The German chemist Beckmann, known for the thermometer named after him, began
using the freezing point of milk in as early as 1895 to detect if it had been adulterated
with water. The American Hortvet worked intensively with this method in 1920 and im-
proved some of its essential features. The first thermistor cyroscopes were brought to
the market in the 1960s. However, they had to be operated entirely by hand. At the be-
ginning of the 1970s, the first automatic thermistor cyroscopes became available. With
this development it was possible to determine the freezing point automatically at the
push of a button.
A decisive step in the improvement of thermistor cryoscopy was displayed at the “Food-
Tec” tradeshow in 1984: Funke-Gerber introduced the first device with automatic calibra-
tion. This successful and intensive development work reached a new peak at the “Food-
Tec” in 1988, where Funke-Gerber presented a fully automatic freezing point determina-
tion mechanism with a capacity of 220 samples per hour. With the introduction of an
indirect freezing point measurement device (e.g. LactoStar) for routine analysis, interest
was focused primarily on reference devices which are able to determine freezing points
in accordance with the applicable standards and regulations. These devices must satisfy
the strictest requirements with regard to measuring accuracy. For this reason Funke-Gerber
developed a programmable cryoscope with a resolution of 0.1 m °C. This instrument has
proven its accuracy and reliability in countless laboratories all over the world. The product
range has been expanded with a multi-sample device (CryoStarautomatic). Since January
2007, these instruments have been equipped with a graphic colour display. This makes it
possible to show the entire freezing curve, in particular the process of the plateau search,
with a patented screen presentation.
The freezing point of pure water is the temperature at which ice and water are in equi-
librium. If soluble components are added to this liquid, the freezing temperature lowers
(becomes colder) because the ability of the water molecules to escape from the surface
diminishes. As fat is not water soluble, it has no influence on the freezing point.
The milk is cooled to -3°C (super-cooled) and crystallisation is induced by mechanical
vibration. As a result of this freezing process, the temperature increases due to the
released lattice energy and stabilises at a certain plateau which corresponds to the
freezing point.
The freezing point of liquids is not just any temperature, but the exact temperature at
which one part of the sample is in a solid state and another part is in a frozen state,
whereby the parts are in equilibrium. To measure the freezing point, the sample must
therefore be brought into this state. In order to do this, a certain procedure must be
followed, which is carried out in the following way:
First the sample must be cooled to under the actual freezing point while being stirred.
Stirring is necessary for 3 reasons:
- The sample is kept in motion so that it can not freeze on its own.
- The sample is thoroughly mixed so that all parts of the sample have the same tempe-
rature.
- The warmth contained in the sample is transported out where it can be dissipated by
the cooling mechanism.
When a liquid is colder than its actual freezing point, this state is instable. This state is
called “metastable”. Even the smallest influences, such as the impact of a hard object on
the glass wall, cause freezing to set in. This continues like an avalanche until the re-
leased fusion heat increases the temperature of the sample so much that the freezing
point is reached and the frozen parts of the sample are in equilibrium with the not
yet frozen parts of the sample.
A cryoscope must therefore trigger freezing when the sample is sufficiently colder than
its actual freezing point. But what is “sufficiently colder”? The aim here is that so much
ice builds up during freezing that there are normal-sized ice crystals all throughout the
sample but that the sample is not completely frozen. With milk, it has been proved
optimal to trigger the freezing at about -2°C to -3°C. After triggering freezing, the tem-
perature of the sample climbs because the fusion heat created during freezing is released.
It stabilises at a certain value, which is called the plateau. The cooling bath continues to
pull warmth out of the sample, and to the same degree that this happens, more parts in
the sample freeze and release their fusion heat. Therefore the temperature remains the
same – at least as long as there are still liquids parts in the sample. This plateau lasts
for a few minutes. The cryoscope determines the freezing point from the temperature
measurement values of the plateau. There are rules regulating this.
If the heat withdrawn from the sample is too little, the cooling takes too long. The rea-
son for this is either the cooling bath or the stirring rod. The cooling bath must be at
least 6°C and circulate well in order to beable to transfer the heat out of the sample.
