7/21/2019 Centrifuges Training Guide http://slidepdf.com/reader/full/centrifuges-training-guide 1/19 Home Equipment E le ct ro nic s O rg anis at io n Co nt ac t Anaesthesia Autoclaves Automated Analyzer Centrifuges » Equipment » Background » User Manuals » Service Manuals Defibrillators Dental Units ECG Electrosurgery Endoscopy Fluid Warmer Hospital Beds Incinerators Infant Incubators Infusion Pumps Laboratory Equipment Light Medical Gas Supply Microscopes Microtomes Operating Tables Ophthalmology Oxygen Concentrators Patient Warmer Photometer & Analyzer Physiotherapy Pulse Oximeter Sphygmomanometer Spirometry Suction Pumps Surgical Instruments Thermometer Ultrasonographs Ventilators Weighing Scales X-Ray Laboratory Centrifuges by Frank Weithöner A centrifuge is a motor-driven equipment which is used in hospital laboratories. A centrifuge prepares body liquids from a patient for further examinations. For diagnosing diseases, the examination of the components of body liquids like blood and urine are important. Each component has a specific use in the body and therefore a different test is required. But before particles of body fluids can be examined they first have to be extracted from the supporting fluid. This is what the centrifuge does. The liquid samples are spun around at high speed, which causes large centrifugation forces and consequently the substances separate due to their different densities. The separated components can then be examined. Types and applications Laboratory centrifuges differ in size, sample capacity, speed and application. First we distinguish between preparative and analytical centrifuges. Most centrifuges are preparative centrifuges. Their task is to separate the solids of a biological sample from the supporting fluid for further examinations. Analytical centrifuges do both, extracting the components and doing an analysis at the same time. The analysis result is already given after centrifugation. In hospital laboratories we mostly find small tabletop centrifuges. These centrifuges are called general purpose centrifuges, micro centrifuges or preparative centrifuges. They are used for separating cells, parasites and bacteria from body fluids like blood and urine. The micro centrifuge
A centrifuge is a motor driven equipment which is used in hospital laboratories. A centrifuge prepares body liquids from a patient for further examinations.
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The relative centrifugal force or the speed can also be determined with the help of a
nomogram (download).
Some manufacturers also offer an online calculator. This is the easiest way to calculate the
speed or the force. link
Using the centrifuge
Not only the laboratory personnel need to know the correct usage of the centrifuge but also
the hospital technician. During the repair and later while making a test run after a repair the
technician has to know all the functions of the centrifuge in order to test them.
Important: When you are not familiar with the equipment and not sure about the
correct usage read the user manual first.
Use only accessories and tubes which are designed for your type of centrifuge.
Before using the centrifuge, make sure the rotor is tightened.
Check the tubes for cracks before inserting them into the rotor.
Samples should be closed with a cap before centrifugation.
The rotor has to be balanced at all times. Therefore, the rotor has to be filled with tubes
symmetrically. A tube with exactly the same weight/volume has to be put into the
opposing hole of the rotor. If there is an odd number of samples, prepare one additional
sample with water.
Do not open the lid while the rotor is still spinning (for all simple centrifuges without a
safety lock).
Switch off the centrifuge immediately if it makes abnormal sounds or starts shaking or
wobbling.
After a sample spillage clean the rotor and the buckets immediately with a mild multi-
purpose cleaner. See ↓Cleaning.
User manual
User manuals are always delivered with the centrifuge. I strongly recommend copying or
scanning the manual before leaving it with the user. The copy should be kept together with
other technical manuals in the hospital workshop.
If the manual is missing, you are probably able to download it from the manufacturer's
website. If this is not possible, try it here
Cleaning during usage
Cleaning of the centrifuge is in the first place the task of the user. The laboratory assistant
has to ensure that the centrifuge is clean and operational at all times.
But cleaning a centrifuge is also one part of the maintenance and repair procedure. Unlikeother hospital equipment the cleaning of a dirty centrifuge should be done before starting
with the service. The dirt could come from infectious samples and we should avoid getting in
contact with it. Therefore it is recommended to wear rubber gloves when removing a filthy
rotor and when washing the parts.
