Apogee Instruments SQ-214 User manual

APOGEE INSTRUMENTS, INC. | 721 WEST 1800 NORTH, LOGAN, UTAH 84321, USA
TEL: [phone] | FAX: [phone] | WEB: APOGEEINSTRUMENTS.COM
Copyright © 2020 Apogee Instruments, Inc.
OWNER’S MANUAL
QUANTUM SENSOR
Models SQ-214 and SQ-224
Rev: 28-Oct-2020

TABLE OF CONTENTS
Owner’s Manual ............................................................................................................................................................................... 1
Certificate of Compliance......................................................................................................................................................... 3
Introduction ............................................................................................................................................................................. 4
Sensor Models ......................................................................................................................................................................... 5
Specifications ........................................................................................................................................................................... 6
Deployment and Installation.................................................................................................................................................... 9
Cable Connectors................................................................................................................................................................... 10
Operation and Measurement ................................................................................................................................................ 11
Maintenance and Recalibration............................................................................................................................................. 17
Troubleshooting and Customer Support................................................................................................................................ 19
Return and Warranty Policy................................................................................................................................................... 20

CERTIFICATE OF COMPLIANCE
EU Declaration of Conformity
This declaration of conformity is issued under the sole responsibility of the manufacturer:
Apogee Instruments, Inc.
721 W 1800 N
Logan, Utah 84321
USA
for the following product(s):
Models: SQ-214, SQ-224
Type: Quantum Sensor
The object of the declaration described above is in conformity with the relevant Union harmonization legislation:
2014/30/EU Electromagnetic Compatibility (EMC) Directive
2011/65/EU Restriction of Hazardous Substances (RoHS 2) Directive
2015/863/EU Amending Annex II to Directive 2011/65/EU (RoHS 3)
Standards referenced during compliance assessment:
EN 61326-1:2013 Electrical equipment for measurement, control and laboratory use –EMC requirements
EN 50581:2012 Technical documentation for the assessment of electrical and electronic products with respect to
the restriction of hazardous substances
Please be advised that based on the information available to us from our raw material suppliers, the products
manufactured by us do not contain, as intentional additives, any of the restricted materials including lead (see
note below), mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBB), polybrominated
diphenyls (PBDE), bis(2-ethylhexyl) phthalate (DEHP), butyl benzyl phthalate (BBP), dibutyl phthalate (DBP), and
diisobutyl phthalate (DIBP). However, please note that articles containing greater than 0.1% lead concentration are
RoHS 3 compliant using exemption 6c.
Further note that Apogee Instruments does not specifically run any analysis on our raw materials or end products
for the presence of these substances, but rely on the information provided to us by our material suppliers.
Signed for and on behalf of:
Apogee Instruments, October 2020
Bruce Bugbee
President
Apogee Instruments, Inc.

INTRODUCTION
Radiation that drives photosynthesis is called photosynthetically active radiation (PAR) and is typically defined as
total radiation across a range of 400 to 700 nm. PAR is almost universally quantified as photosynthetic photon flux
density (PPFD), the sum of photons from 400 to 700 nm in units of micromoles per square meter per second (µmol
m-2 s-1, equal to microEinsteins m-2 s-1). While microEinsteins and micromoles are equal (one Einstein = one mole of
photons), the Einstein is not an SI unit, so expressing PPFD as µmol m-2 s-1 is preferred. Daily total PPFD is typically
reported in units of moles of photons per square meter per day (mol m-2 d-1) and is often called daily light integral
(DLI).
The acronym PPF is also used and refers to the photosynthetic photon flux. The acronyms PPF and PPFD refer to
the same variable. Both terms are used because there is not a universal definition of the term flux. Flux is
sometimes defined as per unit area per unit time and sometimes defined as per unit time only. PPFD is used in this
manual.
Sensors that measure PPFD are often called quantum sensors due to the quantized nature of radiation. A quantum
refers to the minimum quantity of radiation, one photon, involved in physical interactions (e.g., absorption by
photosynthetic pigments). In other words, one photon is a single quantum of radiation.
Typical applications of quantum sensors include measurement of incident PPFD on plant canopies in outdoor
environments or in greenhouses and growth chambers, and reflected or under-canopy (transmitted) PPFD
measurement in the same environments.
Apogee Instruments SQ-100 and SQ-300 series quantum sensors consist of a cast acrylic diffuser (filter),
photodiode, and signal processing circuitry mounted in an anodized aluminum housing, and a cable to connect the
sensor to a measurement device. Sensors are potted solid with no internal air space, and are designed for
continuous PPFD measurement in indoor or outdoor environments. SQ series sensors output an analog signal that
is directly proportional to PPFD under sunlight (e.g., model SQ-214) or electric lights (e.g., model SQ-224). The
analog signal from the sensor is directly proportional to radiation incident on a planar surface (does not have to be
horizontal), where the radiation emanates from all angles of a hemisphere.

