c. For thermal converters which are used to measure small quantities of
current, smaller heater wires can be used.
Because the frequency at which skin
effect first becomes appreciable is higher for smaller wires, low current
thermocouples are generally unaffected up to approximately 30 megacycles.
Most
thermal converter meters are designed to measure large currents and the high
to handle the larger current, the type of heater wire rather that the size is the
factor involved in frequency compensation. Ribbon heaters are normally associated
with large currents, however, they have relatively large skin effects.
For this
reason solid wire or thin-walled tubing is used in the heater element because their
electrical behavior is instrumental in reducing skin effect.
d. At extremely high frequencies where length is an appreciable fraction of a
wavelength, the current developed along the heater will not be uniform.
The
indication given for the signal being measured becomes inaccurate. For this reason
the heater and its leads must be kept extremely short in respect to a wavelength if
high accuracy is to be expected in the measurement.
e. Thermocouples can be easily destroyed. There is no visible warning that a
thermocouple is overloaded. In fact, a thermocouple meter may not ever reach full
scale before the heater is burned out. When using this equipment, never exceed the
maximum voltage on a given range, even for an instant. Thermocouples that you will
encounter are rated at predetermined resistances.
Excessive current will cause a
solder effect to occur in which the junction rapidly deteriorates and often melts.
Always connect thermocouple instruments into the circuit last and ensure that the
attenuator is set to its highest range. Always down range and increase the input
slowly, allowing time for the instrument to react. Many thermocouples are matched
to the circuit in which they are installed at the factory. One simple mistake can
result in a large expenditure and an extended down time.
13
Previous Page
