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HYDROGRAPHY COMMI'ITE

A FIELD INTERCOMPARISION BE1WEEN A MINIATURE CTD AND THE NEIL BROWN SERIAL 1223 CTD- PROFILER

BY

Trygve Gytre Inst of Marine Research P.O. Box 1870.

N-5024 Bergen -Nordnes Norway

ABSTRACT

During a survey in Lofoten in may 1989 a new miniature CTD which is being developed at the Inst. of Marine Research was mounted to the ship .. s ordinary serial 1223 CTD- profiler on 10 different CTD- stations. Using the NB CTD as a reference and compensating for differences in sampling rate, the data

presented from the two instruments when being in the same water volumes were compared. The experiments showed occational differences in the measurements of maximum 0.18 degr. C in temperature and 0.18 mmho/cm in conductivity.

lf the miniature CTD is calibrated against the NB CTD and the water is homogenous,the differences in reading between the two instruments may be reduced to within +/- 0.02 degrees C and+/- 0.025 mmho/cm.

INTRODUCTION

The Neil Brown CTD- profiler has become a standard instrument in Norwegian research vessels for fast acquisition of high quality hydrographic information .

When operated in accordance with procedures given by Blindheim (l) , the in -field data precision is re gard ed to be:

-Conductivity: +/- 3/1000 mmho/c -Temperature +/- 3/1000 degrees C

-Depth +/-2.0 m (Total range 6000m)

At the ICES- meeting in 1988 , Gytre (2) described a new

miniature "personal" CTD instrument.

This instrument is basically shaped as a 60 mm thick, 450 mm lang molded polyurethane cylinder containing electronic

interfacing, processing, .memory and displaying circuits. During operatlon data are processed and recorded inside the instrument . The recorded data may be wieved from a built in display or transferred to a PC for processing. Alternatlvely the data may be fed directly to a computer via cable.

The MINI- CTD can be programmed by the user to present CTD,STD CTDc or STDc data. (c= sound velocity). During the intercomparision the CIDc- made was selected.

During a survey in Lofoten with M/S G.O. SARS during may 1989, a prototype of this instrument was tested against the ship"s

standard N eill Brown serial 1223 CID- profiler.

BASIC INSTRUMENT PROPERTIES

The Neil Brown profiler and the miniature CTD are quite different in design.

Fig. l illustrates the most significant differences. Basically the NB CTD is designed to generate a vast amount of high speed raw data which are transmitted to a deck unit and computer via cable.

The MINI- instrument is designed to measure,process and record a moderate amount of raw or processed data at a programmable repetition rate ranging from from ane measurement each 5.

second to ane measurement every 3 hour. Although the

instrument was initlally design ed. for environmental monitoring in bouys, it can also be be used for profiling.

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FIG. l BASIC DESIGN OIFFERENCES

Fig. l Basic differences in design

When in use the NB CTD must be operated with a winch from a relatively large vessel, while the MINI instrument is is small enough be operated from boats of any size without a winch.

Fig. 2 shows how the MINI- instrument was mounted close to the

the NB CTD by frxing it to the protective cage that surrounds the NBCTD.

UNDER- WATER UNIT

PROTECTIVE CAGE

FIG.2 MOUNTING OF MINI CTD ON PROTEXTI VE CAGE

Fig. 2 Sensor positions during the measurements.

During the experiments the MINI - instrument was programmed to measure and record at its fastest possible recording rate- each 5 second.

To eliminate the dynamic effects of differences in sampling rate, and to be sure to intercompare data from the same water

masses,the winch was stopped at each 10 meter for 10-20 seconds. During this interval the NB CTD data were read and noted down from a display in the deck unit.

After each profiling the recorded data in the MINI- CTD memory were transferred to a PC from which the CTD-data that

corresponded to to stable depth - readings were sorted out and noted down.

Fig. 3 shows a typical data against time - diagram showing the "

plateaus" generated by each stop. The intercalibration data were collected from these flat regions.

IUnut.ts

Fig. 3 Data-time. printout showing stops for each 10m.

CALIBRATION CONTROL.

Befare the survey start ed, the actual calibration of the MINI -CTD temperature, conductivity and pressure sensors were checked in

3 points against the laboratory standards used by Inst. of Marine Research.

Table l shows the results.

Referance MINI CTD DIFFERENCE

24.385 24.461 -0.076 MMHO/CM

42.270 42.349 -0.079 "

45.940 45.811 +0.129 "

+0.600 +0.580 + 0.02 DEGR. C

15.080 15.030 +0.05 "

25.090 25.010 +0.08 "

0.00 0.01 -0.01 db ar

98.1 98.3 -0.02 "

181.3 181.8 -0.05 "

During the C and T- calibration control, which was done in well mixed, stable water

bath, the displayed vartations in temperature and conductivity were

observed to be within +/- 0.01 degrees C and +0.01 mmho/cm respectively.

Fig. 4 illustrates the temperature and conductivity calibration errors that were observed.

In particular the initial conductivity calibration proved to be inaccurate for high conductivity values.The resident calibration equations were not adjusted at this time.

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Fig. 4. Observed calibratlon errors.

During the survey a total of 10 intercompartngs were made. The Lofoten region is relatively shallow (50-300 m) so the full depth range could not be tested on this occasion.

Fig. 5 shows the PC printouts for temperature and conductivity against depth for a typical station.( SARS7). The corresponding readings from the NB CTD are marked for each 10m stop.

·Ref-r.uMber-:---30 - File-nal're-:-sars7

Measure~ent series nu~ber: 1 Interval tiMe: 5 seconds.