The stirring rod must stir uniformly with an amplitude of 3.4 mm. When cooling errors
occur, the cooling bath temperature must therefore be measured with a thermometer,
then the cooling bath circulation is checked with an empty sample tube. Then, it is de-
termined whether the stirring rod can move freely and that it does not strike or grind
against anything. Finally, the stirring rod amplitude is tested. There is a special menu
in the device for this purpose. The valid reference value is not simply some number on
the display; this is only meant to be an indication. The tip of the oscillating stirring rob
is observed and adjusted so that the points of regression are only about 3-4 mm apart.
Then 2.5 ml of water is poured into a sample tube which is held under the thermistor so
that the stirring rod stirs the water. It is determined whether the stirring rod oscillates
well in the water.
When everything has been tested and adjusted, a sample measurement with water is car-
ried out and the temperature value in display is observed. The time that the device takes
to cool one sample from room temperature (20 °C to 25°C) to -2°C should be almost
exactly one minute. If this is the case, it means that the cooling bath and the stirring rob
are adjusted correctly. If cooling takes less than 45 seconds, then the cooling bath is too
cold or the stirring rod setting is too high. If cooling takes longer than 75 seconds, the
cooling bath is too warm or is circulating poorly or the stirring intensity is too low.
If an “error during cooling” occurs after the cooling bath and stirring rod have been tested
and determined to be functioning correctly, then the thermistor and the calibration of the
instrument must be tested. If the instrument has been incorrectly calibrated, it will not find
its temperature scale and therefore cannot measure the temperature correctly.
The state of the sample is instable when it is below its freezing point. It can therefore hap-
pen that the sample freezes due to an unintentional influence or on its own before the de-
vice triggers freezing. There are many possible reasons for this. If stirring is too strong or
if the stirring rod is grinding against something, jolting can occur and trigger freezing. The
longer cooling takes the more time the sample has to freeze on its own. Therefore the coo-
ling should be carried out as quickly as possible. If the sample is contaminated, freezing
may be triggered.
If the temperature set for supercooling (the “trigger temperature”) is reached, the device
beats against the glass wall of the sample tube to trigger freezing. The temperature should
then start to rise. A criterion for this is a rise in temperature of at least 0.1° C. This is al-
ways the case with water or calibration solutions if the stirring rod is set in such a way that
it beats hard against the glass wall. This is not always the case with milk. Some milks
freeze slowly. Should this error occur rarely with individual milk samples, the milk in ques-
tion should be heated to approx. 40°C, cooled and measured again. However, if this error
occurs often in a certain region, then it is better to lower the trigger temperature so that
the samples are supercooled more aggressively, causing them to freeze easier. If this er-
ror occurs with calibration solutions, then the calibration of the device is incorrect or coo-
ling bath liquid has leaked into the sample.
This error can only occur when the “Plateau Search Method“ in accordance IDF is used to
determine the freezing point. With this method, the temperature value must be within the
defined range for a certain time during the plateau. It can so happen that a certain milk
sample does not fulfil this criterion. Then a second sample of this milk must be measured.
If this error occurs frequently even though the device is otherwise functioning correctly,
the error is either with the thermistor or the result of external disturbances.
The instrument tests the current thermistor value when starting a measurement or cali-
bration. Its electrical resistance is known to be a function of the temperature. This elec-
trical resistance is translated with an ADC (analogue digital converter) into a number
which is then used by the instrument. If the thermistor has a short circuit or a disrup-
tion, its resistance is zero or infinite, both of which conditions are impossible for a proper-
ly functioning thermistor. In this case, the thermistor will not start themeasurement. If
the temperature which is given from the current thermistor value together with the cali-
bration constant stored in the device is lower than +1°C (which is not possible with a
thermistor which is located in a new, i.e. still warm sample), the device will also fail to
start the measurement.
Switching on: The device must show the starting message „CryoStar I (or. CryoStar
automatic), Funke Gerber“ on the display when it is switched on.
