Lesson 8BME 301 - Bioinstrumentation

By the end of this lesson students will be able to:• Explain how a thermocouple and thermistor work.• List advantages and disadvantages of a thermocouple and

thermistor.• Calculate output voltage and the Seebeck coefficient of a

thermocouple.• Calculate the output and NTC of a thermistor.• Calculate the radiant power emitted by an object.• Given the emission spectra of three of radiation source, filter,

sensor, and output, find the fourth.

I. Temperature sensorsA. Single measurement is a useful clinical indicator

1. shock, infectionB. Need to monitor (many measurements) for some

procedures1. newborn baby incubator2. during surgery3. ablation

C. Clinical problem: how do you get a good estimate of the core temperature? Which is best?1. Mouth2. Under arm3. Ear4. Rectal

D. We focus on three approaches1. thermocouples2. thermistors3. radiation methods

II. ThermocouplesA. Thermocouples make use of the Seebeck Effect. If you

make a loop out of wires made from two different metals and heat one junction you get a voltage difference and current flow. This is the Seebeck Effect.

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B. Every metal has a Seebeck coefficient, . Basically, this is a measure of how fast the electrons in the metal can move when heated. Some examples (with respect to Pb):

C. If we only had one metal, we wouldn’t get any current flow, because there is no net potential voltage difference:

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D. Use metals with different and now we can get current flow because there is a net potential voltage difference (because the metals change voltages at different rates when heated).

E. The cold junction of the thermocouple must always kept at a known temperature, otherwise you don’t know how much the temperature increase is (because you have no reference). The cold junction used to be an ice bucket, but now devices use a microchip called an “electronic cold junction” to simulate the ice bucket.

F. Here is an example of a thermocouple and digital thermometer

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G. And of just a Type K thermocouple. Thermocouples come in a variety of types which means different metal combinations. Type K = nickel-chromium/nickel-alumel with an overall Seebeck Coefficient of = ~40 µV/ºC.

H. We get the voltage, E, produced by a thermocouple empirically, that is by measuring it in a lab and fitting a curve to it:

I. Take the derivative to get the Seebeck coefficient for the thermocouple (not for the metal!)

J. Typical values are 6.5–80 µV/ºC with accuracies from 0.25% to 1%.

K. Connect many thermocouples in series to make a thermopile. In a thermopile, the voltage differences add, so if you have 10 thermocouples attached in series, you will get 10x larger voltage changes. Microfabrication lets you create dozens of thermopiles in series.

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L. Can detect insanely small temperature changes this way (on the order of 100 µK)! For example, the thermopile shown above can be used to measure the changes in temperature when DNA polymerase incorporates a dCTP nucleotide into a template. (From Nestorova, G. and E. Guilbeau. “Thermoelectric method for sequencing DNA,” Lab Chip, 11 (2011):1761–1769.)

M. Thermocouple advantages1. Fast response (1ms)2. Small (can microfabricate ~12 µm)3. Easy to make4. Stable5. Very high or low temps!

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N. Thermocouple disadvantages1. Small output voltage, low sensitivity, need for ref temp

O. Student problem. For the simplified thermocouple shown in (D) above what change in µV would you expect to see for a 10 ºC rise in temperature?

III. ThermistorsA. The electrical resistance of all conductors changes with

temperatureB. True for copper and even resistors used in circuitsC. So, another way to make a temperature sensor is to

measure change in resistance as temp changesD. This change is especially large for semiconductors (10–100

x greater than metals)E. Semiconductors change change resistance two ways:

1. NTC - negative temp coefficient - resistance decreases (most common)

2. PTC - positive temp coefficient - resistance increasesF. Example of semiconductor thermistor:

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G. Here is a representative graph of the relationship between thermistor resistances and temperature.

H. Note two things:1. You can get thermistors with different base resistances.2. The change in resistance is not linear!

I. There is an empirical relationship between thermistor resistance and absolute temperature (NTC only):

J. Get the temperature coefficient (NTC) by differentiating:

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material constanttemperature (in K)resistance at reference temp. can assume 298K

β =T =R0 = T0T0 =

K. It is possible to self-heat the thermistor, which reduces the linear behavior.1. In other words, if you heat the thermistor too much,

you can’t use V=IR to figure out the resistance (and temp).

L. Advantages1. Small2. Large sensitivity to temp (3–5%/degC)3. Good stability

M. Disadvantages1. Time delay (ms - several minutes)

N. Student problem. A NTC thermistor is characterized by K and K. How much of a temperature

increase is required to cause the resistance to decrease by half?

IV. Consider a standard oral digital thermometer. How does it work?

β = 4000 To = 293

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A. Block diagram

B. A thermistor shows a first-order response, so it can take several minutes to get to the actual temp.

C. The patient wants a reading ASAP, so what are some ways to make the thermometer quicker?1. Thermometer gets around this problem by adding 1

degree C to the output once the rate of change < 0.1 C/s

2. If you know the time response of the system, then you could predict what the final temp will be once you have enough data points

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V. Radiation ThermometryA. You’ve seen these lately

B. But among all body locations they are routinely the least accurate (~3 ºC or more off). Don’t trust them.

C. The most accurate, especially when measuring the temperature of children:

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D. Both pictures show thermometers that use “radiation thermometry”. Why is one more accurate than the other?

E. How it works1. Overview of system (from Webster, J.G., ed.

Bioinstrumentation, Wiley, 2004).

2. Every object that has a temperature > 0 K radiates electromagnetic power. For objects at or near room temperature, this usually means “heat” or infra-red (IR) electromagnetic radiation. You can estimate the amount of radiation from the Stefan-Boltzmann law:

where

A is the surface area of the radiator (m2) is the Stefan-Boltzmann constant (

W/m2 K4) is the emissivity of the radiator (~ 1)

is the object temperature (K) is the ambient temperature (K)

3. A type of sensor called a pyroelectric sensor generates an amount of current proportional to the amount of heat radiation that it measures (i.e., to the power). Think of it as a piezoelectric crystal for heat.

4. The pyroelectric sensor cannot measure a constant temperature, only changes (like a piezoelectric crystal),

P = Aσϵ (T 4obj − T 4

amb)

σ = 5.67 × 10−8

ϵTobjTamb

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so a shutter is used to briefly expose the sensor to heat coming from the ear.

5. An ambient sensor is also included so that the thermometer can correct for the air temperature around the patient.

F. These thermometers have a response time on order of 0.1 sec and accuracy of ~0.1 ºC.

G. Contact-based methods are much slower because the probe must equilibrate with the skin

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