image You can read the whole article in the book:

Introduction to Medical Equipment Repair

The book also contains a chapter on preventive maintenance, with recommendations on how to perform the various tests.
Click here for more information.

Repair of Blood Pressure Measurement Devices

          by Frank Weithöner

Blood pressure measurement is a routine examination that determines the pressure in the arteries. The measurement provides information about the pumping capacity of the heart and the condition of the blood vessel walls. Blood pressure measurement is a harmless and easy-to-perform diagnostic measure.
image The most common blood pressure measurement is the non-invasive measurement. For this purpose, a mechanical sphygmomanometer or an electronic blood pressure monitor is used. A non-invasive blood pressure monitor is also called a NIBP (Non-Invasive Blood Pressure) monitor.
In both cases, an inflatable cuff is needed, which is placed around the patient's upper arm. By inflating the cuff, pressure is applied to the artery (brachial artery) and the blood flow is stopped. The pressure is then slowly released in a controlled manner. The pressures at which the blood starts to flow again and when it starts to flow freely are measured.
Manual sphygmomanometers are always used together with a stethoscope, electronic blood pressure monitors work automatically without any additional equipment.
This non-invasive method of measurement does not require an instrument to be inserted into the body. However, blood pressure can also be measured invasively by inserting a catheter directly into an artery. This method is sometimes used during operations, for example, because it allows continuous measurement.

Blood pressure
Blood pressure (BP) is the pressure of the circulating blood in the large arteries and is therefore the result of the pumping action of the heart. Blood pressure is one of the vital signs that indicate the status of the body's life-sustaining functions.
Two values are used to express arterial blood pressure: systolic pressure, the maximum pressure during one heartbeat, and diastolic pressure, the minimum pressure between two heartbeats. During each heartbeat, the blood pressure varies between the systolic and diastolic pressures. The pressure is measured in millimetres of mercury (mmHg).
In addition to systolic and diastolic pressure, mean arterial pressure (MAP) is sometimes used. This is the average arterial pressure during a cardiac cycle.
Blood pressure is not static, but varies throughout the day. Normal resting blood pressure for an adult is about 120 mmHg systolic and 80 mmHg diastolic. This result is expressed as '120 over 80' and written as BP 120/80.
A patient has hypotension, or low blood pressure, when the blood pressure is lower than 90/60. High blood pressure is called hypertension and is defined when the blood pressure is consistently higher than 140/90. Both conditions are indicators of disease.

Principles of operation
There are two main methods of measuring blood pressure: the auscultatory method and the oscillometric method.

Auscultatory method
Systolic and diastolic pressures are measured by listening to the flow of blood in the brachial artery, the main blood vessel in the arm, with a stethoscope. To do this, the cuff of a sphygmomanometer is wrapped around the patient's upper arm and a stethoscope is placed over the brachial artery. When the cuff is pumped up and the pressure becomes higher than the patient's systolic blood pressure, the blood flow through the artery stops. No sound can be heard at this point.
The pressure is then slowly released. When the pressure in the cuff is lower than the pressure produced by the heart, blood begins to flow through the artery and a flow noise can be heard. This noise, which is the result of turbulence in the blood, is also called the Korotkoff sound. The pressure at which this sound first appears is the systolic blood pressure. If the pressure in the cuff drops but is still high enough to narrow the artery, the turbulence can be heard. When the sound finally disappears, the pressure in the cuff has reached the diastolic blood pressure.
Flow noises are nothing unusual. It can also be heard in everyday life when water pipes are partially blocked or water valves are not fully opened.

The beginning of the flow sound marks the systolic pressure and the last audible sound marks the diastolic pressure.

Oscillometric method
Automated blood pressure monitors work a little differently. Instead of listening to the sound of the blood flowing, a sensor detects oscillations in the blood flow. These oscillations can be detected by a pressure sensor as small changes in pressure. This method also uses a cuff to control the flow of blood through the artery. Again, the beginning of the oscillation marks the systolic pressure and the last detectable oscillation marks the diastolic pressure.