Cleaning of the inside of the centrifuge, the buckets and the adapters should be done by the
user frequently or at least on a monthly basis, and of course immediately after a sample
spillage. It is the task of the hospital technicians to introduce the laboratory personnel to the
correct cleaning procedure.
Remove all tube adapters from the buckets.
Wash buckets and adaptors in warm soap water. Mild multi-purpose cleaner or
The output signal of a Hall-sensor is similar to the signal of a photoelectric sensor. Butinstead of a slotted disk a magnet is mounted on the shaft. A Hall-effect sensor responds to
the magnetic field of this magnet. The output again is a square wave signal with a frequency
depending on the speed.
It looks like a small transistor, but it is not.
This is the hall sensor. The shaft with the
magnet is removed.
Speed control
The speed control process happens in the control unit. The control unit does two things. First,
it converts the square wave signal from the sensor and compares this with the set speed
from the control panel and adjusts the voltage for the motor until the actual speed reaches
the set value.
The speed signal is also needed for activating the safety lock. As long as the sensor delivers
a signal the control unit knows that the centrifuge is spinning and it keeps the lid locked.
Brake
When the centrifugation time is over the rotor slows down and comes to a complete stop
after a few seconds. This is only possible if the motor is decelerated actively by a brake. The
brake is nothing else but a resistor that bypasses the rotating motor. The rotating motor the
acts as a generator and produces energy which is absorbed by the resistor. The 'generator'
then runs under heavy load and slows down. The electrical energy is transformed in the
resistor into heat. Because the generated power is quite big, the resistor also has to be big.
Indeed it is the biggest resistor inside the centrifuge and it is easy to find. Sometimes it is
mounted with some distance on the control board and sometimes somewhere else away
from the board where it is better cooled by the surrounding air.
The lid has to close and lock easily. If problems occur, readjust the lid hinges or lock
assembly.
Check the lid gasket if there is one.
Check the sample buckets, their carrier or the rotor for cracks and corrosion.
Check the rubber cushions in the buckets if they all are in place.
Check that the rotor is not loose. The centre nut has to be tightened fir mly.
Test run
Make a test run without samples. Listen for any unusual noises .
Check the speed adjustment, the timer and the brake function. When stopping the
centrifugation, the centrifuge should decelerate.
Try to open the lid while the centrifuge is still spinning - it should be not possible.
Check the lid security switch. Try to switch on when the lid is open. This should also not
be possible.
Check from the inside
Make sure you have disconnected the power cable.
Take off the unit housing.
Look for spilled liquids and corrosion of the metal parts.
Take a close look at the circuit board. Examine the electronic components for burns,
breaks and leakages. Search the solder side of the board for cold soldering points and
disconnections.
Check relay contacts for burns. Clean the contacts with a fibre glass brush when the
contacts got black.
Motor
Check the motor attachment. All screws have to be tight. Usually the motor is mounted in
rubber buffers. Check the buffers for ruptures. Rubber buffers also become hard or
crumbly after some years.
Check the brushes and the commutator for wear. Take out the brushes and check the
wear. The surface of the carbon brushes must be smooth and even and should shine
silvery. Replace them if they are less than half of their original length. A motor with newbrushes should run for a while without load and at low speed so that the brushes can get
shaped. This should be done later after reassembly of the centrifuge.
Check the bearings for clearance. Exchange them if necessary. The bearings are in
principle always sealed so that they are permanently lubricated. Lubrication is not
needed, they are maintenance free.
Lubrication
In general lubrication means the use of grease. Apply a small amount of grease and move
the joints or bearings. Wipe off unnecessary grease with a cloth afterwards. Use a cloth that
you only use for greasy works or better use paper tissues.
Avoid using oil. Oil washes out the grease. Use oil only in exceptional cases or when it is
demanded in the service manual.
Lubrication areas include lid, hinges, lock mechanism, bucket carrier and sometimes
bearings, if they are not sealed.
After the maintenance
Assemble the centrifuge and make a last visual check.
Make a test run under laboratory conditions. Therefore set back all the controls to the
user's settings.
Load the rotor with some tubes and run the centrifuge for 30 minutes.
