Sensors and Measurement systems at Bremen

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What effects do you know for temperature sensing?

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Effect Measurand Response
  • Thermoelectric
  • Thermoresistive T  
  • Pyroelectric 

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Explain the thermoelectric effect
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Exemplary flashcards for Sensors and Measurement systems at the Bremen on StudySmarter:

Explain the Thermoresistive Effect

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How does a thermopile work? (Thermosäule)

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Write down Nyquist’s equation for thermal noise.
Explain the terms

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Bolzmankonstante = Konstant
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What can we learn for noise reduction from this formula?


U_N = sqrt( 4 k_B R T ∆f )

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What material do we use for the thermopile membrane? Why?

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What do we use as thermocouple material in the thermopile? Why?

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What is the difference between accuracy and resolution?

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Name three distortions of the sensor output caused by slow response.

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Exemplary flashcards for Sensors and Measurement systems at the Bremen on StudySmarter:

Sensors and Measurement systems

What effects do you know for temperature sensing?
- Thermoelectric Effect (Seebeck Effect)
- Thermoresistive Effect
- Pyroelectric Effect

Sensors and Measurement systems

Effect Measurand Response
  • Thermoelectric
  • Thermoresistive T  
  • Pyroelectric 
Thermoelectric: ∆T Temperature difference U Voltage 

Thermoresistive: T Absolute temperature R Resistivity

Pyroelectric: dT/dt Change of temperature with time

Sensors and Measurement systems

Explain the thermoelectric effect
Unter Thermoelektrizität versteht man die gegenseitige Beeinflussung von
Temperatur und Elektrizität und ihre Umsetzung ineinander.
Bei einem Stromkreis aus 2 verschiedenen Leitern entsteht bei einer
Temperaturdifferenz zwischen zwei Kontaktstellen eine Elektrische
Spannung.
U = k_AB(T1 − T2)

Sensors and Measurement systems

Explain the Thermoresistive Effect
Der Wiederstand eines Leiters ist Themperaturabhängig
R = R0(1 + α∆T + β∆T^2 + . . .)

Sensors and Measurement systems

How does a thermopile work? (Thermosäule)
It uses the thermoelectric effect!

Eine Thermosäule oder Thermokette ist ein elektrisches Bauelement, das thermische Energie in elektrische Energie wandelt. Es besteht aus mehreren Thermoelementen, die thermisch parallel und elektrisch in
Reihe geschaltet sind, wodurch die sehr geringen Thermospannungen addiert werden.

Sensors and Measurement systems

Write down Nyquist’s equation for thermal noise.
Explain the terms
U_N = sqrt( 4 k_B R T ∆f )

Sensors and Measurement systems

Bolzmankonstante = Konstant
- Der Wiederstand R erhöht die Rauschspannung

- Die Temperatur T erhöht die Rauschspannung

- Die Bandbreite erhöht die Rauschspannung

Sensors and Measurement systems

What can we learn for noise reduction from this formula?


U_N = sqrt( 4 k_B R T ∆f )

- möglichst kleiner Widerstand

- Sensor kühlen
- keine unnötig hohe Bandbreite


T: The noise increases with temperature, since the movement of the electrons increases with temperature. Thus, the noise can be reduced by cooling the sensor. This is done in laboratory equipment. When thermopiles are used for IR spectroscopy or astronomy, they are cooled with liquid nitrogen to 77 K.


R: Due to Ohms law, the noise increases with the resistivity of the sensor. Unfortunately, the thermopile has a high internal resistivity due to the interconnects in thin-film technology and due to the many contacts between the two metals. A typical value is 50 kΩ. Therefore, we would like to make sensors with less resistance to reduce Nyquist noise. We could reduce the number of thermopairs in a thermopile, let us say, in half (1/2). The noise would reduce to 1/√ 2 = 71%, but the sensitivity would reduce to 50%. Obviously, it does not pay out.


∆f: The noise increases with the bandwidth of measurement. To reduce noise, the bandwidth can be reduced using a filter (like the “treble” filter in an audio amplifier). There is a trade-off between noise and response time. Small bandwidth reduces noise, but increases response time

Sensors and Measurement systems

What material do we use for the thermopile membrane? Why?

membrane from very thin (300 nm) silicon nitride
Nichtleitend sonst Kurzschluss, Standartprozess, Geringe Termische leitfähigkeit, hohe intrinsische
Zugspannung, Chemisch stabiel => widerstandsfähig gegen chemisches ätzen

Sensors and Measurement systems

What do we use as thermocouple material in the thermopile? Why?

polysilicon versus tungsten–titanium oder Aluminium


- polysilicon- Hoher Thermoelektrischer Koeffizient, Standartprozess (keine Verunreinigung)


- Aluminium Niedriger Thermoelektrischer Koeffizient


- tungsten–titanium - Nidriger Thermoelektrischer Koeffizient (+Hohe temperaturen)

Sensors and Measurement systems

What is the difference between accuracy and resolution?

Accuracy is the maximum difference between the sensor output and the physically true value, which is guaranteed by the supplier under all specified circumstances (e.g., working temperature: -40 ◦C to +120 ◦C).

The resolution can be measured. The accuracy is specified by the supplier.


The resolution is the smallest signal that can be distinguished from a neighboring value. Limits of resolution may be:

– Noise
– Number of bits in A/D conversion
– Number of decimal places on the display

Sensors and Measurement systems

Name three distortions of the sensor output caused by slow response.

1. Phase shift: The electric signal will be a little later than the measured physical effect.


2. Reduced sensitivity: The electric signal will not reach the full amplitude.


3. Signal distortion: A rectangular signal will be distorted in the form of a “shark fin”.

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