Dry Mass or Dry Weight
Dry Mass is a more reliable indicator of biomass due to the fact that fresh measurements are dependant on fluctuating water concentrations in the biological material measured.
Dry Weight (dried material) is the plant, animal, or other material containing the chemical of interest is dried to remove all water from the material. The amount of the chemical found in subsequent analysis is then expressed as weight of chemical divided by weight of the dried material which once contained it.
Dry weight is the weight measured after all the water from the specimen has been removed. For the case of photosynthesis, this dry weight will be used as a measure for the amount of carbohydrate produced by photosynthesis. Dry weight is a much more accurate way of measuring than using normal weight as the amount of water present in the plant may fluctuate and give incorrect readings.