Finally fill out your maintenance log and put a new service sticker on the centrifuge
Controller) for an AC motor. The focus of troubleshooting should be the power
transistor/MOSFET/TRIAC of this stage. High voltages, high currents and big power losses
stress these components a lot so that failures are common.
To test the motor and the driver stage you can bypass the driver transistor/MOSFET of the
PWM control or the TRIAC of a PFC control. The motor has to spin then with maximum speed
To bypass the driver transistor connect the collector with the emitter or in the case of a
MOSFET, drain with source. But be careful, make sure that you do not take the base or gate
connector. A TRIAC of a PFC can also be bypassed. Therefore connect the two anodes. If you
are not sure about the pin connection check the data sheet of your type here.
Caution! TRIAC controls work with mains potential! Do not touch any components!
If the motor just runs slow, check also the brake resistor. Disconnect it and start the
centrifuge again.
Here is an example of a typical motor control of a micro centrifuge.
The motor is running on 230 VDC. In this case the AC side is regulated and then rectified.
The TRIAC controls the AC-voltage to the motor. It gets the control signal via the optocouplerfrom the microprocessor. The optocoupler is needed to decouple the mains potential of the
TRIAC from the potential-free low voltage of the rest of the control unit (which is not seen
here). The regulated AC now gets rectified by the four diodes before it gets to the motor.
The brake circuit in this example is quite tricky because the brake relay is activated when
the centrifuge does not brake and in standby mode the brake resistor (100 Ω in the above
circuit) is connected across the motor. The advantage is, that in case of a power outage the
relay would release, the resistor by-passes the motor and slows down the centrifuge.
When the centrifuge has been started the relay is activated, the contacts switches over and
the motor is connected via the 1 Ω resistor (vertical) to the regulated power supply. The
brake resistor then is by-passed by the relay. The 1 Ω resistor has no influence on the motor
regulation or the brake function. The resistance is just too small. But it creates a small
voltage drop depending on the current through it, which is the motor current. If the motorcurrent gets too big due to a fault, the voltage drop across this resistor gets bigger and the
A tricky source of trouble: When the lid switch is not exactly adjusted the centrifuge will not
start. You should therefore not only focus on the motor and its driver when a centrifuge does
not work, but also take a look at the lid switch.
Speed sensor
The speed sensor is placed close to the motor shaft and is either a forked photoelectric
sensor or a Hall-effect sensor.
It usually has 3 cables: plus, minus and the control signal which is a square wave signal
when the rotor turns. The supply voltage is probably 5 V and should be checked first. For
checking the output signal you will need an oscilloscope. When you turn the shaft slowly by
hand, then also a voltmeter will also work.
After the repair
When the centrifuge is repaired, assemble it and continue with a full maintenance as
described under ↑Maintenance including a thorough cleaning.
Make a test run under laboratory conditions. Therefore set back all the controls to the
user's settings.
Load the rotor with some tubes and run the centrifuge for 30 minutes.
Finally fill out your job card/service report and put a new service sticker to the centrifuge
showing the next maintenance date.
Before returning it to the laboratory, make sure the centrifuge is complete with all cables
and accessories.
Service manual
You find the correct repair and maintenance procedures in the service manual of the
manufacturer. Unfortunately it is difficult to get the particular manual. While user manuals
are always delivered with the new equipment, service manuals are never provided by the
manufacturer. Also on the websites of the companies you will not find any service manual.
The policy is clear, the companies want to do all repairs by themselves. Only a certified
technician who took part in a special training for certain equipment gets a service manual
and technical support.
Nevertheless, I made the experience that it is worth to write a friendly email to themanufacturer when a service manual is needed. Most of the time I received an answer with
the needed manual attached.
All the manuals I have collected, you can find here.
Spares
There is no need to keep special spare parts for centrifuges in stock. Sometimes a fuse is
needed, but spare fuses should always be available in the workshop anyway.
Life expectancy
Hospital centrifuges are in principle robust and durable. Low and high-speed centrifuges can
easily be used for 10 years. But there is no reason why a quality laboratory centrifuge could
not run for another 10 years if it is regularly maintained.
Manufacturers
Important manufacturer of laboratory centrifuges are: