Application Note No. 13-1, rev B - Revised April 1993
SBE 13/22/30 DISSOLVED OXYGEN SENSOR CALIBRATION AND DEPLOYMENT
SEA-BIRD ELECTRONICS, INC.
1808 - 136th Place Northeast, Bellevue, Washington 98006
A. General Description
Sea-Bird Electronics uses either a Beckman sensor element or a modified YSI 5739
oxygen probe in its oxygen sensors. Present Sea-Bird oxygen sensors have two 0 to +5 volt outputs. One
of these is proportional to the internal temperature of the sensor and the other is proportional to
the oxygen current. SBE 13 sensors produced before February 1992 have a 0 to +5 volt output
(oxygen current) and a -5 to +5 volt output (sensor temperature). CTD instruments made by Sea-Bird that
are equipped with oxygen sensors record these voltages for later conversion to oxygen
concentration using the algorithm by Owens and Millard (1985).
Oxygen sensors determine the dissolved oxygen concentration by `counting' the number
of oxygen molecules per second (flux) that diffuse through a membrane from the ocean environment
to the working electrode. By knowing the flux of oxygen and the geometry of the diffusion path
the concentration of oxygen in the environment can be computed. The permeability of the membrane
to oxygen is a function of temperature and ambient pressure and this is taken into account in the
calibration equation. The algorithm to compute oxygen concentration requires that the following
measurements be made: water temperature, salinity, pressure, oxygen sensor current, and oxygen
sensor temperature. When the oxygen sensor is attached to a Sea-Bird CTD all of these parameters
are measured by the CTD.
At the working electrode (cathode) oxygen gas molecules are converted to hydroxyl
ions (OH-) in a series of reaction steps where the electrode supplies four electrons per molecule to
complete the reaction. The sensor counts oxygen molecules by measuring the electrons per
second (amperes) delivered to the reaction. At the other electrode (anode) silver chloride is formed
and silver ions (Ag+) are dissolved into solution. Consequently the chemistry of the sensor
electrolyte changes continuously as oxygen is measured, and this produces a slow but continuous change of
the sensor calibration with time (the slope coefficient, Soc, changes by a factor of two after about
1000 hours of powered-up use in `Beckman' sensors and after a few hundred hours in YSI sensors).
Oxygen sensors have operating characteristics that require certain procedures be followed to
insure that accurate and reliable measurements of oxygen concentration are obtained. These
characteristics include:
1. When power is applied to the oxygen sensor it takes up to three minutes for the
sensor to polarize and come to a stable reading. This implies that when a CTD is
turned on it must be held at the surface for at least three minutes before a cast is
started to insure accurate oxygen readings.
2. The oxygen sensor consumes the oxygen in the water near the sensor membrane. If
there is not a adequate flow of new water past the membrane, the sensor will give a
reading which is lower than the true oxygen concentration. This requires that the sensor
be moving through the water or that water be pumped past the sensor.
3. Temperature differences between the water and the oxygen sensor can lead to errors in the
oxygen measurement. When profiling through areas of high temperature gradients this
error can be substantial. Because of its different construction the Beckman sensor element
is more susceptible to this error source than is the YSI sensor. Aligning the
oxygen data in time with the ALIGNCTD program can minimize this problem
and also correct for the water transit time in the plumbing on pumped
systems and for the relatively slow response time of oxygen sensors in com
parison to other CTD sensors.
B. Oxygen Algorithm
SEASOFT uses the algorithm by Owens and Millard (1985) to convert SBE 13/22/23/30
oxygen sensor data to oxygen concentration but treats the coefficients differently. Only Soc and Boc,
the scale and offset coefficients, are allowed to be variable. The other four coefficients (tcor, pcor,
tau and wt) are fixed at reasonable physical values. Sea-Bird provides two programs to compute
the values for Soc, (sensitivity or scale) and Boc (offset). OXFIT uses the zero oxygen value and
air saturated water readings. OXFITW uses the zero oxygen value and an oxygen value measured
by Winkler or other methods.
