310201d RTD's

Question 1

What does the acronym RTD refer to?

Answer 1

Resistance temperature detector

Question 2

Name another term that describes Ro reference temperature or nominal resistance at 0°C

Answer 2

Ice point resistance

Question 3

List 4 materials used to construct RTD elements and include their alpha values

Answer 3

1. Platinum; laboratory grade alpha of 0.003925ohms, and industrial grade alpha of 0.00385ohms
2. Nickel with an alpha of 0.00672ohms
3. Balco with an alpha of 0.00518ohms to 0.00527ohms
4. Copper with an alpha of 0.00472ohms

Question 4

What is the temperature range for a copper RTD and why is it not possible to go above the upper temperature limit?

Answer 4

-200°C to 150°C
You do not use copper for temperature measurement above a 150°C because copper tends to oxidize rapidly above this temperature

Question 5

What must be done in the construction of a wire wound element RTD to prevent temperature measurement error?

Answer 5

The coefficient of thermal expansion of the winding core material must match the sensing wire to minimize any mechanical strain

Question 6

Define stability in relation to an RTD

Answer 6

Stability refers to the temperature coefficient and the nominal resistance, not changing over time.

Question 7

What is the major source of errors when a 2 wire circuit is used with an RTD

Answer 7

Lead wire resistance

Question 8

What does the term “self heating” refer to?

Answer 8

To measure resistance, a small current passes through the RTD. This current causes the RTD temperature to rise, which introduces error

Question 9

Which RTD measurement circuit eliminates errors from lead wire resistance?

Answer 9

4 wire current source circuit

Question 10

Name the feature of smart transmitters that reduces or minimizes errors due to self heating

Answer 10

Some smart RTD temperature transmitters can pulse the excitation current for a short duration through the RTD

Question 11

List 4 styles of common thermistors

Answer 11

1. Bead type
2. Disc type
3. Rod type
4. Washer type

Question 12

How does lead wire resistance affect thermistors?

Answer 12

With a sensor resistance of about 10000ohms, you can neglect lead wire resistance

Question 13

Explain the temperature versus resistance relationship as it relates to an NTC (negative temperature coefficient) thermistor

Answer 13

The resistance decreases as temperature increases

Question 14

When you use a thermistor to sense temperature, what is the major advantage of its small size?

Answer 14

They respond very quickly to a change in temperature

Question 15

What is the most common type of temperature measurement circuit used for thermistor sensors?

Answer 15

A DC Wheatstone bridge circuit

Question 16

Define the terms “sensor trim” and “output trim” as they apply to smart transmitters

Answer 16

Industrial transmitters use them to provide cold Junction compensation or housing temperature. This is necessary when using thermocouples for temperature measurement