MATELECT DCM-2 INSTRUCTION MANUAL
8
specimens would be sufficiently high enough to effect the local electrochemistry and even if they were,
the reactions occurring may not be fully reversible in any case.
For now, the matter remains unresolved but for those researchers that prefer the concept of reversing-
DCPD, the DCM-2 can be specified at the time of purchase (or retrofitted) with a reversing module. This
works in harmony with the pulsed DCPD operation of the DCM-2 to produce the World’s first pulsed-
reversing-DCPD instrument. In this way, the thermoelectric, specimen heating and possible
electrochemical effects are all countered.
The DCM-2 acts as a stable current source but also must amplify the resultant voltages that the current
generates across a crack, or between two points on a specimen. Even with large currents, these voltages
are never usually more than micro-volts in magnitude, and demand final digit resolutions that are in the
nano-volt regime. For sensitive work, carefully designed voltage amplifiers are thus required. These
need to be highly stable and as immune to noise as possible.
Highly specified pre-amplifiers, of novel design, are built into the DCM-2 and these have the effect of
greatly reducing the electrical noise that superimposes on the processed signal. Noise can be generated
in a variety of ways –it can appear at the signal inputs as a result of the interaction with the test
environment, and it can be generated within the very circuitry that is used to measure the signal.
If unchecked, external noise will be amplified by an instrument and simply appear at the output. By
employing advanced analogue circuit design methods, such noise can be reduced early on in the
amplification process. This does not simply constitute application of a filter, as these cannot be employed
with any great success on signals that change rapidly (as a pulsed voltage would be expected to). Once
analogue noise reduction has been carried out, signal averaging methods are further used to reduce noise.
Signal averaging can be thought of as a form of digital filtering –and the DCM-2 offers end-users
considerable flexibility in setting the “filter” parameters for this process.
As standard, the DCM-2 houses its analogue pre-amplifiers within its chassis. Optionally these can be
specified as external units and supplied as mini-preamp modules in rugged, shielded metallic enclosures.
These can then be mounted close to the specimen and are particularly effective where long cable runs are
anticipated. The preamplifiers can also be specified to be used in environments where the main unit (and
its operator) would not normally wish to be located (e.g. radiation “hot” zones).
Temperature can markedly alter the characteristics of the voltage amplification circuitry. It is therefore
important to make the pre-amplifiers as temperature insensitive as possible. Some amplifier designs
incorporate heaters in order to maintain a constant temperature and hence reduce the drift, however, the
advent of modern electronic techniques have made this unnecessary. This results in a simpler and more
reliable measurement system. After pre-amplification, concerns about temperature effects become less
important, but the DCM-2 is still equipped with extensive design elements to counter thermal effects,
including thermally activated forced cooling and ultra low drift voltage references.
A further enhancement of the DCPD technique is to use two specimens. One of these is used as the test
specimen, whilst the other is used as a reference so that changes in the DCPD due to environmental
changes (e.g. temperature) are eliminated. The specimens are connected in series so that the same current
flows through both. Two voltage measurement channels are required in this case. This technique is
known as normalisation. The DCM-2 , like its predecessor the DCM-1, is supplied as standard with two
entirely separate signal processing and amplification channels, including two electrically independent
pre-amplifiers.
Two channel normalisation can still be performed on a single specimen by using specialist formulae
(available in published scientific literature). In such cases it is usual for the reference channel to be sited
in an area that is not subject to cracking, although formulae have been developed to handle two channels,
both of which straddle the crack site. Matelect can advise on some of these methods.
Historically, two channel operation has always been a feature of DCPD apparatus, although in the early
years such an implementation was encountered as a means of compensating for the fluctuations in the
current supply to the specimen, rather than any thermal compensation. Changes in the supply would thus
affect both active and reference channels equally and hence be normalised out. The disadvantage of this
was that single channel operation was not feasible. The current supply of the DCM-2 , however, is
designed to be inherently stable, and hence it is perfectly possible to use only one of its two channels and
