8-1 MEASUREMENT OF TEMPERATURE, RH AND WEIGHT LOSS L.S. Herbert by lindayy

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MEASUREMENT OF TEMPERATURE, RH AMD WEIGHT LOSS .
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L.S. Herbert & D.A. Lovett
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CSIRO Meat Research Laboratory.
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TEMPERATURE
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Really accurate temperature measurements, to better than 0.5 C, can
be made only under well controlled conditions, for example, using a ...,
thermometer of proven accuracy in a large bulk of well stirred liquid
at a temperature close to that of the surroundings. Plenty of time, and a
careful observer are also required. Probably the only time such conditions
occur in the meat works situation is when they are deliberately contrived
for calibration of thermometers used around the plant.
Thermometers, whether they be simple mercury in glass or modern.wide-
range electronic instruments, are capable of giving false readings. If
the error is large it is obvious and appropriate action will be taken. If,
. howeverit is only a degree or two, it may go unnoticed and incorrect
information will continue to be obtained. All thermometers should be checked
at regular intervals - ideally once a month, but certainly at least once
a year. For most purposes a single calibration point will suffice. For
chiller work, thermometers can be checked by immersing them in an ice
distilled water slush in a Dewar flask - provided ice and water are both
present and the slush is stirred regularly, the thermometer should read
0°C after a minute or so, and remain at 0 C at the end of 15 mins.
Thermometers used for hot water and in canneries etc are more appropriately
calibrated at around 100°C by immersion in the steam space above boiling
distilled water. The bulb or sensing portion of the thermometer must be
shielded- from radiation losses by wrapping aluminium foil around the flask
in which the boiling Is taking place. A barometric pressure boiling point
correction must be applied. Alternatively, a works thermometer can be.
checked against a 'standard' thermometer at a number of points in its
operating ranges The latter is a specially selected liquid-in-glass
thermometer which has been calibrated at National Measurement Laboratory
in Sydney;. It can be purchased as such, or a suitable thermometer can be
sent to NML, who charge $10 for the calibration.
Even with accurate thermometers, several factors contribute to
difficulties in obtaining true temperatures in applications outside the
laboratory. Positional errors are probably the most common. For example,
when a deep butt temperature' is to be measured at the end of a chilling
cycle, the meat temperature is not uniform; What is generally required
the temperature of the warmest meat so that the sensor must be positioned
a small zone furthest removed from the cool surface layers. Accurate
positioning is more difficult because the leg is two and a half metres
above the floor. It requires determination, experience and practise to
place the sensing tip of a thermometer at the correct point. Assuming
the thermometer is correctly positioned, possibilities of error from
other sources arise. In the 'Teltru' thermometer (a commonly used
bimetallic strip instrument), the active portion of the sensing element
is about 5 cm long, so that it will almost certainly pass through the
warmest meat zone, and sense the cooler meat on either side, thus giving
A further error is introduced by conduction of
heat from the warmest part of the thermometer, along the stem, to the
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a falsely low reading.
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cooler meat nearer the surface, again resulting in a falsely low.reading.
Thermometers are often deliberately constructed with thick stems for
strength, and these will suffer most from stem conduction. In another
example, that of measuring the centre temperature of a carton of frozen
meat, a hole drilled in;the frozen slab enables the thermometer to
be accurately located at the thermal centre (which is some 1 to 2 cm above
the geometric centre of the slab). Temperature?gradients in the slab are
large, from minus 2 Q at the thermal centre to iinus 20 C or lower at the
surface. It is recommended that the hole'for the thermometer is drilled
along one of the longer axes of the slab, preferably from the centre of a
short end to the centre. Even then, -errors caused by length of the active
portion of:the sensor and stem conduction.mayJhe?high especially if part ,
of the stem is clear of the block, and exposed to warm ambient conditions.
A further problem is the -difficulty of makikg good thermal contact ,
•between'the thermometer and the side of the hole, drilled in the ..block, .
During our work' in tilast freezers, we have noted that centre temperatures
measured by Teltru type thermometers have been substantially lower than
the true centre temperature of the block. In chilling experiments, it
-would not be difficult to measure deep-butt temperatures up to 5 C ;
lower than warmest .meat temperature. In both cases, even greater..errors
could occur if insufficient time-were allowed for temperature stabilisation.