SENSOR MODELS
This manual covers the amplified analog output quantum sensors models SQ-214 and SQ-224 (listed in bold
below). Additional models are covered in their respective manuals.
Model
Signal
Calibration
SQ-214
4-20 mA
Sunlight
SQ-224
4-20 mA
Electric light
SQ-110
0-800 mV
Sunlight
SQ-120
0-800 mV
Electric light
SQ-212
0-2.5 V
Sunlight
SQ-222
0-2.5 V
Electric light
SQ-215
0-5 V
Sunlight
SQ-225
0-5 V
Electric light
SQ-311
0-800 mV
Sunlight
SQ-321
0-800 mV
Electric light
SQ-313
0-800 mV
Sunlight
SQ-323
0-800 mV
Electric light
SQ-316
0-800 mV
Sunlight
SQ-326
0-800 mV
Electric light
SQ-420
USB
Sunlight and Electric light
SQ-421
SDI-12
Sunlight and Electric light
SQ-422
Modbus
Sunlight and Electric light
Serial Numbers 10054 (SQ-214), 9914 (SQ-224)
and above: Sensor model number and serial
number are located on the bottom of the sensor.
If you need the manufacturing date of your
sensor, please contact Apogee Instruments with
the serial number of your sensor.
Serial Number 0-10053 (SQ-214), 9913 (SQ-224):
Sensor model number and serial number are
located near the pigtail leads on the sensor
cable. If you need the manufacturing date of
your sensor, please contact Apogee Instruments
with the serial number of your sensor.

SPECIFICATIONS
Calibration Traceability
Apogee SQ series quantum sensors are calibrated through side-by-side comparison to the mean of transfer
standard quantum sensors under a reference lamp. The reference quantum sensors are recalibrated with a 200 W
quartz halogen lamp traceable to the National Institute of Standards and Technology (NIST).
SQ-214/SQ-224-SS
Power Supply
7 to 24 V DC with a maximum current draw 22 mA (2 mA quiescent current draw)
Sensitivity
0.004 mA per µmol m-2 s-1
Calibration Factor
250 µmol m-2 s-1 per mA (reciprocal of sensitivity)
Calibration Uncertainty
± 5 % (see calibration Traceability below)
Calibrated Output Range
4 to 20 mA
Measurement
Repeatability
Less than 0.5 %
Long-term Drift
(Non-stability)
Less than 2 % per year
Non-linearity
Less than 1 % (up to 4000 µmol m-2 s-1; maximum PPFD measurement is 4000 µmol m-2 s-1
Response Time
Less than 1 ms
Field of View
180°
Spectral Range
410 to 655 nm (wavelengths where response is greater than 50 % of maximum;
see Spectral Response below)
Spectral Selectivity
Less than 10 % from 469 to 655 nm
Directional (Cosine)
Response
± 5 % at 75° zenith angle (see Cosine Response below)
Temperature Response
0.06 ± 0.06 % per C (see Temperature Response below)
Operating Environment
-40 to 70 C; 0 to 100 % relative humidity; can be submerged in water up to depths of 30 m
Dimensions
Serial # 10054 (SQ-214),
9914 (SQ-224) and above
30.5 mm diameter, 37 mm height
Dimensions
Serial # 0-10053 (SQ-
214), 9913 (SQ-224)
24 mm diameter, 33 mm height
Mass (with 5 m cable)
Serial # 10054 (SQ-214),
9914 (SQ-224)and above
140 g
Mass (with 5 m cable)
Serial # 0-10053 (SQ-
214), 9913 (SQ-224)
90 g
Cable
5 m of four conductor, shielded, twisted-pair wire; TPR jacket; pigtail lead wires; stainless steel
(316), M8 connector

