FAQ's - and answers from the Temperature Measurement Experts...

Q: How can I determine the RTD accuracy at my specific usage temperature?

A:

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.

Q: What’s the best way to measure fluid temperature in a 1/2” tube?

A:

There are several solutions to this and one of the best is to install a Model SWE thermowell at a location where there is a right angle turn in the tubing.  Standard spring loaded probes of either RTD or thermocouple can be mated to the thermowell.  For more detail and other solutions you can view our paper on the topic.

Q: What is the best method of measuring fluid temperature inside a large diameter pipe - surface mount or immersion?

A:

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 separate probe.  An immersion length of 4” or greater will typically give the most accurate measurement.

Q: Why are there 3 and 4-wire connections for RTDs? Which is best?

A:

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.  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. 

Q: My HART communicator will not communicate with your T55 transmitter. How do I fix it?

A:

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:  

Burns               PR

T55                  5335

T57                  5337

T65                  6335

T67                  6337

T75                  5337

Q: How do I know if the thermowell I specified will work with my process conditions?

A:

Burns Engineering can provide wake frequency calculations that will indicate a safe immersion length and thermowell configuration for a given set of process conditions.  When the calculations are ordered with the thermowell, we will provide a certificate stating that the thermowell will pass the process conditions.  If it is found that the thermowell will not pass with the given process conditions, Burns Engineering will make a recommendation on what changes can be made to the thermowell for it to pass.  Specify option WE05 in the assembly part number to include calculations.

Q: What is the purpose of the extension in thermowell/RTD assemblies? Can I just not use one?

A:

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. 

Q: Over what temperature range should I get my secondary standard recalibrated at?

A:

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.

Q: How do I identify what type of temperature probe I have?

A:

Begin by removing the sensor from the thermowell or process.  Look for a 6 digit serial number on the probe sheath or wrench hex.  It may be electro-etched or engraved by hand and may be difficult to read. Next note the number and color of the lead wires. If there are two wires it is most likely a thermocouple.  The color codes will indicate the thermocouple type. Red is negative on all the standard types and the positive leads are: blue = type T, white = type J, yellow = type K, and purple = type E.

If there are 3, 4, or 6 wires it is probably an RTD.  Common lead wire colors are two reds and one white for one sensing element, and two greens and one black for the second element if present.  Measure the overall length and diameter of the sensor and note if there is a process connection, exposed spring, or plain sheath.  Knowing only a couple of these identifying characteristics we can help unravel your temperature sensor mystery.

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: 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: 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: 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 dependent 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: Are RTDs and thermocouples intrinsically safe?

A:

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.

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: Can thermowells be made from Tantalum?

A:

Tantalum Cover on SST Flanged WellYes, 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.

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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: 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.

Q: I lost my original calibration/material/conformance certificates and have an audit. What now?

A:

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 original certification. 

Questions? Contact us anytime at info@burnsengineering.com or submit a contact form.