Measuring the size of bubbles from a picture of material particles - Python-OPenCV

Question

I am trying to measure the size and count of bubbles available in image taken from real world material particles.

I have tried the approach present Here (second one with opencv). But some how it doesnt work probably because of the bubbles, because image is kind of an average quality.

Can some one please direct me to correct direction or provide recommendation?

The reference image, i am trying process is

Result: It doesn't detects the circles, in fact at finding contours step, it gets a zero therefore the test is not passed.

Made some changes. Able to detect some bubbles but still not good. Detected bubble image is below

Thanks

Code:

import cv2

image = cv2.imread('....')

# Gray, blur, adaptive threshold
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
blur = cv2.GaussianBlur(gray, (11,5), 0)
thresh = cv2.threshold(blur, 0, 255, cv2.THRESH_BINARY + cv2.THRESH_OTSU)[1]

# Morphological transformations
kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (3,3))
opening = cv2.morphologyEx(thresh, cv2.MORPH_OPEN, kernel)
# Find contours
cnts = cv2.findContours(opening, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
cnts = cnts[0] if len(cnts) == 2 else cnts[1]
for c in cnts:
    # Find perimeter of contour
    perimeter = cv2.arcLength(c, True)
    # Perform contour approximation
    approx = cv2.approxPolyDP(c, 0.04 * perimeter, True)
    if len(approx) > 6:

            # Obtain bounding rectangle to get measurements
            x,y,w,h = cv2.boundingRect(c)

            # Find measurements
            diameter = w
            radius = w/2

            # Find centroid
            M = cv2.moments(c)
            cX = int(M["m10"] / M["m00"])
            cY = int(M["m01"] / M["m00"])

            # Draw the contour and center of the shape on the image
            cv2.rectangle(image,(x,y),(x+w,y+h),(0,255,0),4)
            cv2.drawContours(image,[c], 0, (36,255,12), 4)
            cv2.circle(image, (cX, cY), 15, (320, 159, 22), -1)

            # Draw line and diameter information
            cv2.line(image, (x, y + int(h/2)), (x + w, y + int(h/2)), (156, 188, 24), 3)
            cv2.putText(image, "Diameter: {}".format(diameter), (cX - 50, cY - 50), cv2.FONT_HERSHEY_SIMPLEX, 3, (156, 188, 24), 3)
cv2.imwrite('...', image)
cv2.imwrite('...', thresh)
cv2.imwrite('...', opening)