The algorithm has the following form:
OX = [Soc*(oc+tau*doc/dt)+Boc]*OXSAT(T,s)
*exp(tcor*[T+wt*(To-T)]+pcor*p)
where
| Computed: | OX | dissolved oxygen concentration [mL/l] |
| Measured Parameters: | T | water temperature [°C ] |
| To | oxygen sensor internal temperature [°°C]. |
| s | salinity [PSU}-[ppt] |
| p | pressure [decibars] |
| oc | oxygen current [microamps] |
| doc/dt | slope of oxygen current [microamps/sec] |
Calibration Coefficients: Boc | oxygen current bias |
| Soc | oxygen current slope |
Constants: wt | weighting fraction
of oxygen sensor
internal
temperature |
| tcor | temperature correction factor for membrane permeability |
| pcor | pressure correction factor for membrane
permeability |
| tau | oxygen sensor response time |
Calculated value: OXSAT(T,s) | oxygen saturation
value after Weiss
(1970) |
Values for tcor, tau and wt are taken from the Beckman polarographic oxygen sensor
technical memorandum. The value for pcor recommended by Sea-Bird deviates from the Beckman
memorandum and is based on more recent data analysis (see Application Note 13-3).
tcor = -0.033
pcor = 1.50e-4
tau = 2.0
wt = 0.67 (Beckman type sensors)
wt = 0.85 (YSI type sensors)
C. Oxygen sensor Calibration
The calibration method used by Sea-Bird is to measure the oxygen current output in a
zero oxygen environment and the oxygen current and oxygen temperature outputs in either
air-saturated water (OXFIT) or in water where the oxygen content is independently measured (OXFITW).
The voltage outputs are converted to sensor temperature and oxygen current using the k and c
coefficients for temperature and the m and b coefficients for current. The conversion coefficients are
found on the original factory calibration sheet for the oxygen sensor. OXFIT and OXFITW calculate
the coefficients Soc and Boc that are used in the oxygen algorithm. Use the SEASOFT
module SEACON to enter the computed values for Soc and Boc.
The oxygen sensor can be calibrated by itself using a voltmeter to measure the sensor
outputs and a power supply to provide power to the sensor. Alternatively the CTD system can be used
to provide power to and acquire data from the oxygen sensor. In this methods the SEASOFT
software can be used to display real time data from the instrument including oxygen concentration.
If the oxygen sensor is on a CTD system with a pump it is recommended that the entire
CTD be submerged in the bath but not powered for at least one hour prior to the calibration. Supply
power to the CTD, oxygen sensor, and pump for 15 minutes prior to the calibration. The oxygen
sensor power must not be interrupted for 15 minutes prior to the calibration so that full polarization
and equilibration can be established.
SBE 19 SeaCat Profilers with a pump and SBE 25 SEALOGGER CTDs have
adjustable pump start frequencies which will be set to zero using the appropriate terminal program and the
SP command for the SBE 19 and the CC command for the SBE 25. This will insure that the pump
will start in fresh water. The SBE 9 CTD contains circuitry that turns the pump on when the
conductivity sensor enters salt water. To insure that the pump will turn on in a fresh water bath, remove the end
of the tygon tubing going between the conductivity cell and the oxygen sensor from the
conductivity sensor and place a loop of tubing filled with salt water over both ends of the conductivity cell (or
TC duct and conductivity cell). Please note that if salt water is in the conductivity cell and the
oxygen sensor is in fresh water the CTD will compute salinity based on the water in the conductivity cell.
In this case be sure to enter 0 for the salinity value in OXFIT or OXFITW and be aware that the
values of OXSAT and Oxygen computed by the software will be incorrect because the wrong value
of salinity will be used for the oxygen computation. Once the sensor has soaked for the required
one hour period, power will be applied to the sensor either by turning on the external power supply or
the CTD. If a pump is being used that is not connected to the CTD, power will be applied to it.
