I get questions from people who build experimental (or not experimental) solar devices, and would like to measure their performance.  Measuring performance  can be very helpful to see if the device is working well, and/or to measure the benefits of design changes.


So, here are some simple ways that I use to measure stuff.  Since most of us do not have the test budget of a GE or Boeing, the emphasis is on cheap ways to get good data. 


There are probably better, easier, and cheaper ways to do all of this, so if you know of any lets hear about them!  Gary.


Since most of my work is in the solar thermal area, there is not much (nothing in fact) on measurements for PV systems, wind generators, or the like.  If you have some experience in these areas and would like to pass along some tips, techniques, or advice -- please do.


Directory to this Measurements page:


Air Flow Measurements
Dwyer $25 Vane Air Velocity Meter
then search for "Vaneometer"



Dwyer vane type air flow meterThis very simple and inexpensive Dwyer gage measures air velocities in the range from 10 fpm up to 400 fpm.  The incoming air stream deflects a vane up the meter scale.  It has just about the right amount of damping to give steady readings, but still react to changes.  I use this meter and like it a lot.  Note that the upper limit of 400fpm is to low for typical flow in fan forced system.   It is ideal for the flow velocities encountered in  thermosyphon or natural convection systems.  About the only negative is that it can only measure horizontal flow, but you can change the flow direction with something like  a gutter pipe elbow so it is horizontal, and measure this.  Only $25.
Micro Turbine Wind Meters

Other suppliers make similar meters -- a search under "Wind Meters" will turn some up.



Kestrel turbine type air velocity measurement deviceThese wind meters use a small turbine whose rotation rate is measured to determine air velocity.   This Kestrel works well from about 70 fpm up to several thousand fpm.

I have used the one in the picture for several years and think it works quite well.  There are several brands and many models.  This model is about $80.  They also come in models that include other sensors for temperature, humidity, altitude, ...  I think that some now even offer some form of logging.

Garbage Bag Airflow Measurements

Evaluation of Flow Capture Techniques for Measuring HVAC Grille Airflow

Or, how to use garbage bags and laundry baskets to measure airflow

Lawrence Berkeley National Laboratory

Full How-To Report (pdf)

More on Same (pdf)

Measuring air flow with garbage bags and laundry basketsMeasuring air flow with garabage bags





This is a great and practical piece on how to measure airflows accurately using timed filling of garbage bags, or simple pressure difference across a flow resistance, where the pressure difference is generated with a simple laundry basket.

Adventures in Measuring Airflow for a Solar Collector... Some notes on various ways to measure airflow for a solar collector, and how well they work (or don't).
Temperature Measurements
Ordinary Thermometers


Simple alcohol thermometer works fine for solar collector temperature measurementsOrdinary hardware store alcohol thermometers work fine for measuring temperatures in most solar thermal systems.

When you buy the thermometers, try to select ones from the bin that read the same temperature.  In most cases, you will be interested in the difference between the readings on two thermometers, so the absolute accuracy is not usually critical, but having thermometers that are all off by the same amount helps.

When you place them, try to get the bulb into the air stream you are measuring, and keep them out of direct sun (this may require a shade of some type).

For most projects, you will get better results if you take and record several readings over a time span.   If more accurate measurements are needed, the thermometers can be calibrated as described below.

Every once in a while, bring them all together and see if they read the same temperature -- throw away any that don't.

Here is what happens when plastic thermometers  get a bit too warm :)

Better thermometers


hanna thermometerIf you want to move up to a more accurate and precise thermometer, here is one example.

This is the Hanna Checktemp. It is thermistor based with a stainless proble that can be inserted in air or liquid. It maintains an accuracy of +- 0.5F over its -58F to +302F range. It comes with a calibration certificate and it has a built in calibration test you can run any time.

About $25.




Thermistor for solar thermal temperature measurementsThermistors are semiconducting devices whose resistance varies as a function of temperature.  This resistance change is used to measure temperature.  They are relatively cheap, can be quite accurate (e.g. 1/4F over the solar thermal range), and are fairly rugged when packaged well.

Picture of logger with thermistor above, and
thermistor packaged in stainless can.

They require some circuitry and a display to show the temperature, or a data logger to record the temperature readings.  Or, an ohm meter can be used to find the resistance, and then look up the temperature in a table.