Spectral Response
Temperature Response
Mean temperature response of eight
SQ-100 series quantum sensors (errors
bars represent two standard deviations
above and below mean). Temperature
response measurements were made at 10
C intervals across a temperature range of
approximately -10 to 40 C in a
temperature controlled chamber under a
fixed, broad spectrum, electric lamp. At
each temperature set point, a
spectroradiometer was used to measure
light intensity from the lamp and all
quantum sensors were compared to the
spectroradiometer. The
spectroradiometer was mounted external
to the temperature control chamber and
remained at room temperature during the
experiment.
Mean spectral response of six SQ-100
series quantum sensors (error bars
represent two standard deviations
above and below mean) compared to
defined plant response to photons.
Spectral response measurements were
made at 10 nm increments across a
wavelength range of 300 to 800 nm with a
monochromator and an attached electric
light source. Measured spectral data from
each quantum sensor were normalized by
the measured spectral response of the
monochromator/electric light
combination, which was measured with a
spectroradiometer.

Cosine Response
Mean directional (cosine) response
of six apogee SQ-100 series
quantum sensors. Directional
response measurements were
made on the rooftop of the Apogee
building in Logan, Utah. Directional
response was calculated as the
relative difference of SQ-500
quantum sensors from the mean of
replicate reference quantum
sensors (LI-COR models LI-190 and
LI-190R, Kipp & Zonen model PQS
1). Data were also collected in the
laboratory using a reference lamp
and positioning the sensor at
varying angles.
Directional (cosine) response is defined as the
measurement error at a specific angle of radiation
incidence. Error for Apogee SQ-100 series quantum
sensors is approximately ± 2 % and ± 5 % at solar
zenith angles of 45° and 75°, respectively.

DEPLOYMENT AND INSTALLATION
Mount the sensor to a solid surface with the nylon mounting screw provided. To accurately measure PPFD incident
on a horizontal surface, the sensor must be level. An Apogee Instruments model AL-100 Leveling Plate is
recommended to level the sensor when used on a flat surface or being mounted to surfaces such as wood. To
facilitate mounting on a mast or pipe, the Apogee Instruments model AL-120 Solar Mounting Bracket with Leveling
Plate is recommended.
To minimize azimuth error, the sensor should be mounted with the cable pointing toward true north in the
northern hemisphere or true south in the southern hemisphere. Azimuth error is typically less than 1 %, but it is
easy to minimize by proper cable orientation.
In addition to orienting the cable to point toward the nearest pole, the sensor should also be mounted such that
obstructions (e.g., weather station tripod/tower or other instrumentation) do not shade the sensor. Once
mounted, the green cap should be removed from the sensor. The green cap can be used as a protective covering
for the sensor when it is not in use.
Nylon Screw: 10-32x3/8
Nylon Screw: 10-32x3/8
Model AL-100
Model AL-120
Important: Only use the nylon screw provided
when mounting to insulate the non-anodized
threads of the aluminum sensor head from the
base to help prevent galvanic corrosion. For
extended submersion applications, more insulation
may be necessary. Contact Apogee tech support for
details.

