calculating the area dimension in image

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is there a why to measure different areas of black object in image ? the goal is to detect if tube has a bobbin or not. here's an image example

i have tried to use binary image that contains a known number of blobs that vary in shape and size, it doesn't seems to get good results. here's my code

#!/usr/bin/env python3


import numpy as np
import cv2

# Read input image
img = cv2.imread('img22.jpg')

# Convert from BGR to HSV color space
hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)

# Get the saturation plane - all black/white/gray pixels are zero, and colored pixels are above zero.
s = hsv[:, :, 1]

# Apply threshold on s - use automatic threshold algorithm (use THRESH_OTSU).
ret, thresh = cv2.threshold(s, 0, 255, cv2.THRESH_BINARY+cv2.THRESH_OTSU)

# Find contours in thresh (find only the outer contour - only the rectangle).
contours = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)[-2]  # [-2] indexing takes return value before last (due to OpenCV compatibility issues).

# Mark rectangle with green line
cv2.drawContours(img, contours, -1, (0, 255, 0), 2)

# Assume there is only one contour, get the bounding rectangle of the contour.
x, y, w, h = cv2.boundingRect(contours[0])

# Invert polarity of the pixels inside the rectangle (on thresh image).
thresh[y:y+h, x:x+w] = 255 - thresh[y:y+h, x:x+w]

# Find contours in thresh (find the triangles).
contours = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)[-2]  # [-2] indexing takes return value before last (due to OpenCV compatibility issues).

blank = np.zeros(img.shape, dtype=np.uint8)

# Iterate triangle contours
for c in contours:
    area = cv2.contourArea(c)
    print("area ",area)
    hull = cv2.convexHull(c)
    cv2.drawContours(img, [hull], -1, (0, 0, 255), 1) 

        # Mark triangle with blue line
        #cv2.drawContours(img, [c], -1, (255, 0, 0), 2)
        #pass

# Show result (for testing).
cv2.imshow('img', img)
cv2.imshow('thresh', thresh)
cv2.waitKey(0)
cv2.destroyAllWindows()

any suggestions or guide lines please !? thanks in advance.