9. F.A.Q.’S
72
1. Q. How many metres of T/C wire can I run?
A. For a specifc instrument, check its specifcations to see if there are any limits to the input impedance.
However as a rule of thumb, limit the resistance to 100 Ohms maximum, depending on the con-
ductor diameter of the wire: the larger the diameter, the less resistance/metre, the longer the run
can be. If the environment is electrically noisy, then a transmitter may be required which transmit a
4-20 mA signal that can be run a longer distance and is more resistant to noise.
2. Q. Should I use a grounded or ungrounded probe?
A. It depends on the instrumentation. If there is any chance that there may be a reference to ground
(common in controllers with non-isolated inputs), then an ungrounded probe is required. If the instru-
ment is a handheld indicator, then a grounded probe can almost always be used.
3. Q. Can I split my one thermocouple signal to two separate instruments?
A. No. The T/C signal is a very low-level millivolt signal, and should only be connected to one device.
Splitting into two devices may result in bad readings or loss of signal. The solution is to use a
dual T/C or convert one T/C (output a 4-20mA signal) by using a transmitter or signal conditioner
which can send the new signal to more than one instrument.
4. Q. What are the accuracies and the temperature ranges of the various thermocouples?
A. They are summarized in the tables of the International Standards (page 18).
It is important to know
that both accuracy and ranges depend on a.o.: the thermocouple alloys, the temperature being
measured, the construction of the sensor, the material of the sheath, the media being measured,
the state of the media (liquid, solid or gas) and the diameter of either the thermocouple wire (if it is
exposed) or the sheath diameter (if the thermocouple wire is not exposed but sheathed).
5. Q. How can I choose between thermocouples, resistance temperature detectors (RTD’s) and
thermistors?
A. You have to consider the characteristics and costs of the various sensors as well as the available
instrumentation. Thermocouples generally can measure temperatures over wide temperature ran-
ges, are inexpensively and very rugged. They are not accurate or stable as RTD’s and thermistors.
RTD’s are stable and have a fairly wide temperature range but are not as rugged and inexpensively
as thermocouples. Since they require the use of an electric current to make measurements, RTD’s
are subject to inaccuracies from self-heating. Thermistors tend to be more accurate than RTD’s or
thermocouples, but they have a much more limited temperature range.
F.A.Q.’S