FAQ's - and answers from the Temperature Measurement Experts...
Q: What are the major differences between a thermocouple and an RTD?A: The most notable difference between a thermocouple and an RTD is the principle of operation and manufacturing. A thermocouple is made of two dissimilar metals joined so that a potential difference generated between the points of contact is a measure of the temperature. An RTD, operates on the principle that electrical resistance of certain metals changes in a predictable way depending on the rise or fall in temperature. The two measurement tools each have their own advantages and disadvantages.
Advantages of the thermocouple include a wide range from -300°F to 2300°F, fast response time (under a second in some cases), low initial cost and durability. Overall, thermocouples are able to withstand rugged applications. Disadvantages for thermocouples are their wide accuracy range, especially at elevated temperatures, difficult to recalibrate (seeing as though they are dependant upon the environment) and, finally, installation can be expensive if long lengths of thermocouple wire are needed.
On the flip side, advantages for RTDs include stable output over a long period of time, ease of recalibration, and accurate readings over narrow temperature spans. Disadvantages, when compared to the thermocouples, are: smaller overall temperature range ( -330°F to 930°F), higher initial cost and they are more fragile in rugged, industrial environments. To determine whether you have an RTD or a thermocouple, refer to the common wire color chart.
Q: What does RTD and PRT mean?A: An RTD is a resistance temperature detector. It may use platinum, nickel or copper for its element. A PRT (platinum resistance thermometer) is a type of RTD that uses platinum for its element.
Q: Is there a difference in accuracy between a .00385 and a .003902 probe?A: No, as long as the measuring device is consistent with the probe's specific alpha value. The only difference is the amount that the resistance changes per degree of temperature. For example, both probes will read 100 ohms at 0°C, but at 100°C, the .00385 probe will read 138.5 ohms and the .003902 probe will read 139.02 ohms.
Q: Regarding spring loaded RTDs for thermowell applications, what is the difference between immersion length, bore depth and probe length?A: First of all, let's discuss the components that are used in an RTD- thermowell assembly. Starting on the process side there is the thermowell, next is the extension which connects the thermowell to the connection head. These 3 parts connected together with an RTD is considered an assembly. The thermowell alone makes up 2 of the 3 dimensions in question. The immersion length, also referred to as the "U" dimension, is the length that is actually immersed in the process. The bore depth is exactly that, the depth of the bore hole inside the thermowell. This dimension is a combination of the immersion length (minus the 1/4 inch sealed tip), and the connection end with the addition of any lag. (See illustration) The actual RTD runs the entire length of the assembly. To calculate the probe length, you must add the bore depth, the length of your extension and add the portion of the probe consumed in the connection head. Typically this dimension is 1.5 inches with a spring loaded probe.
Q: What is the maximum distance that a PRT can be from a recorder or controller without using a transmitter?A: There is no definitive distance. Burns Engineering recommends no more than 250 feet of at least 18 AWG leadwire without a transmitter. Further information may be available from the manufacturer of the controller/recorder. When a 3-wire connection is made to the PRT, there is a maximum error of +.16°F per 100 feet of 18 AWG leadwire. This error is caused by the manufacturing tolerances of the leadwire. If the resistance of each of the three leads is exactly the same, there is no error.
Q: Is there a cost impact related to measurement accuracy?A: Yes, although we describe the impact as the cost of in-accuracy. When the measurement is not accurate, the process control will typically either over compensate or under compensate relative to the desired temperature of interest. In a heated process, if the measurement is reading low, (measured temp below the actual temp) the system will consume additional energy to drive the measured temperature to the required set point. The cost associated with this error is two fold. 1) The energy cost due to the system over compensating and 2) The risk of damage either to the system or the product being produced. Here is a technical paper we developed to demonstrate the real potential cost of an error of as little as 1 degree. This is the main reason that the Burns team focuses on the Measurement as well as the most appropriate sensor for the application.
Q: If I use a Burns transmitter, how far can I run the signal?A: There is no limit. The transmitter requires a minimum of 12 VDC at the terminals and this is the only limiting factor. A power supply will have to be capable of overcoming the leadwire resistance. Remember that a long leadwire can act as an antennae causing radio frequency and electromagnetic interference with the transmitter. Twisted shielded wire should be used for long runs or if the wires run next to other wires or electric motors.