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• It is possible consistently.,to obtain, t'emperature readings within
0.5 C of the correct values, using good Pleasuring techniques.. . For our .
chilling & freezing'wdfk, we used 26 gatige;copper constantan thermocouples,
with active tips of approximately spherical shape 1 to 2 mm dia. In chiller
tests we inserted thermocouples into as many as 50 carcasses at the start of
the chilling cycle, using a special slotted spear (fig. 1) to place the tip
in the desired position. The' outputs of all thermocouples were measured
continuously throughout the chilling cycle, and the data were plotted as a
smooth curve of temperature against time. In another investigation, we wanted
to measure temperature^ of deep meat and air during container transport of
Vacuum packed primal cuts from an abattoir-in Queensland to the buyer, in
England. The measurement system had to be completely self-contained, and .
capable of functioning for the expected 8 week transit period. Two 8 point
Grant recorders (sold locally by Selbys Scientific Ltd.) were ..used with the
thermistor sensors located in cartons and in air spaces around the load.
Eight measuring points became inactive after. 10 hours runningj and ...
continuous records were obtained -for only 7 points, two of which appeared
to he of rather doubtful accuracy. It is optimistic-'to expect any type
of recording equipment to work continuously under practical conditions
without regular attention.	..
1 . For freezer tests on cartons of; meat-pieces, we made up a special.
locating device for thermocouples. An insert of corrugated fibrelboard
was cut to the dimensions of the length and depth of a carton, and the
thermocouple tip was secured in the.correct thermal centre position of this -
hoard. The insert- was then covered in plastic film (Gladwrap) as shown ...
in fig. 2. In the carton packing area,' the insert was- placed. Ve^icaily
along the long-centre-line of' a carton full of meat , and the meat pieces ,.
carefully repacked on each side before closing the carton and sending.it
to the blast freezer. We have measured centre temperatures in more than
50 test cartons during a 72 hp freezing period, enabling accurate .	.
temperature histories'to he'obtained.; Not .only does this method of.fixing
the thermocouple give accurate location of the tip, unaffected by movement,
of the,meat?.during freezing, but it also greatly simplifies the removal of
the thermocouple from the frozen block of meat - the two halves break	....
cleanly, away from the'insert. ; ■	;	;	'
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WEIGHT LOSS
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Weight 'is lost, "by evaporation of water from small stock or "beef
: carcasses during chilling, at a high rate -during the first h or 5 hours
after-leaving-the slaughter floor. Evaporation proceeds at a much reduced
rate during chilled:storage and during freezing and frozen- storage of
carcasses. 'A typical weight loss from a beef side would be 0.5$ "from wash
booth at end of the slaughter floor-to chiller entry 1.7$. from. start to
end of a 2h h chilling cycle, and a" further 0.5$ during chilled storage
for a.weekend,'prior to entering the boning room. Different abattoirs
have different lengths of corridors between slaughter floor & chiller entry,
different chiller-'characteristics & chilling cycles, and therefore weight
losses can be: expected to vary from abattoir to abattoir. It is clearly of
interest tooksow how^ much weight is lost, and during what stage of processing
or storage>rfifj:MeasUrement of weight loss can be carried out with simple
equipment, but it is by no means simple to achieve satisfactory accuracy.
The problem is that weight losses are only 2 or 3$, that is 2 to 3 kg on
a 100 kg beef side. To obtain reasonable accuracy in comparing weight
losses, measurements must be made to around 0.2$, that is 0.2kg on the
beef side. In our chiller investigations, we used a Salter Model 236
suspended weigher of 200 kg capacity calibrated in % kg intervals for
beef side and another Salter weigher of 30 kg capacity, calibrated in
50 g intervals for small stock. All weighings were made on these scales,
for example, beef sides entering the chiller were weighed into the
.chiller, and weighed out 2k hr later, using the same, carefully calibrated
weigher. Our technique of transferring the beef sides from the works
trolley to the hook of the weigher and then back to the- trolley become
-so well developed that we required less than 1'minute for a weight :
measurement. The weigher was hung alongside the rail, and a blbck and
tackle positioned nearby. The test side was lifted off the rail,' complete
with trolley, using a long hook through the Aitch Bone hole. 'The trolley
was removed from the Achilles tendon, and the hook of the weigher inserted
through the Achilles tendon. The side was lowered and the Aitch bone,
hook disconnected, so that the' weight- could b'e measured - we found'it1
possible to estimate to the nearest 0.25 kg. The procedure was then '
reversed and the side, duly labelled, was put on its trolley and back
onto the rail. The handling of small stock onto and off the weigher was
"simpler, because it could be1 accomplished manually. It is, However, a
job for tall men!