If you decide to invest in a data logger, the loggers that use thermistor sensors tend to be among the most affordable.

The picture shows a thermistor sensor enclosed in a stainless steel can, and connected to a cable that is plugged into a data logger.




Various types of thermocouple junction end stylesA thermocouple is a junction of two different metals.  Temperature changes at the junction generate a voltage that is proportional to the temperature. 


IR Thermometers IR thermometers are ideal for surface temperature measurements

IR thermometers measure surface temperature by measuring the IR radiation emitted from the surface.  About $50.

You can use them for measuring surface temperatures in solar thermal experiments, and also to see if your tires are overheating, or your beef stew is up to temperature -- no household should be without one.

They are often the easiest way to measure surface temperatures.  Some caution must be used, in that they assume that the IR emissivity of the the surface is 0.95.  Many surfaces are close to this, but some surfaces (e.g. polished metals) are much less.  In some cases this ambiguity can be removed by putting a piece of dark colored tape on the surface, and doing the measurement on the tape.  You can get air temperature by waving a sheet of paper around in the air, and then measuring the paper surface temperature with the IR thermometer.

I have a little matchbook size one that I carry around some of the time -- they are great for getting a better understanding of the thermal physics of the world around us.  The matchbook size  model costs $25..

Thermal Imaging Cameras FLIR thermal cameraThermal imaging cameras offer an unmatched capability to view the temperatures over a whole surface.

They have been almost prohibitively expense, but over the last few years have come down a lot as some entry level lines have been introduced with lower prices that have been steadily dropping.  The lowest priced ones are now a bit over $1000 -- still a lot, but much better than $10,000.

I recently took the plunge and got a FLIR IR camera.  My impressions of the FLIR so far are here... 

 An Arduino based homemade thermal imager for cheap...

homemade thermal imagerThis is an IR (thermal) camera that you can build for a couple hundred(?) dollars.

Instead of the array of sensors that most thermal imagers use, it uses a single IR sensor that is scanned over the target object, then an image is put together from the set of readings taken during the scan.

The upside is the price and the downside is the time it takes to scan an image (about 6 minutes?). Pretty clever.

Water Flow Measurements
Timed Filling of A Bucket


If you can arrange it so that the fluid flow to be measured can be diverted to a container of know size you can get very accurate flow rate measurements by just timing how long it takes to fill the container.  A stopwatch is helpful for accurate measurements.  Make sure that whatever you do to divert the flow does not itself change the flow rate.   If you do this carefully, it will provide better accuracy than all but the most sophisticated flow meters, and do it for zero cost.

In the picture, there is a submersible pump in the bucket that pumps water around the collector, and back into the bucket via the line with the flow control clamp on it.  To measure flow, the line going back into the bucket is redirected to the 2 qt measuring bowl in the background, and the fill time measured.

Using the DLJ Water Meter for Flow Measurement


An example of using these water meters to measure and log flow rate and total flow of water (including hot water).

More ...

$5 DIY Flow Meter



A simple flow meter made from a couple of standard CPVC 3/4 by 1/2 reducers installed back-to-back.


Some Commercial Flow Meter Possibilities


A sampling of commercially available flow meters at the low end of the price scale.
Pressure Measurements
Water Manometer


A water manometer can be used to measure pressure differences in inches of water.  A manometer can be easily made from transparent plastic tubing bent into a U shape.   The height of the U should b a bit greater than the maximum pressure differential you will want to measure.  To use it:
  1. Fill the U tube up about half way full with water.
  2. Connect the top of one side of the U to the pressure source to be measure using a tube.  Leave the other end of the U open. 
  3. With the pressure applied, measure the difference in height between the water columns in the two arms of the U.  This is the pressure in inches of water.  One inch of water is 0.036 psi.

Ordinary ink pen ink can be used to dye the water.  Adding a little bit of detergent prevents breaking up of the water column.

Water manometers for measuring inflation pressure in inflatable heliostatTwo water manometers used to measure the top dome and bottom dome inflation pressures of an inflatable heliostat.  The difference in pressure is used to focus a flexible aluminized Mylar mirror.  You can see that the pressures are nearly equal, and about 6 inches of water.

Accuracy is excellent, and cost is near zero.