Before power is applied it will be verified that no air is trapped in the plumbing system. Trapped air
will prevent the pump from establishing a good flow. Most oxygen sensors will come to within 1%
of their asymptotic stable reading in five minutes after the application of power. This reading (either
in units of current or voltage) will be recorded. To obtain oxygen readings that are within +/- 1% of
the true reading the oxygen sensor temperature must be within 0.25°C of the bath water temperature
as measured by the CTD or a thermometer.
1. Zero Oxygen Reading (OXFIT and OXFITW)
It is recommended that the zero oxygen point be taken first. This can be done by two
different techniques; one can flush the sensor with a continuous stream of inert gas (e.g. Nitrogen
or Argon), or place the sensor in a 5% - 10% by weight solution of Na2SO3 (sodium sulfite).
Sea-Bird recommends the sodium sulfite methods. It is simpler and is not subject to errors that can
occur when using an inert gas such as poor temperature control and incomplete displacement of
oxygen gas diffusing out from inside the oxygen sensor. On Sea-Bird CTD systems that are equipped with
a pump the oxygen sensor is provided with a plenum. This plenum can be filled with sodium
sulfite solution and closed off with a piece of tubing (or alternatively inert gas can be flushed through
the plenum). When using the sodium sulfite solution make sure that there are no air bubbles trapped
on the oxygen sensor membrane. Insure that power has been applied to the sensor for several
minutes before the inert gas or sodium sulfite solution is placed in the sensor. Watch the output of the
sensor decrease rapidly towards zero volts. At some point the rapid change will stop, usually within one
to two minutes. Record the output after three minutes seconds. This will be the zero value to use in
the calibration. Often, depending on the individual sensor. the output will slowly drift towards
zero volts. For the purposes of the calibration this slow drift is not considered. The original
calibration sheet that accompanied the oxygen sensor will contain the zero oxygen current that was
obtained during the factory calibration. If the sodium sulfite solution was used, rinse the oxygen
sensor thoroughly several times to remove all traces of the solution and carefully clean your hands.
2. Air Saturated Reading (OXFIT)
The theory is to read the sensor's output in water which is exactly saturated with atmospheric
gases. The saturated value of dissolved oxygen at atmospheric pressure and at a given temperature
and salinity is computed with the program OXSAT. In practice this is accomplished by immersing
the oxygen sensor in a volume of air saturated water and drawing water past the sensor with a
small submersible pump. If the CTD system is equipped with a pump, this will be used for the
calibration along with the plenum that was provided with the oxygen sensor. If another pump is used it will be
a submersible type and configured to pump at a rate of 20 to 30 mL/s. In this case a plenum will
be purchased from Sea-Bird to insure a reliable and repeatable flow of water past the membrane.
The water is air saturated by aerating with an aquarium pump and air stone for 24 hours prior to
the calibration. The air stone will be located within 10 cm of the surface. The air stone positioned
at greater depths will tend to supersaturate the water because the air is injected at a pressure higher
than atmospheric pressure. The water will be stirred during aeration and before measurements to
insure that the whole volume contains saturated water. Stirring that is two vigorous can inject air
bubbles deep into the bath supersaturating the bath water. For the highest accuracy work it is preferable
that the temperature of the water used for the calibration be as close as possible to the temperature of
the water where the measurement will be taken. Care will be taken to minimize the ambient
temperature changes that the container of water is subjected to. As water is heated its capacity to hold air
is diminished and air will come out of saturation and form bubbles. These bubbles if present on
the oxygen sensor membrane will interfere with the measurement. As the water heats it will also tend
to supersaturate. If the container is cooled it will tend to drop below saturation. Since OXFIT
assumes that the water is neither over or under saturated if the water temperature in the container
changes faster than the oxygen can equilibrate the computed values of Soc and Boc will be incorrect. It
may be necessary to wait more than fifteen minutes per liter of water in the container for every degree
of temperature change.
