Applied Sensor Technologies, Division of United Electric Controls

Frequently Asked Questions

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 instrument is a handheld meter, then a grounded probe can almost always be used.

Q.    How many feet of T/C wire can I run?
A.    For a specific instrument, check its specifications to see if there are any limits to the input impedance. However as a rule of thumb, limit the resistance to 100 Ohms resistance maximum, and this depends on the gage of the wire; the larger the diameter, the less resistance/foot, the longer the run can be. However, if the environment is electrically noisy, then a transmitter may be required which transmits a 4-20 mA signal that can be run longer distances and is more resistant to noise.

Q.    Can I split my one T/C signals 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 to two devices may result in bad readings or loss of signal. The solution is to use a "dual" T/C probe, or convert one T/C output to a 4-20 mA signal by using a transmitter or signal conditioner; then the new signal can be sent to more than one instrument.

Q.    How can I choose between thermocouples, resistance temperature detectors (RTD's),
       Thermistors when measuring temperature?

A.    You have to consider the characteristics and costs of the various sensors as well as the available instrumentation. In addition: THERMOCOUPLES generally can measure temperatures over wide temperature ranges, inexpensively, and are very rugged, but they are not as 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 inexpensive as thermocouples. Since they require the use of 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. They are also subject to selfheating.

Q.    What is a Thermistor?
A.    Thermistors are thermally sensitive resistors and have, according to type, a negative (NTC), or positive (PTC) resistance/temperature coefficient.

Q.    What are the best ways of overcoming electrical noise problems?
A.   1) Use low noise, shielded leads, connectors and probes.
       2) Use instruments and connectors that suppress EMI and RF radiation.
       3) Consider using analog signal transmitters, especially current transmitters.
       4) Evaluate the possibility of using digitized signals.

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