CABLE CONNECTORS
Apogee started offering cable connectors on some
bare-lead sensors in March 2018 to simplify the
process of removing sensors from weather stations
for calibration (the entire cable does not have to be
removed from the station and shipped with the
sensor).
The ruggedized M8 connectors are rated IP68, made
of corrosion-resistant marine-grade stainless-steel,
and designed for extended use in harsh
environmental conditions.
Cable connectors are attached directly to the head.
Instructions
Pins and Wiring Colors: All Apogee connectors have
six pins, but not all pins are used for every sensor.
There may also be unused wire colors inside the cable.
To simplify connection to a measurement device, the
unused pigtail lead wire colors are removed.
If a replacement cable is required, please contact
Apogee directly to ensure ordering the proper pigtail
configuration.
Alignment: When reconnecting a sensor, arrows on
the connector jacket and an aligning notch ensure
proper orientation.
Disconnection for extended periods: When
disconnecting the sensor for an extended period of
time from a station, protect the remaining half of the
connector still on the station from water and dirt with
electrical tape or other method.
A reference notch inside the connector ensures
proper alignment before tightening.
When sending sensors in for calibration, only send the
sensor head.
Tightening: Connectors are designed to be firmly
finger-tightened only. There is an o-ring inside the
connector that can be overly compressed if a wrench is
used. Pay attention to thread alignment to avoid cross-
threading. When fully tightened, 1-2 threads may still
be visible.
WARNING: Do not tighten the connector by twisting
the black cable or sensor head, only twist the metal
connector (yellow arrows).
Finger-tighten firmly

OPERATION AND MEASUREMENT
Connect the sensor to a measurement device (meter, datalogger, controller) capable of measuring and displaying a
4-20 mA signal. In order to maximize measurement resolution and signal-to-noise ratio, the input range of the
measurement device should closely match the output range of the quantum sensor. DO NOT connect the sensor
to a power source greater than 24 V DC.
VERY IMPORTANT: Apogee changed the wiring colors of all our bare-lead sensors in March 2018 in
conjunction with the release of inline cable connectors on some sensors. To ensure proper connection to your
data device, please note your serial number or if your sensor has a stainless-steel connector 30 cm from the
sensor head then use the appropriate wiring configuration listed below. With the switch to connectors, cables
were also changed and now have four or seven internal wires. To make sensors easier to connect to a
measurement device, unused wires at the end of the cable are clipped. If the cable is cut or the original pigtail
modified, wires inside the cable that are not used with the sensor may be revealed. Please disregard the extra
wires and follow the color-coded wiring guide provided.
Wiring for SQ-214 and SQ-224 with Serial Numbers 7603 and above
Wiring for SQ-214 and SQ-224 within Serial Number range 0-7602
White: 4-20 mA signal output
Green: Signal ground
Red: Input power
Black: Input ground
Clear: Shield/ground
White: 4-20 mA signal output
Red: Input power (7-24 V DC)
Black: Ground (for sensor signal and input power)
Clear: Shield/Ground

Sensor Calibration
Apogee SQ-214, SQ-224 models have a standard PPFD calibration factor of exactly:
250.0 µmol m-2 s-1 per mA with a 4 mA offset
Multiply this calibration factor by the difference of the measured mA signal and the 4.0 mA offset to convert
sensor output to PPFD in units of µmol m-2 s-1:
Calibration Factor (250 µmol m-2 s-1 per mA) * [Sensor Output Signal (mA) –Offset (4.0 mA)] = PPFD (µmol m-2 s-1)
250 * (12.0 –4) = 2000
Example of PPFD measurement with an Apogee
quantum sensor. Full sunlight yields a PPFD on a
horizontal plane at the Earth’s surface of
approximately 2000 µmol m-2 s-1. This yields an
output signal of 12.0 mA. The signal is converted to
PPFD by subtracting the 4.0 mA offset and then
multiplying by the calibration factor of 250 µmol m-2
s-1 per mA.
Sensor Output
12.0 mA
Full Sunlight
(2000 µmol m-2 s-1)