Dwyer Magnehelic Differential Pressure Gage



Dwyer Magnehelic gage to measure differential pressures These Dwyer Magnehelic gages provide an accurate and relatively cheap way to measure small pressure differences (as commonly occur in solar air systems).  About $50, but often available on ebay for much less.  Available with full scale readings from 0.25 up to more than 100 inches of water.  They are for measuring gas pressures only (not water).
Power Measurements



Kill-A-Watt power consumption meter This amazing gadget measures voltage, current and power (in watts) consumed by a 120VAC appliance plugged into it.  It will also accumulate energy used (KWH) over the period of time the appliance is plugged into it.

The claimed accuracy is 0.2, which is amazing.  If you bought the equivalent of this as a "lab instrument", it would probably cost a thousand dollars. often has them for about $20.

The Electric Clock Method If you want to know how much power a device is using, and the device has a fixed and known power consumption when its on, then you can hook up an electric clock across the connection that goes hot when the device powers on.  The clock will then only run when the device is powered up.  Just set the clock for midnight at the start, and note what time its pointing to 24 hours later, and you have the daily on time. 

The picture shows using an electric  timer to record how many hours my water heater runs each day by powering the timer from the fan that runs whenever the burner flue exhaust fan is on (which runs only when the burner is on).

Using your house electric meter to estimate power used by a load

And, reading your electric and gas meters


The link describes a way to use the utility company watt-hour meter on your house to estimate the power consumed by an electrical load of interest.  This is based on each turn of the meter disk representing a fixed number of watt-hours -- often 3 watt-hours per turn.

The 2nd link tells you how to read the totals on your electric and gas meters.

Reading your gas meter... gas meterInteresting article on pilot lights gas usage that also shows you how to read a gas meter.
Power Cost Monitor


This easy to install power meter tells you how much electricity your house is using (and has used).
The idea being that getting instant feedback when you turn on lights, appliances, ... will help you cut down on electricity use.
Kind of like driving a Prius and seeing what your current mpg is all the time.
Solar  Intensity
Apogee Pyranometer


I finally bit the bullet and got an Apogee Pyranometer in order to be able to measure sun intensity accurately.  The Apogee outputs 2 mv per watt/m^2 of solar radiation (so about 2 volts for full sun).  It is factory calibrated to +- 5%, and is cosine corrected.  About $180.  It works well with the Onset logger which will convert the voltage readings directly to watt/m^2.  So far, I have been quite happy with it, and have a number of projects planed for it.
Shown here set up to determine if the corrugations on SunTuf glazing reduce its transmission  Details ...
Measuring Sunlight at Earth's Surface: Build Your Own Pyranometer,
David Brooks

Update: I built one of these kits and tested it against my Apogee Pyranometer for recording solar radiation during collector tests.... 


Information on making and using pyranometers. 

Material on how to order and assemble a $20 pyranometer kit.

The References section at this link contains some in depth papers on measuring and modeling solar radiation at the earths surface.

Measuring Solar Radiation -- A Cheap Solar Radiation Meter


How to understand, build, calibrate and use a Pyranometer based on an inexpensive PV cell.  Details...

From Chuck Wright's website -- other interesting solar projects, and renewable energy project data logging systems:

Micro Circuit labs Solar Meter and Logger



Micro Circuit Labs offers a solar data logger and a solar meter both in kit or assembled form.

Battery powered logger has a silicon photodiode as the sensor, and will log up to 16K sun measurements over up to 170 days.  Said to have 5% accuracy.   Price -- $70 to $150.

Onset Pendant Light Meter


(search their site for "Pendant" if you have trouble finding them)

This compact and self contained logger measures light intensity in lumens/ft^2  and temperature.   It will handle full sunlight, and you could correlate lumens/ft^2 to watts/m^2 to get a rough idea of solar intensity.  It has an internal battery, and requires no connection to computer during logging.  Case is a bit like a small pill bottle, and is water resistant.  Price is about $40, but you also need the coupler and software, which add many dollars -- so this option might be best if you have or plan to use other Onset loggers. 
Motor On Time
Logging Motor On Tim

The small logger in the picture (white box) logs the on time for the motor its attached to.  No wiring connections -- the logger senses the motor magnetic field.