, The weighers were calibrated in the position in which they were to be
used, allowing at least 30 minutes for stabilisation under local ambient
conditions before calibration.- - They were calibrated before each'Series of
. ;weighings, with emphasis;-on-the range of weights of the carcasses' 'under test.
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Our results showed fairly -wide variations in chilling weight loss,
generally around +^0.5$ but sometimes greater. Some of the variation'is due
to weighing inaccuracy, but-position of carcass in the	'^c'over,
and other carcass factors alfeo- contribute. It is-	tcf rta£4plh'e' •
average of weight losses from as many carcasses as possible (for example',
10$ of the number in a chiller) if a reliable figure is required.	;
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AIR VELOCITY
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Air velocity over carcasses in chillers is a major factor affecting
chilling rate.and weight loss. . Air velocity can vary greatly in different
, places, in a chiller, for example,..in a. sheep chiller, we noted k m/s close
to the FDCs and less than 0.2. m/s at the other end of the room. , In a beef
chiller, at the stare of a chilling...cycle, steam could he seen .drifting
slowing upwards from sides under a centrally mounted FDC unit, while
velocities of 3 m/s downward were measured at the walls
of this nature can often reveal gross variations in air velocity around
. a chiller. Less-obvious variations in air-velocity can only be determined
by on air velocity meter or anemometer. Vane anemometers and hot-wire
and thermo-anemometers can be used, and are described in the paper '
'Measurement Techniques® by Lovett and Garseldine, presented to the
School for Meat Industry Engineers in 1973. It should be emphasised
that one reading of velocity at one position in a chiller is of little
' value. A velocity survey taken at various.locations, and at various	,
heights (above rails, below sides and at the narrowest gap between
sides), gives more useful information, which can be useful for example
in redirecting .air circulation to eliminate dead-spots.	. '
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Observations
RELATIVE HUMIDITY
An excellent description of various instruments for measuring Relative
Humidity (RH) is given in the paper by Lovett & Carseldine referred to in
the last section. Instruments based, on the wet, and dry bulb principle
are available commercially (Assman, and sling type psychrometers) and are
most commonly used in the meat industry. Given good maintenance and
intelligent use, they are capable, of giving results to _+ 1% RH at around
" 20 C but it is difficult to obtain accurate measurements of RH-at •
temperatures near 0 C (a difference of i°C between wet & dry .
bulb at 1 C dry bulb corresponds,to a 11% difference in RH). They are not
suitable for remote reading or control purposes. Hair hygrometers have a
long response time and a negative coefficient of expansion and are therefore
not satisfactory for measurement of fluctuating RH, and air temperature.
An Instrument, developed at the CSIRO. Meat Research Laboratory in 1973,
has a sensor which relies on changes of electrical conductance of a thin
calcium sulphate layer with changes of RH, Speed of response and accuracy
at low temperatures are good and the electrical output can be read remotely
or recorced in analog or digital-form. Prototypes have performed very
. well during our chiller investigations, but the instrument has not been
developed commercially. An instrument also working on change of electrical
conductance with RH, has been recently developed by Nova Sina AG, a Swiss
company, and is available from Selby's Scientific Ltd. in Australia. It
has a measuring range of 20 to 90% RH with an accuracy of _+ 2.5$ at •
temperatures 0 .to 60. C, and is self-contained and very simple to use.
However, its measuring range and accuracy make it of doubtful value for
work in chillers.. In our.opinion, there is still no commercially available
instrument which will measure RH values in the range 80 to 100% at	^
temperatures of 0 to 5 C with reasonable accuracy.
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