Spectral Error
The combination of diffuser transmittance, interference filter transmittance, and photodetector sensitivity yields
spectral response of a quantum sensor. A perfect photodetector/filter/diffuser combination would exactly match
the defined plant photosynthetic response to photons (equal weighting to all photons between 400 and 700 nm,
no weighting of photons outside this range), but this is challenging in practice. Mismatch between the defined
plant photosynthetic response and sensor spectral response results in spectral error when the sensor is used to
measure radiation from sources with a different spectrum than the radiation source used to calibrate the sensor
(Federer and Tanner, 1966; Ross and Sulev, 2000).
Spectral errors for PPFD measurements made under common radiation sources for growing plants were calculated
for Apogee SQ-100/300 and SQ-500 series quantum sensors using the method of Federer and Tanner (1966). This
method requires PPFD weighting factors (defined plant photosynthetic response), measured sensor spectral
response (shown in Spectral Response section on page 7), and radiation source spectral outputs (measured with a
spectroradiometer). Note, this method calculates spectral error only and does not consider calibration, directional
(cosine), temperature, and stability/drift errors. Spectral error data (listed in table below) indicate errors less than
5 % for sunlight in different conditions (clear, cloudy, reflected from plant canopies, transmitted below plant
canopies) and common broad spectrum electric lamps (cool white fluorescent, metal halide, high pressure
sodium), but larger errors for different mixtures of light emitting diodes (LEDs) for the SQ-100 series sensors.
Spectral errors for the SQ-500 series sensors are smaller than those for SQ-100 series sensors because the spectral
response of SQ-500 series sensors is a closer match to the defined plant photosynthetic response.
Quantum sensors are the most common instrument for measuring PPFD, because they are about an order of
magnitude lower cost the spectroradiometers, but spectral errors must be considered. The spectral errors in the
table below can be used as correction factors for individual radiation sources.
Spectral Errors for PPFD Measurements with Apogee SQ-100 and SQ-500 Series Quantum Sensors
Radiation Source (Error Calculated Relative to Sun, Clear Sky)
SQ-100/300 Series
PPFD Error [%]
SQ-500 Series
PPFD Error [%]
Sun (Clear Sky)
0.0
0.0
Sun (Cloudy Sky)
0.2
0.1
Reflected from Grass Canopy
3.8
-0.3
Transmitted below Wheat Canopy
4.5
0.1
Cool White Fluorescent (T5)
0.0
0.1
Metal Halide
-2.8
0.9
Ceramic Metal Halide
-16.1
0.3
High Pressure Sodium
0.2
0.1
Blue LED (448 nm peak, 20 nm full-width half-maximum)
-10.5
-0.7
Green LED (524 nm peak, 30 nm full-width half-maximum)
8.8
3.2
Red LED (635 nm peak, 20 nm full-width half-maximum)
2.6
0.8
Red LED (667 nm peak, 20 nm full-width half-maximum)
-62.1
2.8
Red, Blue LED Mixture (80 % Red, 20 % Blue)
-72.8
-3.9
Red, Blue, White LED Mixture (60 % Red, 25 % White, 15 % Blue)
-35.5
-2.0
Cool White LED
-3.3
0.5
Warm White LED
-8.9
0.2
Federer, C.A., and C.B. Tanner, 1966. Sensors for measuring light available for photosynthesis. Ecology 47:654-657.
Ross, J., and M. Sulev, 2000. Sources of errors in measurements of PAR. Agricultural and Forest Meteorology
100:103-125.