3. Winkler Titration Value (OXFITW)
With this method the amount of dissolved oxygen in the water is independently measured so it is
not necessary to aerate the water. For accurate results the oxygen concentration in the bath needs to
be stable and constant over the period of the calibration. To insure this observe the following
precautions: a) do not use freshly drawn water; it is typically supersaturated in gas and not equilibrated
with the atmosphere, b) sir the bath vigorously (without mixing in air bubbles) to allow the water
opportunity to come in contact with atmosphere and equilibrate to the atmospheric gas concentrations,
and c) the bath temperature must remain stable to better than 0.1 deg C per hour prior to and during
the calibration. If the CTD system is equipped with a pump, this will be used for the calibration
along with the plenum that was provided with the oxygen sensor. If another pump is used it will be
a submersible type and configured to pump at a rate of 20 to 30 mL/s. In this case a plenum will
be purchased from Sea-Bird to insure a reliable and repeatable flow of water past the membrane. For
the highest accuracy work it is preferable that the temperature of the water used for the calibration be
as close as possible to the temperature of the water where the measurement will be taken.
Allow enough time for the oxygen sensor to reach temperature equilibrium and then determine the
amount of dissolved oxygen [mL/l] in the water using the Winkler or some other independent
measurement method.
D. OXFIT Prompts
local barometric pressure (millibars)
this is the pressure that would be read on a barometer (not corrected to sea level)
water temperature (°C)
water temperature read by the temperature sensor
oxygen current in air saturated water (microamps)
when displaying oxygen current with SEASAVE make sure the m and b
coefficients from the dissolved oxygen sensor calibration sheet are entered using
SEACON. If oxygen current voltage was recorded, use the m and b coefficients to
convert the voltage to a current.
oxygen current in zero oxygen water (microamps)
enter the value determined when using the inert gas or the sodium sulfite solution
E. OXFIT Calculation
OXFIT calculates Soc and Boc as follows:
Soc = nsa(T,bp)/[exp(tcor*)*(oc-zoc)]
Boc = -Soc*zoc.
oc = air saturated water current (microamps)
zoc = zero air water current (microamps)
See Table F-1 for the definition of nsc(T,bp).
F. OXFITW Prompts
oxygen serial number =
enter the serial number from the original calibration sheet
m = enter the value from the original calibration sheet
b = enter the value from the original calibration sheet
k = enter the value from the original calibration sheet
c = enter the value from the original calibration sheet
salinity [PSU] =
enter the salinity of the water in the container
water temperature [deg °C] =
enter the temperature of the water at the time of the measurement
Winkler value [mL/l] =
enter the measured amount of dissolved oxygen in milliliters per liter. The
Winkler methods is described in Carritt, D.E. and J.H. Carpenter. 1966.
Comparison and evaluation of currently employed modifications of the Winkler
method for determining dissolved oxygen in seawater. J. Mar. Res. 24(3), 286-
318, and Standard methods for the examination of water and wastewater, editors
Clesceri et al.
oxygen current voltage for xx[mL/l] =
enter the voltage output by the oxygen current channel after the sensor has
equilibrated in the water bath.
oxygen current voltage for air =
enter the voltage from the oxygen current channel when the sensor is in air. This
value is for reference only and is not used to calculate the coefficients.
oxygen temperature voltage for xx [deg °C] =
enter the voltage output by the oxygen temperature channel after the sensor has
equilibrated in the water bath.
oxygen current voltage for zero oxygen =
enter the voltage output by the oxygen current channel after the sensor has
equilibrated to sodium sulfite or an inert gas.
G. OXFITW Calculation
Soc = measured oxygen / [oxsat(T, S)*exp(tcor*T)*(oc-zoc)]
Boc = -Soc*zoc
oc = air saturated water current (microamps)
zoc = zero air water current (microamps)
See Table F-2 for the definition of oxsat(T, S).
A file named SERIALNO. CAL will be written to the current directory containing a
summary of the calibration data and computed coefficients.
