Comparison of a Smartfin with an Infrared Sea Surface Temperature Radiometer in the Atlantic Ocean

Round 1

Reviewer 1 Report

This publication presents a study made to assess the differences between a Smartfin, a CTD and an ISAR during an Atlantic Meridional Transect. The Smartfin is equipped with a low cost Maxim temperature component advantageously located to about 10 cm under the surface. This small distance from the skin layer makes of this device, a very interesting instrument to compare SST measured by radiometers.  The measurement results give rise to interesting interpretations. That justifies this study. However, a few remarks can be made:

p4: the ref [32] does not allow to obtain technical details. It just shows a photo of a Smartfin. What are the dimensions of a Smartfin and what is the distance between the sensible part of the temperature sensor and the surface?

The Maxim Integrated documentation of the MAX31725, gives an accuracy of +/- 0.5 °C for this component. MAX31725 is a temperature sensor embedded in a package containing a 16 bit ADC and electronic components necessary to digitize and transmit the temperature data. Its response time is therefore dependent on the time needed by the electronic circuitry to take the temperature of the seawater (see https://datasheets.maximintegrated.com/en/ds/MAX31725-MAX31726.pdf). The sensor measures the temperature of the component and not the true temperature of the water. Its resolution is 0.004 K, but the temperature conversion noise is 0.0625 K, giving a typical accuracy of +/- 0.1 K, according to the datasheet.

lines 112-118: the accuracy of the instrument used as reference is given (0.002 K), but the stability of the bath  temperature is not given. Is it constant over the range of the calibration? This stability is fundamental to assess the calibration uncertainty of the sensors to calibrate. Could you give more details?

Line 114: The Thermo Scientific NESLAB RTE7 circulating bath/chiller used in this study seems to have a small volume (22.4 x 20.6 x 22.9 cm for the biggest bath).  What are the dimension of the bath you used and was it sufficient to place a Smartfin without having leakage with the air temperature?

Line 128: you say that SOD deployments were made without the iButton and Tidbid sensors, but in the abstract you give difference values removed for these sensors. Is it useful to give such details in the abstract as these sensors were not used in the comparison?

Line 140: “All temperature data were converted to Kelvin (K), the SI unit for temperature ». °C is also the SI unit for temperature. Per convention, °K are used for low temperatures of the scale and °C is used for temperatures from 0°C, but the two units are allowed. Therefore, this sentence is not completely true.

Line 150: why the accuracy of the reference probe is given to only one temperature (25 °C) so that it is used on the range 17 °C – 36 °C?

Line 151: I don’t see the relation between ref [36 -38] and the use of a recirculating bath, and what do you call ‘contrasting temperatures’?

Line 154: why the calibration was limited to 290 K (17 °C ) so that on figure 3(a) temperatures of 283 K were measured? Were any corrections applied to the sensor and in what form: offset-slope, 2sd degree polynomial?

Line 163: ‘the ISAR was shutter was closed’. Remove one ‘was’.

Line 166: ‘at a nominal depth of around 5m data’. Remove ‘data’.

Line 179: the temperature sensor being embedded in an electronic component and this component being embedded in a fin made probably of glass fiber (which is a thermal insulator), its response time must be important. I am very surprised that it is only 2 min. For TidBit, the manufacturer (https://www.onsetcomp.com/products/data-loggers/utbi-001/) gives 5 min and not 9 min. Where comes the difference from? About the iButton, could you tell us which model it is and what are its specifications?

Formula (2): this relation is not often used. The relation of variance or its square root, the standard deviation, is preferred generally because it allows the evaluation of the deviations probability density function.

Line 198: one time again, what is the relation between the recirculating water bath and the reference [36]?

Line 199: same remark that for line 150, and this is still written line 150.

Line 213-216: “Considering the Smartfin had been used for over a year following the initial calibration (and surfed over 80 times), this close agreement with the Themometrics ES 225 temperature probe demonstrates the calibrated MAX31725 temperature circuit used on the Smartfin is stable and accurate.”

 I do not completely agree with this statement. This is probably a demonstration of the reliability of the sensor and its mounting in the Smartfin, but, I have already seen instruments drift and end up with very small deviations when they are calibrated. It is difficult to judge the stability of an instrument with a single check. It would have been interesting at least to know the offset of this sensor before or after its first calibration.

Line 223-224: I agree with this sentence.

Line 327-328: The big problem that remains is the calibration or verification of Smartfin sensors if they are to be used as a reference for validating satellite data. It is one thing to have a large amount of data available to sample a large area regularly. Guaranteeing the quality of these data is another. Your publication is an element to bring to the file, but it is not an irrefutable proof of the reliability of this method.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

This is a well-written manuscript that contributes to “(…)to the use of Smartfin as a citizen science tool for evaluating satellite SST data, and data collected using the SOD and ISAR were shown to be useful for quantifying near-surface temperature gradients (…)”

Besides the technical oceanographic issues, very well developed in the Introduction this a pertinent science subject in what respects citizen science. I question if a paragraph about citizen science in the marine environment could better situate potential readers, …, sice authors last sentence is “ Our results support the use of Smartfin as a citizen science tool for evaluating satellite SST data”

Author Response

Please see the attachment.

Author Response File: Author Response.pdf