Comparing Thermocouple Sensor Probes and RTD Sensor Probes for Temperature Applications

One of the questions often asked to TEGAM is, “Which temperature sensor should I use, an RTD probe or a Thermocouple probe?”

The answer used to be easy – a thermocouple sensor for industrial use and an RTD sensor for the laboratory. Today it has all changed.

Historically, an RTD element was large and fragile; platinum wire wound on a bobbin and encased in glass. Today, with the application of semiconductor manufacturing techniques, we have thick film RTD ceramic-based elements that are very small and almost rival the size of a thermocouple bead.  Because the platinum is thick film deposited on a ceramic base, they are very rugged and not susceptible to shock and vibration like the earlier wire wound devices.

RTD temperature probes can be manufactured in almost all the form factors of a thermocouple temperature probe and share the same ruggedness and durability.

To make the decision today, you need to go much deeper and look at the parameters of the two sensors and choose the one with the best set of attributes that fits your measurement needs.

The key parameters that can affect the decision are:

• Temperature Range
• Accuracy
• Accuracy at Zero
• Stability
• Form Factors
• Cost
• Other Factors

Temperature Range:
Over 1000°F (540°C)

For very high temperatures (without going to expensive platinum thermocouples) Type K and Type N thermocouples have the highest ranges, going up to 2,300°F (1260°C). Type E is also a good choice with a range up to 1600°F (870°C) and it has better accuracy than K or N types. For temperatures above Type K and N, the noble metal (platinum) Type R, S, and B thermocouples can be used as high as 3100°F (1700°C).

There are a few high-temperature RTD probes that will go up to ~1562°F (850°C) but the thermocouple probe is the most popular sensor in these higher temperature ranges.

Below 1000°F, either element is an excellent selection; therefore you need to look at their other characteristics.

Accuracy:

RTD sensors: There are four accuracy categories for RTDs set by IEC 60715
( t is the absolute value of the temperature in °C)

Class AA = ±(0.10 + 0.0017t) over the range of 0 to 150°C
Class A = ±(0.15 + 0.002t) over the range of -30 to 300°C
Class B = ±(0.30 + 0.005t) over the range of -50 to 500°C
Class C = ±(0.60 + 0.01t) over the range of -50 to 500°C

Selected RTD elements can be interchangeable to better than 0.1°C at 0°C.

THERMOCOUPLE SENSORS: Thermocouples have “Limits of Error” specified by ASTM E230-ANSI MC 96.1
There are “Standard Limits of Error” and “Special Limits of Error” (SLE).
The error is “the greater of.”

A quick review shows that RTD probe sensors are typically more accurate and have better zero point accuracy than thermocouple probe sensors.

STABILITY: Staying below 1000°F.

If the RTD element or the thermocouple junction are enclosed in a metal insulated cable, they are both protected against corrosion and generally will not drift at lower temperatures. Exposed elements of either type are subject to contamination or corrosion.

FORM FACTORS:

RTD probes are available in almost all the form factors as thermocouple probes.

COST:

RTD probes generally cost slightly more than thermocouple probes. Looking at one web site, a 12” TC probe is $43.33 while a similar RTD probe is $46.35 – that is about a 7% premium for an RTD probe.

OTHER FACTORS:

Thermocouples

1. Thermocouples are a voltage measurement. They can support long cable runs back to a control room where instrumentation is located.
2. Thermocouple wire must be used for the entire cable run, but less expensive “extension grade” thermocouple wire can be used between the probe and the instruments.
3. Thermocouples require Cold Junction Compensation (CJC). Without going into details, the instrument measures the temperature where the thermocouple is connected to it and uses that data to compute the temperature at the measuring end. For an installed system this does not affect anything. For a portable system, the user must allow time for the instrument to acclimate to the ambient temperature before making measurements. (A well-designed portable instrument can minimize this time.)

RTDs

1. RTDs are a resistance measurement and require a 3-wire or a 4-wire connection to compensate for lead resistance. The length of cable is also limited because it is a resistance measurement.
2. RTD’s use copper wire for connections.
3. SPRTs or Reference Standard RTD probes are also available where the exact RTD curve is programmed into the instrument. They can be used as extremely accurate standards in a metrology lab.

This information should give you a base of knowledge to make the selection between thermocouple sensor probes and RTD sensor probes for your project.

Take a look at our informative, complimentary Temperature Probe Selection Guide to see all the sensor probes that TEGAM has available for purchase. If you have further questions, we’d love to hear from you. Please contact us here and we’ll get back to you shortly.