H. Verification of SOC and BOC
OXFIT and OXFITW calculate and display the new Soc and Boc coefficients. These will
be compared to the original factory calibration or the last calibration that was performed. Typically
the Soc value will slowly increase with time as the sensor is used. The KCI electrolyte in the
oxygen sensor is consumed as part of the reduction reaction. This loss of KCI decreases the sensitivity of
the sensor which is reflected in the slowly increasing Soc value. Application note 13-4 will be
consulted about the life expectancy of Beckman dissolved oxygen sensors. Application note 32
contains additional information about the YSI based oxygen sensors.
The new Soc and Boc values will be entered into the SEASOFT.CON file using
the SEACON program. If the DERIVE program in SEASOFT Version 4 software is being used
to calculate the oxygen concentration after the data has been aligned the Soc and Boc values will
be entered into its configuration file. If the entire CTD was used in the oxygen calibration it can be
run in real time mode to check the calibration results. Display parameters of oxygen concentration
in mL/l, water temperature and salinity are necessary. The program OXSAT can be used to
calculate the saturation value for the measured temperature and salinity and compared with the real
time reading of oxygen concentration. Or if SEASOFT Version 4 is being used the saturated
oxygen concentration can be displayed along with the oxygen sensor reading. If the oxygen sensor is
healthy and the calibration was performed correctly, these values will agree to within 0.1 mL/l. For SBE
9s that must have salt water in the cell to turn the pump on, the real time oxygen readings will be
in error because SEASAVE will assume that the water in the bath has the same salinity as the water
in the tube.
I. Oxygen Sensor Cleaning and Storage
Care must be taken to avoid fouling the oxygen membrane with oil or grease, and it is
recommended that the oxygen sensor be rinsed with a 1% water-solution of Triton X-100 and flushed
with distilled water after each use. With pumped instruments having a clear plastic plenum, loop
tubing from inlet to outlet and partly fill with distilled water between deployments (if there is
freezing danger, shake all excess water out of the plenum). With unpumped instruments, put a few drops
of water in the DO sensor's protective cap and fasten the cap securely. As an added benefit, the
sensor will be kept free of airborne particulates that could otherwise coat the membrane and reduce
the sensitivity.
For routine cleaning, soak the sensor in a 1% solution of Triton X-100 initially warmed
to 50°C (122°F) for 30 minutes. After the soak, drain and flush with warm (not hot) fresh water for
1 minute.
J. Oxygen Sensor Deployment
Connect the pump tubing to the sensor plenum (pumped designs) or remove the
protective cap (unpumped designs) before deployment. NOTE: Failure to remove the cap will result in
the crushing of the cover at depth and will cause destruction of the oxygen sensor.
A large drop of Triton X-100 solution gently placed directly on the sensor membrane
will protect the sensor from oil on the seawater surface. The Triton will quickly rinse away
leaving behind a clean and fully functional sensor membrane.
To allow time for the oxygen sensor to polarize, the instrument to which it is connected must
be powered for at least three minutes before beginning the water-column profile. Failure to wait
will result in erroneously high oxygen readings. When taking water samples using a General
Oceanics rosette and Sea-Bird 9/11 CTD which share a single conductor seacable, wait at least two
minutes after the bottle has been tripped before resuming the CTD profile. Tripping the bottle
momentarily interrupts power to the oxygen sensor which then must repolarize when power is reapplied. A
SBE 911plus CTD which is being to control the rosette does not loose power when a bottle is tripped.
When using an unpumped oxygen sensor, a water flow speed of at least 0.5 meter / second
(horizontal motion, current, or vertical profiling rate) must be constantly maintained to avoid local
oxygen depletion and erroneously low readings.