Yield Photon Flux Density (YPFD) Measurements
Photosynthesis in plants does not respond equally to all photons. Relative quantum yield (plant photosynthetic
efficiency) is dependent on wavelength (green line in figure below) (McCree, 1972a; Inada, 1976). This is due to the
combination of spectral absorptivity of plant leaves (absorptivity is higher for blue and red photons than green
photons) and absorption by non-photosynthetic pigments. As a result, photons in the wavelength range of
approximately 600-630 nm are the most efficient.
One potential definition of PAR is weighting photon flux density in units of mol m-2 s-1 at each wavelength
between 300 and 800 nm by measured relative quantum yield and summing the result. This is defined as yield
photon flux density (YPFD, units of mol m-2 s-1) (Sager et al., 1988). There are uncertainties and challenges
associated with this definition of PAR. Measurements used to generate the relative quantum yield data were made
on single leaves under low radiation levels and at short time scales (McCree, 1972a; Inada, 1976). Whole plants
and plant canopies typically have multiple leaf layers and are generally grown in the field or greenhouse over the
course of an entire growing season. Thus, actual conditions plants are subject to are likely different than those the
single leaves were in when measurements were made by McCree (1972a) and Inada (1976). In addition, relative
quantum yield shown in the figure above is the mean from twenty-two species grown in the field (McCree, 1972a).
Mean relative quantum yield for the same species grown in growth chambers was similar, but there were
differences, particularly at shorter wavelengths (less than 450 nm). There was also some variability between
species (McCree, 1972a; Inada, 1976).
McCree (1972b) found that equally weighting all photons between 400 and 700 nm and summing the result,
defined as photosynthetic photon flux density (PPFD, in units of mol m-2 s-1), was well correlated to
photosynthesis, and very similar to correlation between YPFD and photosynthesis. As a matter of practicality, PPFD
is a simpler definition of PAR. At the same time as McCree’s work, others had proposed PPFD as an accurate
measure of PAR and built sensors that approximated the PPFD weighting factors (Biggs et al., 1971; Federer and
Tanner, 1966). Correlation between PPFD and YPFD measurements for several radiation sources is very high (figure
below), as an approximation, YPFD = 0.9PPFD. As a result, almost universally PAR is defined as PPFD rather than
YPFD, although YPFD has been used in some studies. The only radiation sources shown (figure below) that don’t
fall on the regression line are the high pressure sodium (HPS) lamp, reflection from a plant canopy, and
transmission below a plant canopy. A large fraction of radiation from HPS lamps is in the red range of wavelengths
where the YPFD weighting factors (measured relative quantum yield) are at or near one. The factor for converting
PPFD to YPFD for HPS lamps is 0.95, rather than 0.90. The factor for converting PPFD to YPFD for reflected and
transmitted photons is 1.00.
Defined plant response to
photons (black line, weighting
factors used to calculate PPFD),
measured plant response to
photons (green line, weighting
factors used to calculate YPFD),
and SQ-100 series and SQ-300
series quantum sensor response
to photons (sensor spectral
response).

Underwater Measurements and Immersion Effect
When a quantum sensor that was calibrated in air is used to make underwater measurements, the sensor reads
Correlation between
photosynthetic photon flux
density (PPFD) and yield photon
flux density (YPFD) for multiple
different radiation sources. YPFD
is approximately 90 % of PPFD.
Measurements were made with
a spectroradiometer (Apogee
Instruments model PS-200) and
weighting factors shown in the
previous figure were used to
calculate PPFD and YPFD.
Biggs, W., A.R. Edison, J.D. Eastin, K.W. Brown, J.W. Maranville, and M.D. Clegg, 1971. Photosynthesis light sensor
and meter. Ecology 52:125-131.
Federer, C.A., and C.B. Tanner, 1966. Sensors for measuring light available for photosynthesis. Ecology 47:654-657.
Inada, K., 1976. Action spectra for photosynthesis in higher plants. Plant and Cell Physiology 17:355-365.
McCree, K.J., 1972a. The action spectrum, absorptance and quantum yield of photosynthesis in crop plants.
Agricultural Meteorology 9:191-216.
McCree, K.J., 1972b. Test of current definitions of photosynthetically active radiation against leaf photosynthesis
data. Agricultural Meteorology 10:443-453.
Sager, J.C., W.O. Smith, J.L. Edwards, and K.L. Cyr, 1988. Photosynthetic efficiency and phytochrome
photoequilibria determination using spectral data. Transactions of the ASAE 31:1882-1889.

Immersion Effect Correction Factor
When a radiation sensor is submerged in water, more of the incident radiation is backscattered out of the diffuser
than when the sensor is in air (Smith, 1969; Tyler and Smith, 1970). This phenomenon is caused by the difference
in the refractive index for air (1.00) and water (1.33), and is called the immersion effect. Without correction for the
immersion effect, radiation sensors calibrated in air can only provide relative values underwater (Smith, 1969;
Tyler and Smith, 1970). Immersion effect correction factors can be derived by making measurements in air and at
multiple water depths at a constant distance from a lamp in a controlled laboratory setting.
Apogee SQ-100 series and SQ-300 series quantum sensors have an immersion effect correction factor of 1.08. This
correction factor should be multiplied by PPFD measurements made underwater to yield accurate PPFD.
Further information on underwater measurements and the immersion effect can be found on the Apogee
webpage (http://www.apogeeinstruments.com/underwater-par-measurements/).
Smith, R.C., 1969. An underwater spectral irradiance collector. Journal of Marine Research 27:341-351.
Tyler, J.E., and R.C. Smith, 1970. Measurements of Spectral Irradiance Underwater. Gordon and Breach, New York,
New York. 103 pages

