
known as viscosity.  The more resistant a liquid is to change, the higher 
is its viscosity.  The unit of measure is the poise.  The poise has units 
of dyne-seconds per cm2.  A dyne is the amount of force needed to 
accelerate one gram of mass at the rate of one centimeter per second.  The 
poise defines the distance that a unit volume of liquid will move when a 
known amount of force is applied to it.

So how do you get from the definition to a measurement of the viscosity of 
ketchup using household equipment?  A viscometer is a device used to 
measure viscosity.  One type of viscometer uses the force of gravity (9.8 
meters/second/second) to move a liquid through an orifice.  The viscosity 
is found by measuring the time required for a known volume to pass through 
the opening.  A simple viscometer can be constructed from a disposable 
clear plastic cup.  Put a hole in the bottom of the cup using a drill or a 
punch.  A ¼ inch diameter hole would be a good starting point for 
measuring ketchup.  With a marking pen, mark a line on the outside of the 
cup near the bottom.  Mark a second line somewhere below the top of the 
cup.  

Your viscometer needs to be calibrated.  Some readily available liquids 
(try a pharmacy) of known viscosity are:
Glycerin – 1,490 centipoise,  caster oil – 986 centipoise, and olive oil – 
84 centipoise. 

Cover the hole in the cup with tape to seal it.  Use a long piece of tape 
so that it can be easily removed.  Fill the cup above the top line with 
the liquid to be measured.  Remove the tape and let the liquid drain into 
another container (you might want to do this over a sink in case it 
spills).  Using a stopwatch or the second hand on a clock, measure the 
time it takes for the liquid level to change, starting at the top mark and 
ending at the bottom mark.  

Clean and dry the viscometer.  Repeat the test with a second liquid with a 
known viscosity.  Clean and dry the container and repeat the test with the 
ketchup.  

Plot the viscosity vs time for the two or three known liquids on graph 
paper or using a spreadsheet.  Using the graph, you can estimate the 
viscosity of the ketchup by plotting the time for the ketchup to drain.  
You could use the two or three standards to calculate a slope and 
intercept of a calibration curve for your viscometer.  The viscosity of 
any unknown liquid can then be estimated from the calibration curve or the 
graph.

Chemistry and physics can be much easier to understand through 
experiments.  For most of us it’s more meaningful when you can see the 
results of our experiment.  There are very few people that can accomplish 
experiments in their heads.  And you don’t need fancy or expensive 
equipment to do interesting experiments.  Try measuring the viscosity of 
the ketchup at different temperatures and see what happens.  

Have Fun,
Bob Novak
Specialist, Process R&D
Carpenter Technology