TABLE F-1. CORRECTION FACTOR FOR NON-STANDARD ATMOSPHERE
| nsa(T,bp) | = | (bp/pO) * (1 - pH2O/bp)) / (1 - pH2O/pO) |
| bp | = | barometric pressure in kilopascals |
| pO | = | 101.325 kilopascals |
| ph20 | = | water vapor pressure in kilopascals |
| T | = | water temperature in °C |
| | | |
| pH20 exp[((-216961 * X) -3840.7) * X + 16.4754] | | |
| X | = | 1/(T+273.15) |
| For air saturated water at the surface:
|
| | | |
| oc | = | air saturated water current (microamps) |
| zoc | = | zero air water current (microamps) |
| | | |
| {[Soc*(oc-zoc)] / nsa(T,bp)} * exp(tcor*T) = 1 | | |
| | | |
| Soc | = | nsa(T,bp) / [exp(tcor*T) * (oc-zoc)] |
| Boc | = | -Soc * zoc |
TABLE F-2. COMPUTATION OS OXSAT
OXSAT(T,s)=exp(A1 + A2*(100/T + A3*In(T/100) + A4*(T/100)
+ s*(B1 + B2(T/100)*(T/100)))
The units are mL/1, the oxygen saturation value is the volume of the gas (STP) absorbed from water saturated air at a total pressure of one atmosphere, per unit volume of the liquid at the temperature of measurement where:
| s | = | salinity in parts per 1000 |
| T | = | °C + 273.15 (absolute temperature) |
| A1 | = | -173.4292 |
| A2 | = | 249.6339 |
| A3 | = | 143.3438 |
| A4 | = | -21.8492 |
| B1 | = | -0.033096 |
| B2 | = | 0.014259 |
| B3 | = | -0.0017 |
TABLE F-3. COMPILATION OF OXYGEN SATURATION VALUES.
The following table contains oxygen saturation values at atmospheric pressure calculated using the OXSAT equation found in Table F-2. Units of oxygen are mL/1. To compute units of mg/1 multiply the values in the table by 1.4276.
| Salinity (PSU) |
|---|
| Temp °C | 0 | 5 | 10 | 15 | 20 | 25 | 30 | 32 | 35 |
|---|
| | | | | | | | | | |
|---|
| -2 | 10.82 | 10.46 | 10.1 | 9.76 | 9.42 | 9.1 | 8.79 | 8.67 | 8.49 |
|---|
| 0 | 10.22 | 9.88 | 9.54 | 9.22 | 8.91 | 8.61 | 8.33 | 8.21 | 8.05 |
|---|
| 2 | 9.67 | 9.35 | 9.04 | 8.74 | 8.45 | 8.17 | 7.9 | 7.79 | 7.64 |
|---|
| 4 | 9.16 | 8.86 | 8.57 | 8.3 | 8.02 | 7.76 | 7.51 | 7.41 | 7.26 |
|---|
| 6 | 9.7 | 8.42 | 8.15 | 7.89 | 7.64 | 7.39 | 7.15 | 7.06 | 6.92 |
|---|
| 8 | 8.28 | 8.02 | 7.76 | 7.52 | 7.28 | 7.05 | 6.82 | 6.74 | 6.61 |
|---|
| 10 | 7.89 | 7.64 | 7.41 | 7.17 | 6.95 | 6.73 | 6.52 | 6.44 | 6.32 |
|---|
| 12 | 7.53 | 7.3 | 7.08 | 6.86 | 6.65 | 6.44 | 6.24 | 6.17 | 6.05 |
|---|
| 14 | 7.2 | 6.99 | 6.77 | 6.57 | 6.37 | 6.17 | 5.99 | 5.91 | 5.8 |
|---|
| 16 | 6.9 | 6.69 | 6.49 | 6.3 | 6.11 | 5.93 | 5.75 | 5.68 | 5.58 |
|---|
| 18 | 6.62 | 6.42 | 6.23 | 6.05 | 5.87 | 5.7 | 5.53 | 5.46 | 5.36 |
|---|
| 20 | 6.35 | 6.17 | 5.99 | 5.81 | 5.64 | 5.48 | 5.32 | 5.26 | 5.17 |
|---|
| 22 | 6.11 | 5.93 | 5376 | 5.6 | 5.44 | 5.28 | 5.13 | 5.07 | 4.98 |
|---|
| 24 | 5.88 | 5.71 | 5.55 | 5.39 | 5.24 | 5.09 | 4.95 | 4.89 | 4.81 |
|---|
| 26 | 5.66 | 5.51 | 5.35 | 5.2 | 5.06 | 4.92 | 4.78 | 4.73 | 4.65 |
|---|
| 28 | 5.46 | 5.31 | 5.17 | 5.03 | 4.89 | 4.75 | 4.62 | 4.57 | 4.5 |
|---|
| 30 | 5.328 | 5.13 | 4.99 | 4.86 | 4.73 | 4.6 | 4.47 | 4.43 | 4.35 |
|---|
| 32 | 5.1 | 4.96 | 4.83 | 4.7 | 4.58 | 4.45 | 4.34 | 4.29 | 4.22 |
|---|
LITERATURE CITED
Carritt, D.E., and J.H. Carpenter. 1966. Comparison and evaluation of currently employed modifications of the Winkler method for determining dissolved oxygen in seawater. J. Mar. Res. 24(3): 286-318.
