Measuring Tools

Building Stirling engines is rewarding and fun.  But after a while you will begin to wonder how efficient your engines are.  There are some easy and affordable ways to take measurements of your engines.  This is also helpful for students who are using Stirling engines as a science project, as in a science fair.  Two simple measuring tools that won't cost you too much are a tachometer and a thermometer.  Both of the devices you see pictured here are non-contact measuring tools.  The tachometer uses a laser, and the thermometer is infrared.  Since neigher one has to be in contact with the engine, measurements can be taken without adding any load to the system.

The device on the left is a laser tachometer.  I ordered this from  The price fluctuates depending on the current discount.  They can usually be found for under $15.  

The tachometer uses a small piece of reflective tape attached to the rotating part you wish to measure.  The meter comes with enough tape to install on probably 20 or more items.  When the tape is gone you can probably use white paper or foil.  I placed a small piece of the reflective tape on the flywheels of several of my Stirling engines and took measurements as I made slight changes to configuration.  The tachometer was able to tell me exactly what was happening to the speed of the engine as I made the changes.

The tachometer works by shining a laser on the area where the reflective tape is applied, and it then measures the frequency of the flashes that reflect off the tape.  If you have a very slow rotating flywheel you can add several stickers and then do a little math to come up with the speed.

The device on the right is a non-contact infrared thermometer.  I purchased this at Harbor Freight Tools for about $20.  This tool is very handy for calculating the current operational temperatures of your Stirling engine.  It will tell you the temperature of the hot side of your engine and the temperature of the cool side of your engine.  Subtract one number from the other and you have your temperature differential.

If you have an LTD engine you will want to know what the minimum temperature differential is for it to run.  Just start your engine and get it going, then take away the heat source and measure the temperatures of the hot side and the cool side as the engine starts to cool down.  When your engine is barely moving, that is the lowest temperature differential at which it will operate.

The infrared thermometer is also good for measuring the temperature of your heat source.  If you are trying to get an engine to run from the heat of your hand, you will need to see how warm your hand is!  Some people have hands that are much warmer than others.  If your engine is not running well, it might be the engine's problem, or it might be the hand.

Using these two measuring tools will give you some interesting and meaningful measurements that should help you when designing, fine tuning, or running a Stirling engine.  It also gives you a way to compare your achievements with your friends to see who has the faster motor, or the more efficient LTD engine.

Here is a short video showing the tachometer in action:

Measure RPMs with a Tachometer

Here is a short video that shows the infrared thermometer in use:

Measuring Temperature Differential