Types of blood pressure measuring devices
The traditional mercury sphygmomanometer measures the blood pressure by means of a mercury level in a glass tube. The aneroid sphygmomanometer uses a mechanical pressure gauge. Both systems require a user to manually inflate and deflate the pressure in the cuff. At the same time, the user has to listen with a stethoscope to the sounds of blood flowing through the artery. Manual measurement with a sphygmomanometer is a very accurate technique and is still a very good method of measuring blood pressure. Sphygmomanometers are durable instruments that will last many years if used carefully and checked from time to time.
An automatic blood pressure monitor automatically inflates and deflates the cuff. The blood pressure monitor also measures the systolic and diastolic blood pressure. Automated blood pressure monitors come in two different types: The blood pressure monitor for use in hospitals, which often also monitors other vital signs, and the small, inexpensive, portable device that is made for use in the home care sector. Automated blood pressure monitors for use in hospitals are essential for long-term measurements. Although cheap home-care devices are not very durable and not suitable for long-term use, they are often used in hospitals in developing countries.
The advantage of electronic blood pressure monitors is their easy operation and the simple reading of the measurement result. But this does not mean that they are more accurate or better. In fact, the accuracy of an electronic device depends on several factors, such as the accuracy of the pressure transducer, the software, the tolerance of the electronic components and the quality of the calibration, whereas the reading of a mercury sphygmomanometer is basically always correct.

Mercury sphygmomanometer
In a mercury sphygmomanometer, the weight of the mercury column in the riser tube acts against the cuff pressure and thus against the blood pressure. The height of the mercury column also acts as an indicator. This direct measurement means that no other mechanics are required.
As the working principle is based on the specific weight of mercury, which does not change, mercury sphygmomanometers do not require calibration. They are considered the gold standard. Readings are always correct as long as the riser is vertical and the mercury level is '0'.
Mercury sphygmomanometers are no longer used in Europe and the USA because of the toxicity and environmental impact of mercury. They are being replaced by aneroid instruments. However, mercury sphygmomanometers are still common in developing countries.

Older mercury sphygmomanometer, as still found in developing countries today. When not in use, the instrument can be closed.

Aneroid sphygmomanometer
image The aneroid sphygmomanometer is a mechanical instrument that displays blood pressure on a circular dial. This is why it is called aneroid, which means 'without liquid'. All other components are the same as for a mercury sphygmomanometer. The aneroid type also needs a cuff and an inflating bulb with a pressure release valve. Aneroid sphygmomanometers are also available as large wall mounted units for stationary use.
The advantage of an aneroid sphygmomanometer is that it is smaller, lighter, more portable and, of course, does not contain poisonous mercury. Although the manometer contains some delicate mechanics, it is surprisingly robust. Defective instruments can often be repaired and calibrated easily. The aneroid sphygmomanometer is the most common type of blood pressure device in developing countries.
Unlike a mercury sphygmomanometer, an aneroid needs to be calibrated from time to time, but it is easy to do. By the way, calibration can be done very well with a reference instrument: the mercury sphygmomanometer.

Traditionally a cuff consists of two parts, an inner bladder made from rubber and the outer envelop made from strong fabric, often nylon, with a Velcro fastener. Attached to the bladder are one or two rubber hoses. Cuffs for mercury and aneroid sphygmomanometers have two tubes, one for the pumping bulb and one for the pressure gauge. Electronic home care devices only have one tube.
Cheap cuffs as they are found today in the developing world consist of one piece only. They are made of plastic and are not as strong as those with a separate bladder and fabric. They do not last very long and are almost impossible to repair.

With good cuffs, it is easy to remove the bladder. The fabric sleeve can then be washed or the bladder repaired.

Cuffs are available in various sizes. The size is based on the circumference of the upper arm. The cuff should cover two-thirds of the length of the upper arm. If the cuff is too small, the blood pressure reading will be too high. If the cuff is too large, the result will be too low. The range of arm circumferences is printed on each cuff.
Typical upper arm cuff sizes are:

Infant Child Small adult Adult Large adult
7 9 10 11 12

The size is approximately the width of the cuff in cm. If in doubt, it is better to take the next larger cuff than a cuff that is too small.

The stethoscope is a simple acoustic examination instrument. It is used for listening to the sounds of heart and lung and the blood flow during blood pressure measurement.
image The stethoscope consists of a sound-detecting device, the chest piece, and two earpieces connected by a Y-shaped flexible plastic tube.
The principle is simple: sound is transmitted through air-filled hollow tubes to the user's ears. The sound is picked up by a resonator called the chest piece.
The chest piece is a metal housing that usually has two sides, one with a larger diameter and one with a smaller diameter. The larger side usually has a diaphragm, which is a thin plastic disc. The other, smaller side is called the bell and is shaped like a small cup with a small hole. The larger, flat side with the diaphragm is used to hear higher frequencies and the smaller bell is used to hear lower frequencies.
For examination, the user must cover the opposite side with a fingertip if there is no mechanical closure. The chest piece is placed on the patient, body sounds move the diaphragm and the vibrations cause small changes in air pressure in the tubes that travel to the earpieces.

