Well, you can do by two methods. Either before putting in batter you check the bulk density for your dough flour (moist condition) and again if you want by dry basis you can freeze dry and check it. However, the difference will be significant.

Or if you can immediately ground it and can measure. however I wander about its stickiness.

Good luck.

you may fill cooked idlis in a large enough container, take the dimensions of the container and the weight of cooked idlis. Calculate the volume and find the bulk density. since idlis are almost hemispherical, porosity will be more. Good Luck

For foamed batter density, just fill a cup of even volume and level off the top with a knife before weighing. Check the cup volume by filling to the top with water at 20 C and weighing, subtracting the weight of the empty cup. Weigh to at least 1 decimal place.

To measure unaerated batter density, centrifuge idli batters in a low speed swing-out centrifuge in paired tubes, marking the containers on the side with a line showing the height of foamed batter after filling, and weighing the tube+batter before spinning.

After spinning for 30 min between 3000-5000 g draw a line carefully on the side of the tube for the level of the batter after foam is broken. Tip out the liquid and measure its weight. Draw another line for the level of the pellet. Weigh the tube + pellet. Remove pellet, wash and dry the tube, weighing the tube. Using water at 20 C, fill the tube to each line carefully, noting the weight at each line. They represent the volume of the pellet, the liquid fraction, and the gas volume in idli batter. Replicate tubes give you replicate measures.

From the weights and the volumes at each step you can calculate the gas volume fraction, liquid volume fraction and density , and solids volume and density in the batter.

I have measured idli cooked volume by seed displacement method. Method reference Griswold, R. (1962). Evaluating food by objective methods Experimental study of foods (pp. 540-541). Boston, U.S.: Houghton MiZin Co.

Weigh idlis individually on a sensitive balance (at least 1 decimal place).

Take a large container with an even lip at the top, enough to fill around 0.4 kg seed. Weight the empty container. Fill with water at 20 C to determine container volume.

Dry container, then overfill with small seeds (poppy or mustard). Tap gently to settle, level off the top evenly with a ruler and weigh. Repeat several time to get a reproducible baseline to calculate the relationship between seed weight and seed volume.

Take out a third of the seeds, place in an idli, and refill with seeds to the top, tap and level. The idli must be completely covered. Then carefully remove idli, brushing all seeds back into container, and reweigh container. Repeat to get an idea of idli volume reproducibility.

The idli density can be calculated from idli weight divided by the seed volume displaced by the idli.

You can weigh more than one idli each time, but for scientific publication ans tatistical analysis, each should be weighed separately, with replication, to determine between-idli variability. My microwave idli have about 6-8% standard deviation in volume for a 12 idli batch. Steamed idlis may be less variable.

A simple, non-physical way to estimate volume:

Cut a batch of idlis in half vertically and place on a scanner bed. Cover with a box to omit outside light, adding a photographic intensity strip on the scanner bed to enable later image normalisation. Scan at 300 dpi or more. Save as tiff, not jpeg.

Use an outlining digital image analysis program to measure the area of each idli. Assuming a regular shape, area is proportional to volume. You also have a visual record and can perhaps compare the crumb characteristics later.

The most low tech option is to xerox the idli cut surfaces at 200%, then cut out and weigh each image to estimate area (need an accurate balance).