This experiment was aimed to compare air with fermentative gas for calibrating an in vitro gas production system using digital pressure gauge. 100 ml serum bottles containing 45 ml of in vitro medium were injected with air and fermentative gas produced from incubation of starch (S gas), starch and cellulose (S + C gas) and cellulose (C gas). The pressures in the bottles were read after equilibrating the vials overnight in a 39 °C and for each gas, calibration line was constructed by linear regression of gauge reading (in psi) versus volume fraction (Fv) of added gas, where Fv = milliliters added gas/milliliters bottle headspace volume. For all four gases (air, S gas, S + C gas and C gas), there was a linear relationship between volume and pressure (r2 = 0.99). There was no significant difference between slopes of calibration curve obtained from air, S gas, S + C gas and C gas. Results of present work indicates that calibration of in vitro gas production system using air can give reliable regression equation applicable for measurement of total gas volume of wide ranges of substrates.

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