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
Water Flow measurements
Light intensity measurements
Some sample test setups
Measuring collector performance
|Air Flow Measurements|
|Dwyer $25 Vane
Air Velocity Meter
|This 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.|
Other suppliers make similar meters -- a search under "Wind Meters" will turn some up.
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.
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)
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).|
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 :)
Thermistors 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
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.
|A thermocouple is a junction of two different metals. Temperature changes at the junction generate a voltage that is proportional to the temperature.|
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||
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...
|(Add the IC type
thermometer chips, and the Dallas Semi one wire sensors)
|Water Flow Measurements|
|Timed Filling of
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).|
$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.|
|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
Ordinary ink pen ink can be used to dye the water. Adding a little bit of detergent prevents breaking up of the water column.
Two 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.
Differential Pressure Gage
|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).|
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.
www.NewEgg.com often has them for about $20.
|The Electric Clock Method||
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
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.
|Power Cost Monitor
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.
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 ...
Sunlight at Earth's Surface: Build Your Own Pyranometer,
Update: I built one of these kits and tested it against my Apogee Pyranometer for recording solar radiation during collector tests....
on making and using pyranometers.
Material on how to order and assemble a $20 pyranometer kit.
Radiation -- A Cheap Solar Radiation Meter
understand, build, calibrate and use a Pyranometer based on an inexpensive
From Chuck Wright's website -- other interesting solar projects, and renewable energy project data logging systems: http://chuck-wright.com/projects.html
labs Solar Meter and Logger
Circuit Labs offers a solar data logger and a solar meter both in kit or
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.
(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||
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
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 ...
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.
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.
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.
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
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.
temperature logger from AnotherURL.com
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.
Usage for Inverter & Battery Bank Sizing
Search HP archives for issue 76
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.
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 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
simple setup that measures the output of a small collector built to test a
|Some Additional ones:||Some Additional ones:|
|Measuring Collector Performance|
How to measure:
- Collector Heat Output
- Collector Efficiency
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.