Temperature Detectors

Question 1

State the basic theory of operation for a Bimetallic strip thermometer (thermostat).

Answer 1

Utilizes differing coefficients of thermal expansion for different metals. Two trips of metal are joined, and will always be at the same temperature. When heated the strip will bend as one metal expands more than the other.

Question 2

State the basic theory of operation for a thermocouple.

Answer 2

Range from from sub-zero to over 3,000°F. . The wires are joined at one end, the sensing junction. When the temperature of the sensing junction increases, a voltage or electromotive force (EMF) is developed at the reference junction. The magnitude of the voltage is a direct function of the difference in temperature between the sensing and reference junctions.

Question 3

What temperature range can Bimetallic strip thermometers cover?

Answer 3

-200°F to 1,000°F.

Question 4

State the basic theory of operation for a thermocouple.

Answer 4

The wires are joined at one end, the sensing junction. When the temperature of the sensing junction increases, a voltage or electromotive force (EMF) is developed at the reference junction. The magnitude of the voltage is a direct function of the difference in temperature between the sensing and reference junctions. The reference junction must remain constant

Question 5

What temperature range can a thermocouple cover?

Answer 5

sub-zero to over 3,000°F.

Question 6

If the reference junction temperature of a thermocouple is colder than the sensing junction and the refernce junction temperature increases, what happens to indicated temperature?

Answer 6

Indicated temperature will decrease

Question 7

If the reference junction temperature of a thermocouple is colder than the sensing junction and the reference junction temperature increases, what happens to indicated temperature?

Answer 7

Indicated temperature will decrease

Question 8

What is an advantage of a thermocouple?

Answer 8

It has a fast response to changes in temperature due to the thinness of the sensing wires.

Question 9

What are disadvantages of a thermocouple?

Answer 9

1. It requires compensation for a nonlinear voltage signal. That is, its signal must be adjusted so that equal changes in temperature will cause the indicator to move the samenumber of units.2. It is prone to erroneous readings due to changes in reference junction temperature.3. It is considered less accurate than a RTD.

Question 10

State the basic theory of operation for a Resistance Temperature Detector.

Answer 10

Operates on the principle that resistance to current flow in a metal will change with temperature. This change in resistance is proportional to temperature and can be measured with simple electronic circuits.

Question 11

What metal serves as the standard in RTDs, a reference for checking the calibration of other temperature indicating devices in the plant?

Answer 11

Platinum. It has a linear and stable resistance to temperature relationship.

Question 12

What are two methods of several available for the measurement of the RTD resistance and conversion to temperature?

Answer 12

Wheatstone bridge: Uses a variable resistor to balance against to known resistances and the unknown RTD resistance. Temperature is determined from reading the variable resistor setting when amps are zero. Another method uses three fixed resistors and one variable resistor (RTD). Changes in temperature of the RTD unbalance the bridge, causing current to flow through the ammeter. The ammeter is calibrated in a temperature scale, so bridge balancing is unnecessary and readout is continuous.

Question 13

What happens to indication if the bimetallic strips separate in a bimetallic thermometer?

Answer 13

Indication fails low

Question 14

What happens to indication if a filled fluid system thermometer breaks? i.e. capillary tube, bulb, etc.

Answer 14

Indication fails low

Question 15

What happens to indication if a filled fluid system thermometer breaks? i.e. capillary tube, bulb, etc.

Answer 15

Indication fails low

Question 16

What happens to indication in a thermocouple that has develop an open circuit?

Answer 16

Indication fails low. If an open develops, a path for current flow is no longer available, and therefore the output fails to a low temperature indication.

Question 17

What happens to indication in a thermocouple that has develop a short circuit?

Answer 17

Indication fails low. A short circuit causes no voltage to be developed, and the thermocouple indicator fails low.

Question 18

What happens to indication if a RTD fails open?

Answer 18

Maximum temperature is indicated. The Wheatstone bridge sees a large resistance, which is comparable to a high temperature.

Question 19

What happens to indication if a RTD develops ashort circuit?

Answer 19

Indicates a minimum temperature as the bridge network sees a low resistance.

Question 20

What are the advantages of an RTD over a thermocouple?

Answer 20

• RTDs are better suited for small temperature bands.• RTDs have a higher output voltage which means less auxiliary equipment is required to boost output signals.• RTDs do not require reference junctions.• RTDs’ circuitry is more tolerant to electrical noise.• RTDs have an increased sensitivity to small changes in temperature.• RTDs are more accurate than thermocouples.

Question 21

What are the advantages of a thermocouple over and RTD?

Answer 21

• Thermocouples are more rugged than RTDs.• Thermocouples are well suited for large temperature bands.• Thermocouple sensing wires can be drawn very thin, giving a very fast response to temperature changes.