Data displayed froM 16:41:24 03.May-89 to 16:55:29 03.May-89

~ f"\ T e ^M p e r a t u r e ( °C ) •

(. OI

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15 30 45 t.o 75 9'o io5 120 135 1so 165 1so 195 210 225 24lJ25s 2?0 285 300

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Fig. 5 Printout of a typical CTD-station inside the Lofoten basin made with the MINI-CTD. The corresponding NB CTD observations are marked with crosses.

Fig6, 7 and 8 show the observed differences in temperature,

conductlvity and depth between the NB CTD and the MINI-CTD in

station SARS7. The se differences were typical also for stations SARS l-SARS6.

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-0,2 -t--.---,---.--~-.--~----.--~-.-~---. 0/12

6,0 6,2 6,4 6,6 6,8 7,0 7,2 32 33 34 35

Fig. 6 Differences in temperature Fig. 7 Differences in conductlvitv 36

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Fig. 8 Difference in depth re a dings.

Intercompartng of the pressure sensors was not very meaningful since the pressure sensor in the NB CTD covers 6000 m and the · MINI pressure sensor just c overs 500 m. The ship 's continuous motions also makes a direct intercomparision inaccurate. Fig. 7 basically shows that the pressure data from the NBCTD had an offset of appr. 1.6 m . ( In the MINI- instrument the depth is automatically initialised to zero when the instrument is started) According to the initial calibration control, conductivities around 34 mmho/cm are presented 0.079 mmho/cm too high.

During the intercompartngs made, the MINI- instrument showed appr. 0.13 mmho/cm too high conductivity.

Fig. 6 and 7 show that temperature readings could differ up to+/- O.l degrees C and conductivity could differ up to 0/- 0.025

mmho/cm. The !argest differences were observed in regions were the changes with respect to depth were large.

IN FIELD CALIBRATION ADJUSTMENT .

In the MINI CTD the conductivity is calculated by the internal microprocessor from a equation of the form

C= D+ENc +FNc exp2. (l) where Ne are the conductivity bits and D,E and Fare calibration coefficients. Modifications to these

coefficients can be easily made from a PC via the instrument's RS- 232-communication plug.

After the SARS7- station had been recorded, 0.130 was subtracted from constant D in eq. (l) .

Fig.8 shows the observed anomalies in conductivity in the next station - SARSB ( bottom depth 80 m) Clearly the two conductivity sensors showed a good correlation in the homogenous water near the bottom.

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STATIOH SARS8

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Fig9 Differences in conductivity reading after offset adjustment.

SOUND VELOCITY

Sound velocity is aften a convenient "fringe benefit" parameter in in oceanic research. In the MINI CTD the sound velocity is calculated by the internal microprocessor from salinity, temperature and pressure.

Fig. lO shows an excample of computed sound velocity proftle .

1534.0 1527.5 1521.0 1514.5 1508.0 1501.5 1495.0 1488.5 1482.0 1475.5

-R e f n u "'be r : ·-3 ~ F i l e n a 1"1 e : sa-- s 7

Measurel"'ent series nul'lber: 1 Interval til"'e; 5 seconds.

Dat.a displayed froM 16:41:24 03.11ay-89 to 16:~:2 9 03.May-89 Sound velocity ^(~/s).

Fig. lO Computed sound velocity proftle

DISCUSSION

An intercomparision in the field shows how two different instrument react to the same environment at the same time.

Different results may be caused by different calibration, different quality in sensor design and signal processing, different dynamic response or nonequal interferences from the mooring.

From the calibration control, a permanent offset of appr. 0.03 degrees C and 0.078 mmho/cm could be expected around +7 degrees C and 34mmho/cm. According to fig. 5 and fig. 6, the observed offsets were appr. 0.07 degrees C and 0.15 mmho/cm respectively .These offsets were typical for the 7 first

intercomparisions (when the calibration coefficient were still unchanged).

To sort out the reasons for the observed permanent offsets, a more thorough check of the laboratory calibration routines must be made.

However,such repetititve offsets are very easy to remove.

A large part of the observed scatter in data around the offset values - which represent the ultimate differences- may be due to variations in response to moving water.

When the ship goes up and down due to waves, the instruments are also pulled up and down . The temperature sensor used in the MINI-

CTD has a langer time constant than the temperature sensor in the NB CTD, and this may cause dynamic errors.

The conductivity sensor used in the MINI CTD has a larger diameter than the diameter in the NB CTD. Therefore the flow rates through the two sensors cannot be equal. When measuring in stratified water, the se factors have to genera te small differences in the observed temperature and conductivity data.

CONCLUSIONS

The tests carried out in Lofoten were made in a region with limited span in seawater parameters. Within this limitation, the inter-

comparisons showed that the conductivity data delivered from the originally calibrated MINI CTD followed the data from the NB-CTD with a conductivity dependent offset from appr. 0.13-0.18

mmho/cm. In homogenous water the scatter around the offset was appr.+/-0.02 mmho/cm.

For temperature the corresponding numbers were - 0.02- -0.18 degrees C and+/- 0.02 degrees C.

Stable or predictable offsets may be easily removed by making changes in the MINI-CTD calibration parameters.

The scatter is caused by basic differences in the two instrument's design and sensor quality and can only be reduced by statistical methods.

References:

l Blindheim J. "Procedures for collecting and processing of

hydrographic data in Norway." (ICES C.M. 1985/C:l Hydrography Committee)

2 Gytre T. " Automatic calibration of the sensors used in a

miniature SID-instrument." (ICES CM 1988 l C:2t Hydrography Committee.