Temperature &Heat
Temperature :Is the thermal state of a substance
that determines whether it will give heat to another
substance or receive.
Heat :Is a form of energy transferred from a hotter
subset. to a colder one
Temperature Scales
Temperature Measurement
A. Non_electrical Methods
1.
Liquid Expansion Thermometers
i. Mercury Thermometers
adv:it has linear scale& cheap
disadv.:
1. unsuitable for remote readings & for
measurement of very low temp.
2. Response time is long (2-3min.)
3. Rigid & made of glass which is liable to break.
Temp.Measurement(cont)
ii. Alcohol thermometers
Adv:
 cheaper than mercury
 Suitable for use at very low temp
Disadv:
 Unsuitable for remote reading or recording
results
 Non linear scale
 Unsuitable for high temp
2. Dial expansion Thermometers
Bimetallic strip thermometers
It is used for measuring air temp.
It depends on 2 dissimilar metals with diff.
coefficients of expansion fixed together as a coil.
iii.
temp
diff. in length of the 2 metals
moving a pointer on a scale.
Adv: cheap
disadv:not accurate

i.
ii.
 Bourdon gauge thermometers
i. Used for measuring large temp changes e.g.
autoclaves
ii. Small bulb cont. volatile fl. to a small coiled
tube attached to a pointer.
iii. Temp
bulb pr.
Straighten the
coiled tube
moving the pointer on a
scale.
Adv:Cheap
Disadv.:not accurate
Liquid crystal thermometer
•
•
•
temp.
organic.comp. from solid to
liq.passing by intermed. State with optically
active properties
Exposure of such material (intermed.state) to
light at certain temp.
iridescent colors &the
remaining of light transmitt. By the crystals
The colors form letters & numbers& black
background.
• It consists of a flexible adhesive –backed strip
•
•
•
with plastic-encased liq. Crystals on a black
background.
To use ,the covering over the adhesive is
removed &the strip placed on the skin.
Advs.;convenient,non-invasive,easy to
apply&read,unbreakable,disposable&inexpensive
.
Disadvs.;infrared lamps&sun exposure give high
readings &less accurate
Electrical methods
1. Resistance Thermometer:based on the
fact that :the electrical resistance of a metal
increases with the increase of temp.
Adv:- can be used for calibration of other
thermometers accurately
-remote &continuous reading
-linear scale
Disadv.;-long response time
-expensive& fragile
Thermistor
• The electrical resist. decreases with the
increase of temp.It is formed of a small
bead of metal oxide e.g.zinc oxide.
Adv. :-the bead is of very small size &
diff. shapes,fixed on a needle.
very accurate with remote & continuos
reading.cheap. than platinum resis.therm.
Disadv.the is not linear &resistance
changes with time.
Thermocouple
• Based on the seebeck effect,2 diff. metals are
•
•
•
joined to form a circle the potential diff. bet. the
2 junctions is proportional to the diff.in their
temps.
The 2 metals may be copper or iron &
constantan.
Advs.-accurate& short response time, remote&
cont.reading
Disadvs. Non-linear scale
Thermocouple
• Transistor thermometer
The voltage across transistor js.is temp.dependent &more sensit.than thermocouple
• Optical thermometer
objects emits electro magnetic radiations,the
wave lengths of these radiations are temp.
dependent.
Types; a-infrared ear thermometer.
B-tympanic membrane thermometer
Clinical Applications
• Humans are homeothermic i.e. control body
•
temp. within a narrow range despite a wide
range of environmental temp.
Body temp. :
a-Core temp. i.e.temp. of deeper tissues(2.5 cm
from the skin).It includes brain ,thoracic&
abdominal organs(37˚C).
b-Shell temp.:temp.of tissues within 2.5 cm.from
the skin(32-35˚C).
Sites Of Temp. Monitoring
• Core temp.
Tympanic memb.
Brain temp.
Nasopharynx.
Oesophagus
Cardiac temp.
Rectum(risk of perforation)
Pulmonary a. catheter
Urinary bladder
• Peripheral temp.:at the skin by liq.
Crystal adhesive strip
Indication of temp. monitoring
1. Detection of peripheral perfusion (coreperipheral temp.gradient is 5C )
2. Detection of unintentional hypothermia
3. Detection of elective hypothermia
4. Detection of hyperthermia
Heat Capacity &Latent Heat
• Units of Heat measurement
_The Calorie (cal ):quantity of heat req.to
raise temp.of 1g of water by 1C
_The Joule(J):SI unit of energy
1Joule= 0.24 cal
_Specific heat capacity :amount of heat
req.to raise temp.of a given subst.by 1C
_Heat Capacity :amount of heat req.to
increase temp.of a given subst.by 1C.
Clinical Applications
1. Sp.h.of water=1 K cal/Kg/C=4.2Kj/Kg/˚C
-sp.h.of water is very high in comp.to
other subst.
2. Sp.h.of tissues ~ 3.5Kj/Kg/˚C
the total heat human body of 70 Kg is
70 ×3.5=245 Kj/˚C
3. Sp.h.of bl.~3.6Kj/Kg/˚C
4. Sp.h.of gases;its unit is J/L/˚C
Latent Heat
• Def.:amount of heat req.to change the
state of subst.from one phase to another
at a const.temp.
• Sp. Latent h.:amount of heat req.to
convert 1Kg.of subst.from one phase to
another at given temp.(j/Kg)
Types Of Latent Heat
1. L.h.of melting:amount of h.req.to change a
2.
3.
4.
solid subst.to its liq.phase
L.h.of crystallization:amount of heat given out
to change a liq.subst.to its solid phase without
change of temp.
L.h.of condensation:amount of heat given out
to change subst.from vapour to liquid.
L.h.of vaporization:amount of heat req.to
changa a subst.from liq.to vap.without change
of temp.
Clinical Applications
1. During vap.of volatile anesth.:L.h obt.from the
2.
3.
4.
vol.liq.& wall of vaporizer so;temp.decreases&
vaporizat.
During use of N2O cylinder :
vap.increases&latent h. obt.from wall of
cylinder&reducing v.,further decrease in
temp.&frost formation on valve
Use of ethyl chloride as local anesth.:
when sprayed it vap.lead to cooling effect
Humidification of insp.air:water vap.obt.from
resp.mucosa lead to heat loss
Heat Production
• The basal metabolic rate:It is the minimal
amount of heat produced by fasting
individ.at physical& mental rest at room
temp.(20-25˚C).it is 36.5Kcal/m2/hr.in
female&39.5Kcal/m2/hr in male
• Ignition of food:
1 g fat
9.5 Kcal. 1g protein
5.5Kcal. 1g.carbohyd.
4Kcal.
Heat Loss in Human Body
• There are 4 routes of heat loss :
•
•
convection(30%),radiation(30%),evaporation(30
%)&respiration(10%)
In convection,the layer adjacent to the surface is
warmed by conduction&moved up creat.
Convection current carries heat away.It needs
moving medium.
In radiation,hot object emits radiation carrying
heat energy away from the object. - it can
transf.heat energy bet.2objects not in contact.
Heat Loss (cont.)
• Evaporation: due to loss of L.H.of vap.of
moisture on the skin surface.it depends on
the water vap.pr.gr.from skin to air.
• Respiration:responsible for 10% of heat
loss;8% by evap.&2% by convection.
Preventive measures of hypothermia
1.
2.
3.
4.
5.
6.
ambient temp.& humidity
Enclose exposed viscera
Warm mattress
Low flow anesthesia
Humidification of inspired gases
Warming of IV &irrigating fluids to 37˚C