MAINTENANCE AND RECALIBRATION
Blocking of the optical path between the target and detector can cause low readings. Occasionally, accumulated
materials on the diffuser of the upward-looking sensor can block the optical path in three common ways:
1. Moisture or debris on the diffuser.
2. Dust during periods of low rainfall.
3. Salt deposit accumulation from evaporation of sea spray or sprinkler irrigation water.
Apogee Instruments upward-looking sensors have a domed diffuser and housing for improved self-cleaning from
rainfall, but active cleaning may be necessary. Dust or organic deposits are best removed using water, or window
cleaner, and a soft cloth or cotton swab. Salt deposits should be dissolved with vinegar and removed with a cloth
or cotton swab. Salt deposits cannot be removed with solvents such as alcohol or acetone. Use only gentle
pressure when cleaning the diffuser with a cotton swab or soft cloth to avoid scratching the outer surface. The
solvent should be allowed to do the cleaning, not mechanical force. Never use abrasive material or cleaner on the
diffuser.
Although Apogee sensors are very stable, nominal calibration drift is normal for all research-grade sensors. To
ensure maximum accuracy, recalibration every two years is recommended. Longer time periods between
recalibration may be warranted depending on tolerances. See the Apogee webpage for details regarding return of
sensors for recalibration (http://www.apogeeinstruments.com/tech-support-recalibration-repairs/).
To determine if a specific sensor needs recalibration, the Clear Sky Calculator (www.clearskycalculator.com)
website and/or smartphone app can be used to indicate PPFD incident on a horizontal surface at any time of day at
any location in the world. It is most accurate when used near solar noon in spring and summer months, where
accuracy over multiple clear and unpolluted days is estimated to be ± 4 % in all climates and locations around the
world. For best accuracy, the sky must be completely clear, as reflected radiation from clouds causes incoming
radiation to increase above the value predicted by the clear sky calculator. Measured PPFD can exceed PPFD
predicted by the Clear Sky Calculator due to reflection from thin, high clouds and edges of clouds, which enhances
incident PPFD. The influence of high clouds typically shows up as spikes above clear sky values, not a constant
offset greater than clear sky values.
To determine recalibration need, input site conditions into the calculator and compare PPFD measurements to
calculated PPFD for a clear sky. If sensor PPFD measurements over multiple days near solar noon are consistently
different than calculated PPFD (by more than 6 %), the sensor should be cleaned and re-leveled. If measurements
are still different after a second test, email [e-mail] to discuss test results and
possible return of sensor(s).

Homepage of the Clear Sky
Calculator. Two calculators are
available: one for quantum
sensors (PPFD) and one for
pyranometers (total shortwave
radiation).
Clear Sky Calculator for
quantum sensors. Site data
are input in blue cells in
middle of page and an
estimate of PPFD is
returned on right-hand side
of page.

TROUBLESHOOTING AND CUSTOMER SUPPORT
Independent Verification of Functionality
Apogee models SQ-214 and SQ-224 quantum sensors provide a 4-20 mA output that is proportional to incident
PPFD. A quick and easy check of sensor functionality can be determined using a DC power supply and an ammeter.
Power the sensor with a DC voltage by connecting the positive voltage signal to the red wire from the sensor and
the negative (or common) to the black wire from the sensor. Use the ammeter to measure across the white wire
(signal output) and black wire (signal ground). Direct the sensor head toward a light source and verify the sensor
provides a signal. Increase and decrease the distance from the sensor head to the light source to verify that the
signal changes proportionally (decreasing signal with increasing distance and increasing signal with decreasing
distance). Blocking all radiation from the sensor should force the sensor signal to 4 mA.
Compatible Measurement Devices (Dataloggers/Controllers/Meters)
SQ-214 and SQ-224 quantum sensors are calibrated with a standard calibration factor of 250 µmol m-2 s-1 per mA,
yielding a sensitivity of 0.004 mA per µmol m-2 s-1. Thus, a compatible measurement device (e.g., datalogger or
controller) should have resolution of at least 0.004 mA, in order to provide PPFD resolution of 1 µmol m-2 s-1.
The 4-20 mA circuit design allows the output to drive a resistive load (RL) to within 2 volts of the supply voltage to
the sensor (VS), at 20 mA (0.02 A). The equation to calculate resistive load is RL = [VS–2 V] / 0.02 A. For example, a
sensor with a supply voltage of 12 V DC can drive a maximum load of 500 Ω(RL = [12 V –2 V] / 0.02 A = 500 Ω). The
output voltage from the sensor is calculated by adding the wire resistance to the input resistance of the data
collection system, and then multiplying by 0.02 A.
Effect of Cable Length
Shortening or splicing on additional cable in the field is generally not a problem for the current output of the SQ-
214 and SQ-224. However, adding cable will result in a greater resistive load, which should be taken into
consideration when determining the maximum resistive load that the sensor will drive (see section above on
Compatible Measurement Devices). Apogee model SQ series quantum sensors use shielded, twisted pair cable,
which minimizes electromagnetic interference. This is particularly important for long lead lengths in
electromagnetically noisy environments.
Modifying Cable Length
See Apogee webpage for details on how to extend sensor cable length:
(http://www.apogeeinstruments.com/how-to-make-a-weatherproof-cable-splice/).
Unit Conversion Charts
Apogee SQ series quantum sensors are calibrated to measure PPFD in units of µmol m-2 s-1. Units other than
photon flux density (e.g., energy flux density, illuminance) may be required for certain applications. It is possible to
convert the PPFD value from a quantum sensor to other units, but it requires spectral output of the radiation
source of interest. Conversion factors for common radiation sources can be found on the Unit Conversions page of
the Support Center on the Apogee website (http://www.apogeeinstruments.com/unit-conversions/). A
spreadsheet to convert PPFD to energy flux density or illuminance is also provided on the Unit Conversions page of
the Support Center on the Apogee website (http://www.apogeeinstruments.com/content/PPFD-to-Illuminance-
Calculator.xls).

RETURN AND WARRANTY POLICY
RETURN POLICY
Apogee Instruments will accept returns within 30 days of purchase as long as the product is in new condition (to be
determined by Apogee). Returns are subject to a 10 % restocking fee.
WARRANTY POLICY
What is Covered
All products manufactured by Apogee Instruments are warranted to be free from defects in materials and craftsmanship
for a period of four (4) years from the date of shipment from our factory. To be considered for warranty coverage an
item must be evaluated by Apogee.
Products not manufactured by Apogee (spectroradiometers, chlorophyll content meters, EE08-SS probes) are covered
for a period of one (1) year.
What is Not Covered
The customer is responsible for all costs associated with the removal, reinstallation, and shipping of suspected warranty
items to our factory.
The warranty does not cover equipment that has been damaged due to the following conditions:
1. Improper installation or abuse.
2. Operation of the instrument outside of its specified operating range.
3. Natural occurrences such as lightning, fire, etc.
4. Unauthorized modification.
5. Improper or unauthorized repair.
Please note that nominal accuracy drift is normal over time. Routine recalibration of sensors/meters is considered part of
proper maintenance and is not covered under warranty.
Who is Covered
This warranty covers the original purchaser of the product or other party who may own it during the warranty period.
What Apogee Will Do
At no charge Apogee will:
1. Either repair or replace (at our discretion) the item under warranty.
2. Ship the item back to the customer by the carrier of our choice.
Different or expedited shipping methods will be at the customer’s expense.
How To Return An Item
1. Please do not send any products back to Apogee Instruments until you have received a Return Merchandise