Possible errors:
- locking devices on the network connection block
- locking device on the main conductor board
- main rectifier. Verify that the voltage of the main condenser is at least 11 V.
- power transformer
- error with the main conductor board
- display or a cable leading to the display is defective
Cooling phase: the device should reach a cooling bath temperature of at least -6°C
in a reasonable amount of time. This time depends on the surrounding temperature,
but should not be longer than 20 minutes.
Possible errors:
- air supply is not functioning properly: ventilation slots on the sides of the device are clogged, the inside of the device is contaminated
- a ventilator has failed
- ventilator control system is defective. Verify that the voltage is approx. 24-26 V.
- cooling block has suffered heating damage and is now defective
- cooling block control system is defective. Verify that the Peltier connectors are approx. 6-10 V at full cooling capacity
- no or poor circulation: when an empty sample tube is immersed (with lid removed) into the measuring site and taken back out, the cooling bath liquid should flow back in within approx. 1 to 2 seconds
Possible errors:
- cooling bath liquid has become too thick. Change the liquid
- too little cooling liquid, therefore air in the lead: add liquid
- pump is blocked. Switch off device, open lid, carefully turn the pump motor rotor by hand: it should spin without resistance - If this is not the case (contaminants in the pump): rinse pump and lead
- pump control system is defective. Verify that the voltage on the pump motor connectors is approx. 24-26 V
- pump motor is defective: replace motor.
- axle between pump motor and pump is defective: remove pump motor, check axle
- device reports the signal “lift error“ when starting a measurement.
Possible errors are:
- final position switch on the lift is defective
- cable from measuring head to main conductor board is defective
- device indicates a much too cold value on the display immediately after staring a measurement, beats the sample tube and reports “not frozen”. This only occurs with old firmware versions. Causes:
- thermisor is defective. Change thermistor, install newer firmware version
- stirring rod cannot be properly adjusted
Possible causes:
- stirring rod has been bent during a thermistor change and is touching the thermistor shaft. Bend the stirring rod back into shape and adjust the thermistor so that the stirring rod can oscillate freely.
- upper part of the stirring rod has a fatigue fracture: replace stirring rod.
- stirring rod was assembled backwards. The magnet in the stirring rod must be orientated in such a way that it is pulled by the current-carrying reel and is not pushed away. Assemble the stirring rod in the correct position.
- device measures and can be calibrated, but measurement values are scattered.
Possible causes:
- thermistor is defective. Somewhere on the thermistor, microscopic cracks have formed which moisture can now seep through. This causes the electrical properties of the thermistor to become compromised, meaning that the thermistor must be replaced.
- impure specimen dishes.
- cooling bath liquid has reached the thermistor shaft. A measurement was started without a sample tube. This means that the thermistor was dipped into the cooling bath liquid and the remains of it stuck to the thermistor shaft and have slowly got into the sample.
Most errors that are made during operation of the device are incorrect calibrations. The
calibration of a cryoscope is a precondition for each and every use. For measuring rea-
sons it is necessary to use a thermistor to measure the temperature of a sample. Ther-
mistors have a very strong temperature effect which is necessary for resolutions of
more than 1 m°C. Unfortunately, the production-oriented fluctuation range of the resis-
tance values of these components is so large that the temperature zero point (0°C)
must usually be determined by a pre-calibration before the device can be calibrated
with a new thermistor.
It has to be assumed that the A calibration cannot be successfully executed after a ther-
mistor replacement. The reason for this is that the device first has to reach the set
“trigger tempera- ture” and then must recognize a rise in temperature after the glass
wall has been hit (as a sign that the freezing has started). This does not happen because
the values of the new ther- mistor result in false temperatures being given when calcu-
lated with the calibration constant of the old thermistor. Therefore a socalled pre-calibra-
tion is necessary, in which the device ignores the temperatures and follows a purely time
-controlled measuring procedure. The calibration constants are subsequently adapted to
the characteristics of the new thermistor so that both the A and the B calibration can be
successfully carried out.
Unfortunately, it often happens that during the calibration sample tubes filled with cali-
bration solution are taken for something else or that the incorrect menu item is selec-
ted.