Clesceri, L. A., E. Greenberg, and R.R. Trussell (eds.). 1989. Standard methods for the examination of water and wastewater. 17th edition. Am. Public Health Assoc. Washington, DC. ISBN 0-87553-161-X.
Gnaigner, E., and H. Forstner (eds.). 1983. Polarographic Oxygen Sensors: Aquatic and Physiological Applications. Springer-Verlag, 370 p.
Millard, R. C., Jr. 1982. CTD calibration and data processing techniques at WHOI using the 1978 practical salinity scale. Proc. Int. STD Conference and Workshop, La Jolla, Mar. Tech. Soc., 19 p.
Owens, W.B., and R.C. Millard Jr. 1985. A new algorithm for CTD oxygen calibration. J. Physical Oceanography 15: 621-631.
Weiss, R.F. 1970. The solubility of nitrogen, oxygen and argon in water and seawater. Deep- Sea Res. 17: 721-735.
APPLICATION NOTE NO. 7 - Revised September 1989
CALCULATION OF M AND B COEFFICIENTS FOR
THE SEA-TECH TRANSMISSOMETER
SEA-BIRD ELECTRONICS, INC.
1808 - 136th Place Northeast, Bellevue, Washington 98005
The data sheet supplied by SEA TECH indicates the air calibration voltage (approx. 4.7 volts) and the blocked path voltage (approx. 0.0 volts). These values along with the current air voltage and blocked path voltage are used to derive the M and B coefficients used in SEACON as follows:
To calibrate the transmissometer with the Sea-Bird instrument to which it is interfaced, you must obtain readings with the light path in air (the lenses must be clean and dry for this to be meaningful) and then with the light path blocked. Run SEASAVE, answer ‘y’ to ‘change data acquisition or display parameters (y/n)?’. Answer ‘y’ to the prompt ‘change CRT parameters (y/n)?’. Select ‘fixed display’, and choose ‘voltage’ as the variable type. Enter the transmissometer’s voltage number (see configuration page at beginning of manual); select real time data to get a display of the transmissometer output.
A0 is the AIR CALIBRATION voltage from the SEA TECH calibration sheet
Y0 is the blocked path voltage from the SEA TECH calibration sheet
A1 is the current air voltage
Y1 is the current blocked path voltage
then M = 20(A0 - Y0)/(A1 - Y1)
and B = -M Y!
For example:
If the SEA TECH calibration gave the following values:
A0 = 4.743 volts
Y0 = 0.002 volts
and the current calibration gave:
A1 = 4.719 volts
Y1 = 0.006 volts
then
M = 20(4.743 - 0.002)/(4.719 - 0.006) = 20.119
B = -0.006 * 20.119 = -0.1207
These are the M and B values that are to be entered into SEACON. If your instrument has AV = 2 inputs (used on some SBE 9 configurations) follow the same procedure. You will obtain A1 and Y1 values approximately twice as large as those in the example (9.348 volts and 0.012 volts respectively) leading to M = 10.0594 and B = 0.1207.