FAQ's - and answers from the Temperature Measurement Experts...
Start by finding the interchangeability tolerance for the RTD you have. For Burns catalog RTDs there will be a code in the part number string of either “10” or “05”. Using the following equations you can calculate the tolerance: “05” code: Tolerance ± °C = 0.13 + 0.00185 |t| “10” code: Tolerance ± °C = 0.26 + 0.0037 |t| Where |t| is the absolute value of the temperature of interest in °C (drop minus sign for negative temperatures) For example to find the tolerance at -80°C for a “05” code RTD: ± °C = 0.13 + 0.00185(80) = 0.278°C The interchangeability number represents about 85% to 99% of the total accuracy depending on how and where the RTD is used. To determine any additional error sources refer to the performance specifications.
The surface mount RTD or thermocouple is low cost and easy to install. It will provide a good estimation of the fluid temperature if it is insulated to shield it from the ambient conditions. Error is proportional to the difference in ambient temperature and process temperature. With a high differential, it can be a few degrees. If you need the best accuracy, an immersion-style is the best choice. Use either a direct immersion probe or a thermowell with a separate probe. An immersion length of 4” or greater will typically give the most accurate measurement.
The 3 and 4-wire circuits are used by the signal conditioning device to compensate for lead wire resistance. The RTD can only provide an accurate measurement if the lead resistance is eliminated from the circuit. To accomplish this, a 3-wire circuit has two of the wires (typically red) connected to one side of the platinum sensing element and a third wire (white) connected to the other side. The signal conditioning device will measure the resistance in the two red wires and subtract that from the resistance measured between one of the red wires and the white wire. The accuracy of this method is dependent on each of the three wires having exactly the same resistance. Unfortunately, all three wires never have exactly the same resistance so there is a measurement error with this method. A 4-wire circuit takes the lead wire compensation one step further by using a current-potential method to fully compensate for lead resistance regardless of any differences in the individual lead resistances. This is the method to use for best measurement accuracy.
You will need to install the device descriptor for the T55 into your communicator. It is available at the HART Foundation website or can be downloaded from the PR Electronics website. You will need the PR Electronics part number from the cross-reference list below:
There are several uses for the extension. For certain spring-loaded sensors that can be removed through the connection head, it connects the connection head to the thermowell, such as the Burns ‘C’ or ‘K’ style. If a sensor is often removed for calibration/verification, a union/nipple extension can make it easier to remove the sensor, such as for the Burns ‘L’ Style. The extension is often used to get past insulation where the sensor is being installed and also serves as a spacer from the process temperature that may damage head-mounted electronics such as an indicator or transmitter. The most common reason to not use an extension is if there are space constraints in the area where the probe is being installed into the thermowell.
The calibration range should encompass the full range of use of the sensor. A common calibration that secondary standards will come with is from -196 to 420°C. However, if while using the sensor the temperature never goes below 0 °C and never goes above 300°C, the probe can instead be calibrated over the shorter range. One benefit of this is that there is no unnecessary stress put on the sensor by calibrating it at temperatures that are close to the ends of its usable range.
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.
Yes. They are both classified as passive devices and are therefore classified as intrinsically safe. Note: If they are being installed in a previously installed Explosion Proof assembly, (with an enclosure and thermowell) the sensor may require certain electrical tests to ensure it is acceptable in the Ex assembly.
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.
Yes, but they are very expensive.
Two lower-cost alternatives are to place a Tantalum cover /sheath on a straight stem flanged thermowell made of 304 SS or some other low-cost alloy, and the other is to apply Tantalum over the well with a vapor deposition process.
This can be done to all types of thermowells and gives them a high degree of corrosion resistance.
There is no definitive distance. Burns Engineering recommends no more than 250 feet of at least 18 AWG lead wire 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 lead wire. This error is caused by the manufacturing tolerances of the lead wire. If the resistance of each of the three leads is exactly the same, there is no error.
Contact Burns Engineering customer service. We store all certificates that were originally ordered with the part and can send them to you electronically. You will need to know the original order number or the serial number of the sensor to look up the original certification.