There are several important advances
in this design:
1.Because the gas is measured in small
increments, then released, we can use
larger amounts of substrate in the reactor
than would be possible in a closed system.
2.The system is very flexible. Its sensi-
tivity can be adjusted by changing the
response range and/or trip point of the
pressure transducer, and by changing the
ballast volume.
3.Data are logged on a computer. Once the
experiment is started, no further human
intervention is required until the experi-
ment is finished.
A pressure sensor was used in a
different way by Theodorou et al.(1994).
The method used a three-way stopcock to
connect a syringe (for gas volume measure-
ment), a pressure sensor linked to a
numerical display and a closed incubation
bottle. The setup resembles that in Fig.
10.1 with the syringe connected to the
ballast port. At pre-determined time
intervals, the pressure was read and was
reset to atmospheric by withdrawing gas
into the syringe. This gas was then vented
and the cycle repeated.
A new phase of equipment design
began when less expensive pressure
sensors and commercial computer software
for data input became available in the
1990s. Pell and Schofield (1993) designed a
closed system in which 16 sample bottles
were each connected to a sensor and the
data sent to a computer via a 16-channel
analogue-to-digital (A/D) card. This system
was novel because it measured the pres-
sure increase in each bottle as a function of
time and calculated the corresponding gas
volumes from calibration curves. Bottle
and sample sizes were chosen such that the
maximum pressure increase was <0.6
atmospheres. Because the sample sizes
were small (≤100 mg), these investigators
introduced a micro-method for determin-
ing residual fibre. After digestion, neutral
detergent solution (Goering and Van Soest,
1970) was added to each bottle, the bottles
were sealed and autoclaved, and the
residue was filtered, washed and dried on
glass fibre filters (Pell and Schofield, 1993).
The most recent automated gas produc-
tion apparatus was described by Cone et al.
(1996). This apparatus is essentially a refine-
ment of the design in Fig. 10.1. The article
contains a detailed description of the
apparatus, sources for parts and a useful
discussion of experimental procedures. A
similar system was developed indepen-
dently in the UK (Theodorou et al., 1998).
Summarizing the current state of
affairs, there are three different approaches212 P. Schofield
Fig. 10.1.Schematic view of a vented gas measurement system (redrawn from Beaubien et al., 1988).