Documentation for SeaSoar-mounted OPC data collected from AESOPS cruises

by Mark Huntley and Meng Zhou

Documentation prepared by Yiwu Zhu and US JGOFS DMO

An OPC (Optical Plankton Counter) detects and sizes objects using a light blocking scheme. An OPC returns a digital size value when a target passes through its light beam. The digital size is further converted into an equivalent spherical diameter (ESD) by OPS-DAS software.

In the Polar Front survey cruises, an OPC was mounted on a SeaSoar and undulated along the ship track. The OPC data are integrated into every 10m depth bin and in 40 classes.

The valid maximum and minimum ESD values of our OPC are 14 mm and 0.29 mm, respectively.

US JGOFS DMO note regarding OPC ESD class number

The OPC valid data range was divided into 40 classes; the intervals of all the classes are equal on a log10 basis. For ESD based classes, the increment of any class is dESD = (log10(14) - log10(0.29)) / 40 or 0.042.

ESD class k covers the range from (log10(0.29) + (k-1)*dESD) to (log10(0.29) + (k)*dESD). The middle value of each class is taken to represent the class and is called the class value. As an example, the class value of ESD based class 2 is calculated as (log10(0.29) + (1 + 0.5)*dESD). 

ESD	ESD
Class	Class Value	
1	-0.517
2	-0.475
3	-0.433
4	-0.391
5	-0.349
6	-0.307
7	-0.265
8	-0.223
9	-0.181
10	-0.139
11	-0.097
12	-0.055
13	-0.013
14	0.029
15	0.071
16	0.113
17	0.155
18	0.197
19	0.239
20	0.281
21	0.323
22	0.365
23	0.407
24	0.449
25	0.491
26	0.533
27	0.575
28	0.617
29	0.659
30	0.701
31	0.743
32	0.785
33	0.827
34	0.869
35	0.911
36	0.953
37	0.995
38	1.037
39	1.079
40	1.121