Mix up: A calibration instead of B calibration:
The entire temperature scale of the device is displaced. When re-measuring the calibration solutions, reversed values and a reversed sign are given.
Example: A cal. with 0.000
A cal. with 0.000
B cal. with –0.557
A cal. with –0.557 (faulty operation)
Re-measuring solution B: results in 0.000
Re-measuring solution A: results in 0.557
Mix up: taking the A solution instead of the B solution
At first the A calibration goes as expected. However, when it comes to the B calibration, the devices reports the error “uncalibrated” or “thermistor defective” and remains uncalibrated.
This is a frequently occurring error. There are two possibilities:
1. The thermistor is (was) broken. This can be identified because display constantly
shows a negative value that doesn’t change.
2. The thermistor bonding is permeable. This is can be identified by extremely instable
measurement behaviour. The reproducibility is very poor, e.g. there are variations of
approx. ±0.1°C. In both cases the thermistor must be replaced.
The stirring rod does not oscillate freely:
It must be able to move freely in the slot provided. It cannot be allowed to touch the
thermistor at any place. This must be kept in mind when replacing the thermistor.
The stirring rod amplitude is not high enough:
The cooling of the sample is not carried out uniformly and takes much longer than 1
minute. With a correctly adjusted stirring rod, the cooling time is almost exactly 1
minute. The stirring rod amplitude must be approx. 3-4 mm. If necessary, the stirring
rob must be adjusted accordingly.
The stirring rod amplitude is too high:
Premature freezing of the sample occurs frequently.
Since the liquid relevant for the freezing point only exhibits 60 % sample volume with a cream
of approx. 40 %, it is recommended to increase the sample volume to 3 ml. In addition, the
trigger temperature should be set to -3°C, or -3.2°C if the sample repeatedly fails to freeze. It
is also possible to marginally increase the impact force of the stirring rod.
Suggested set points:
| Name | Settings |
|---|---|
| A calibration | 0,000°C or -0,408°C |
| B calibration | -0,557°C or -0,600°C |
| Base value | -0.520°C (EU boundary value) Serves solely for calculation of the infiltration water content percentage. |
| Trigger temperature | -2,00°C ( -3,00°C minimum) |
| Mode | Celsius |
| Plateau | Plateau search: 0.4 °C / 22 s |
| Fixed time: | 50 s |
| Maximum: | 0,2 m°C |
| Language | free choice |
| Stirring rod / amplitude | 3 - 4 mm |
| Stirring rod / frequency | Note: Do not change the set value! The values lie between 95 Hz and 104 Hz, depending on the device. Stirring rod/impact force. The impact force should be set to be so powerful that a relatively loud noise is heard when the trigger temperature (e.g. -2°C) is reached. However, it should be seen to that the impact force is not too strong, as this could lead to breakage of the sample tube. The set points lie between approx. 40 % and 50 %. |
If the set points are changed, the device must be re-calibrated.
Forward-looking and flexible: fixed-time measurement, plateau search and maximum
search features are available. All parameters relevant to these features can be program-
med freely, and, of course, recorded as well. This means that the device can be adjusted
to all national and international standards.
Easy-to-use: operation is menu-assisted in the language of your choice. Currently, German,
English, French, Greek, Italian, Polish, Portuguese, Spanish, Turkish and Hungarian are
available.
Efficient: a new cooling system provides for quick operational readiness even at high sur-
rounding temperatures (up to approx. 32°C).
Fast: up to 40 samples can be measured per hour, depending on the setting.
Multifunctional: the device has a parallel connection (for standard printers) and can be
hooked up to a PC with a serial interface. This makes it possible to map the freezing curve
on the screen during a measurement and, when necessary, to save it. An efficient zoom
function tops off the image. The software needed for this is included in the scope of delivery.
User-friendly: the operation of this device is uncomplicated. The percentage of infiltration
water is immediately indicated and printed out. The calibration is executed automatically. All
settings and calibrations are permanently saved to non-volatile storage.