This Onset motor on/off monitor is the one I use -- I'm sure there are others.  It is very easy to use -- just tape it to the side of the motor. 
Price of this one is $79 + software.
Lots of uses -- some examples ...
"Fish" Scale Using simple spring scale to determine force deflection curveMeasuring deflection vs force for a model heliostat using a "fish" type spring scale.

Inexpensive digital versions of "fish" or "game" scales are now available.  The one I have is reliable and accurate.

Gravity and pulleys



Using weighted bucket to apply simulated wind force to inflatable heliostat










The weighted bucket applies a known force to the edge of the heliostat via the rope over a pulley.   Not very elegant, but it works!  Used to estimate the heliostat beam displacement for high wind conditions. 
I have found the digital bathroom scales to be an accurate way to measure the weights, but known quantities of water at 8.3 lb per gallon can also be used.

Data Loggers
Data loggers allow you to record temperatures, voltages, events, ... at regular intervals. 
The interval can typically range from once a second or less to hours, and the time period over which data is logged can be minutes to months.   Values recorded by the logger are typically transferred to a computer for processing or plotting.
Data loggers are typically NOT required for most solar thermal experiments -- you can get equally good results by just manually reading and recording sensor values -- the loggers just make life easier.
Onset Computer Data Loggers




Onset 4 channel data loggerThis standalone 4 channel Onset data logger records data from up to 4 sensors of various types with no external power or connections for periods up to months.

I mention this specific brand, because it is the one have used, and have experience with -- I am sure there are other very capable loggers out there.

Home made temperature logger from


Home made temperature logger that uses "one wire" temperature sensorsA home made temperature data logger based on the inexpensive Dallas Semiconductor DS18B20.  This logger supports up to 64 sensors.  The sensors use the Dallas Semi "one wire" bus, which means that the sensors can be a 100 ft or more from the logger, and require minimal cabling, and no separate power.  The logger interfaces the sensors to a Windows PC.  More than 100 of these logger have been built.

There are other versions of loggers that make use of the "One Wire" sensors.  Try a google for the latest.

Measuring Energy Usage for Inverter & Battery Bank Sizing

Mark Patton

Search HP archives for issue 76


Good Home Power article on using an Onset Computer logger to measure power consumption for PV system sizing.
This is a year 2000 article, so some of the hardware may be a bit out of date, but the techniques are still applicable.
1-Wire HVAC Monitoring System for a Residential Geothermal System

The system logs many sensors and provides real-time and historic data over the internet.


Phil describes his system for monitoring the status and performance of his heating system in "Our Cool House".  This system uses the 1-Wire bus (and more) to allow several types of sensors to be logged.  A dedicated micro- processor is used to reduce the power consumption of the system.  The logged values are sent to a web server, and can be accessed from by web browser.
Phil sells kits for the logger main board, and for some of the sensors.  He also provides the software, including source code (so you can customize if you wish).
Our Cool House is Phil and Lisa's very interesting, passive solar, earth sheltered, geothermal house -- their website provides lots of good information on the design an building of their house.  Its in our "I Did It" homes section.
myDTC controller/logger
mydtc loggerThe myDTC is primarily a controller (a very flexible one), but also includes a logging function that is able to store a number of sensor values over a longish period of time.

One of the sensors offered is a solar radiation sensor, which can also be logged.
Some Sample Measurement Setups
Here are a few sample test setups that I had some pictures for.   This is not rocket science -- one of the nice things about Solar Thermal is that you can do good work with simple test setups.   Here are some simple examples that might be helpful to folks who have not done much of this stuff. 
A Test Solar Water Collector



Test of prototype of a solar water collector made from CPVC pipeA simple setup that measures the output of a small collector built to test a new design.


More ...

Some Additional ones: Some Additional ones:

Grey Water Heat Exchanger In A Barrel

Temperature Stratification i Tall Room

Low Tech Garage/Barn Heater

Flow Organizer

Combined Collector and Thermal Shade

Transmittance of Corrugated Glazing

Simple Pool Heating Collector

Measuring Collector Performance
How to measure:
       - Collector Heat Output

        - Collector Efficiency

Common collector design misconceptions.


Some simple techniques to measure collector performance, and determine if design changes that you make actually improve performance. 
A very rough way to estimate collector efficiency.
A list of common collector design misconceptions.