Microsoft LPF-00004 Manuel de l'opérateur télécharger pdf (Page 68)

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380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

Self Heating

When an excitation current or voltage is applied across a temperature measurement sensor, power is

dissipated as heat, causing the temperature of the sensor to rise above the ambient temperature to be

measured. This is referred to as "self heating". For most of the temperature sensors used in

physiology, it is necessary to keep the power dissipation to less than a few milliwatts. If this

condition is met, temperature changes of less than 0.01°C can be measured.

Other transducers, such as strain gauges, are temperature sensitive, and it is also important to prevent

self-heating.

Isolation

In industrial environments, signal leads from transducers may run past machinery that can induce

large voltages. To protect the measurement instrument, isolation amplifiers are used for each

transducer. Induced voltages are not common in animal physiology and isolation is not required. If

human subjects are used, specially designed, isolated probes must be employed.

Insulation Techniques

Electrodes that are implanted for long-term recording eventually fail from rupture of a solder joint,

breakage of flexed wires, or moisture penetrating the insulation. Although Teflon insulated wire is

commonly used, the Teflon must be etched to produce good adhesion when joined to the electrode.

PVC insulated wires are often preferred because the plastic bonds readily, even though it is more

permeable to water over time.

Araldyte AV138M and Epoxylite #6001 both provide excellent water resistant insulation for rigid

applications.

Strain relief should be provided for wires and connections that are repeatedly flexed. A length of

silicon tubing over the stressed region often suffices.

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Microsoft LPF-00004 Manuel de l'opérateur Page 68