The vast majority of power meters should leave the factory properly calibrated, in other words they should be configured to report power output within the limits of the manufacturers
stated accuracy (eg +/-2%) and on average they will not read too high or too low. But not everybody will be so lucky, some power meters will over or under read and for this reason it´s
advisable to calibrate them (or at least check their calibration in the case of units that cannot be self calibrated) at intervals recommended by the manufacturer.
Why Calibrate?
Before considering the specifics of calibration it is insightful to summarise just how the power reporting technology works and why this necessitates calibration. Most designs detect
torque via a number of strain gauges which transmit, electronically, a frequency indicating the degree of torque they are presently experiencing. They measure torque because Torque x RPM
(e.g. cadence) = Power, so power is a calculated value, not something that is measured directly. This further explains why power meters need some way to detect cadence (at the cranks)
or RPM (at the rear wheel).
The implication of measuring torque via frequency information transmitted from strain gauges is that the computer in charge of displaying power output needs to know
- The frequency coming from the strain gauges in a no-load situation (zero torque) and
- How an increase in that frequency relates to a loaded situation when experiencing a real amount of torque.
The no-load frequency is something the rider can reset at the beginning of every ride, SRM for example call this adjusting the “zero offset”, and it´s advisable because the strain gauges
are sensitive enough to be affected by changes in ambient temperature and
chainring bolt de-torquing. Setting the relationship between changes in the frequency and actual torque (SRM call it the “slope” of the power meter)
is what constitutes the more infrequent calibration requirement.
The Calibration Process
All calibration checks involve subjecting the power meter to an amount of torque that is known and then either comparing this with the torque implied by the power meters frequency reading,
or in the case of the PowerTap simply reading the value shown in “torque mode”. Since torque can be defined as a force around an axis, applied at a particular radius, we can create a
known amount of torque by calculating it from a known mass (weight) hung under gravity on a horizontal crank arm of a known length.
The calculators on this page provide the essential maths required to carry out calibration or calibration checks on the 3 most popular power meters - SRM, Quarq, PowerTap - and their usage
is explained with instructions for each of the relevant processes.
Instructions
The common SRM calibration protocol involves suspending a known weight from the
pedal axles to determine the frequency emitted by the torque sensors under a
known torque, comparing this with a zero torque reading "zero offset", and using
that data to calculate the power meters torque response. That response, known as
"slope", can be thought of as a ratio which defines how much torque the strain
guages are assumed to be under for each frequency point over and above whatever
zero torque frequency. The power meter effectively uses this slope to
interpolate or extrapolate torque under any amount of strain.
- Acquire a known weight of 20-30 kilos and some means of suspending it from
pedal axles in a horizontal position, one at a time. This means anything you can
verify the weight of to within 50 grams (such as with postal scales), but in
practise it's convenient to use something dense such as iron weights or rocks.
Before going out and buying a known weight think about the relative merits and
shipping costs of having a reliable set of scales you can use to weigh a bag
full of rocks...or some pieces of iron lying around.
- The dimensions of the weight will determine options for bike positioning. If you
have found something compact enough you could perform the calibration procedure
with the bike on a floor such as in the
Quarq calibration
video. If not you will probably have to stand the bike on a table or workbench
(however makeshift), such that the weight can hang off the side, or better still
clamp the bike into a workstand.
- You
should be familiar with the procedure for reading and resetting the power meters
"zero offset" which should be done before every ride. To recap, you spin the
cranks backward a few revolutions to "wake up" the power meter, leave them in
the horizontal position, press [Mode]+[Set] and compare the number you see, a
frequency reading from the strain guages, with the number set into memory. Do
this leaving the right crank in the 3 o'clock position and put the number in box
(a). Do it again with the right crank in the 9 o'clock position and
put the
number in box (b). If whatever you will be using to suspend your known weight is
not included in the weight itself, hang this on the forward pedal axle when
taking these zero torque readings.
- Spin the cranks backward to keep the power
meter "awake". Suspend your known weight from the right crank (pedal axle) in
the 3 o'clock position while holding the rear wheel still (your hand or the
floor will take care of this). Press [Mode]+[Set] to see the frequency reading.
Slowly creep the rear wheel backwards and forward a little until you find the
highest frequency reading which should occur with the crank perfectly horizontal
(for this reason you dont need a spirit level). Put the number in box (c).
Repeat this step with the weight on the left crank (right crank in the 9 o'clock
position) and put the number in box (d).
- Fill out the rest of the form...
- Enter the weight of your known weight in box (e).
- Select the length of your cranks in box (f).
- Enter the last calibrated slope of your SRM in box (g) if you are interested in percentage error.
- Hit [Calculate].
You can now review the tested slope of your SRM. You may wish to run this or parts of this test 2 or 3
times to work with average values before deciding to accept the new "slope". At
this point it is a good idea to replace the sticker on the inside of your SRM
with something documenting the new slope. You should enter the new slope into
your SRM Training software (see below) and save your settings into the
PowerControl in the normal way.
Calculator
Instructions
At the time of writing Quarq has just released an iPhone application (Qalvin)
which, running on an iPhone equipped with an ANT+ iPhone dongle (eg from
Wahoo Fitness
), not only walks a user through the process of checking a Quarq units
calibration (i.e. slope), but makes it possible to change the slope. Previously
the only way to actually change slope was to send the unit back to the factory,
highlighting a major difference between the SRM and the Quarq: because the SRM
uses it's own head unit and software, slope reset was easy to offer; whereas
because Quarq relies on third party products (eg Garmin head units & Training
Peaks software) - this wasnt supported.
Quarq have suggested there should be little need to recalibrate a units sope,
assuming continuing use with the originally installed, properly torqued
chainrings, but many users would prefer to know that their unit has arrived well
calibrated and not drifted from an accurate calibration by making periodic
checks. These checks can be made without the application, and without a dongle
equipped iPhone, which is why the reader may still be interested in the
calibration check procedure as detailed on the Quarq website long before the app
arrived.
The calibration checking process is amost identical in procedure to the
process applicable to the SRM, save that you will be interested in the frequency
displayed on the "Calibrate Power" screen of a Garmin head unit (eg Settings ->
ANT+ Sport -> Acessories -> Calibrate Power -> Frequency) and that Quarq
recommend taking left and right weighted frequency readings on both the big and
small chain rings, 4 in total.
In case the procedure isnt't made clear enough from the SRM procedure the reader
should take a look at
Quarq calibration
YouTube video intended to illustrate use of the Qalvin app but equally relevant
to the checking procedure you can complete with the calculation form below.
Simply input the applicable parameters and frequency readings, hit calculate,
and you will have an initial indication of how well your Quarq is presently
calibrated.
Calculator
Instructions
Although it isn't officially possible to recalibrate a PowerTap in the field it
is possible and indeed desirable to periodicaly check it's calibration using a
test protocol suggested by Dr Andrew Coggan as the "stomp test",
so-called because "stomping" or at least standing on a pedal was originally
suggested in place of hanging a known weight (rember this is essentially a gross
error check intended to figure out out if your PowerTap needs factory
recalibration, not an exact calibration procedure).
Unlike the SRM & Quarq, which transit frequency data to a head unit and later convert this into torque and power numbers, the PowerTap makes things a little easier by actually
offering a torque display on the head unit. The protocol is thus a simple case of comparing the units torque reading with a known, calculable torque value created by hanging a known
weight on the crank. Note that because the PowerTap measures torque at the rear hub and we are hanging a weight on the crank, gear ratios are involved.
Calculator