NIBP monitor
In addition to manual blood pressure monitors, there are also automated blood pressure monitors. These electronic blood pressure monitors are used in operating theatres, intensive care units and other areas where patients' vital signs need to be monitored around the clock. For this purpose, the devices take readings regularly and independently. This type of automated blood pressure monitor is also known as an NIBP (Non-Invasive Blood Pressure) monitor. NIBP monitors have programmable alarms and also measure mean arterial pressure and pulse rate.

Typical patient monitor measuring blood pressure. Only the cuff is connected and only the blood pressure is shown on the display.

Automated clinical blood pressure monitors are usually combined devices that also measure other vital signs such as blood oxygen saturation, ECG or body temperature. Such a multi-parameter device is also called a patient monitor.
Another type of electronic blood pressure monitor is the small portable unit for home use. These battery-powered devices are also automatic, but are not designed for continuous use. They are designed to be easy to use so that anyone can take a blood pressure reading. Portable home blood pressure monitors are cheaply made, not very robust and therefore not designed for daily use in a hospital for several years.

A typical automatic blood pressure monitor for home use.

image As a biomedical technician, you should know how to take blood pressure with a sphygmomanometer and a stethoscope. It always makes a good impression if you can operate the equipment you are repairing.
Before you start, make sure that the stethoscope is working and that you can hear properly. Put the stethoscope in your ear and tap the chest piece head with your finger.
Also check that the aneroid pressure gauge is zeroed, the pointer should be within the rectangular box on the dial.
Also check the cuff size. Incorrect cuffs will give incorrect blood pressure readings.

  1. The tested person has to be relaxed, calm and should not speak during the measurement.
      The arm should be placed on a table with the palm facing upwards so that the blood
      pressure cuff is at the same height as the heart.
  2. Fasten the cuff around the bare upper arm. The tube connectors should point towards the
      crook of the arm. There should be about 3 cm (1 in) of space between the crook of the arm
      and the cuff for the stethoscope.
  3. Hold the inflating bulb with the pressure gauge with your right hand. Close the release
      valve on the bulb by turning the screw clockwise. Squeeze the bulb and inflate the cuff.
      The pressure should be about 180 to 200 mmHg.
  4. Place the stethoscope in your ears and the stethoscope head between the crook of the arm
      and the cuff where the artery is located.
  5. Open the release valve slightly and slowly deflate the cuff. Listen carefully to the sound of
      the pulses and watch the gauge carefully.
  6. The systolic blood pressure is shown on the dial gauge when you first hear a rhythmic
      thumping sound. In a healthy person, the reading is around 110 to 120. Memorise this
  7. Continue to slowly release the pressure and continue to listen to the sound. When you no
      longer hear the knocking sound, the diastolic pressure has been reached. This reading is
      normally around 60 to 80 mmHg.
  8. The blood pressure measurement is complete. Now deflate the cuff completely.

Example: A reading of 120 systolic and 80 diastolic is expressed as '120 over 80' and written as BP 120/80.

If you have done something wrong, perhaps you released the pressure too quickly, do not inflate the cuff again right away. Wait at least a minute before repeating the measurement.
There is no single reference value for blood pressure. However, the following blood pressure values can be considered normal for adults:

Age Systolic Diastolic
up to 20 years 118 75
20 to 30 years 122 79
30 to 40 years 125 85
40 to 50 years 130 87
50 to 60 years 140 90
over 60 years 150 100

Cleaning by the user
Cuff, tubes and bulb should be wiped regularly with a little soapy water. Some manufacturers also recommend disinfection with 70 % isopropyl alcohol or 0.5 % bleach/water solution. Sphygmomanometer are not autoclavable.
It is not a bad idea to instruct the user on how to do this so that no water gets into the system. After cleaning, components should be allowed to dry before reuse.

Mercury and aneroid sphygmomanometers and digital blood pressure monitors differ in the way they work and display blood pressure. The first two are mechanical devices, whereas NIPB monitors are electronic devices where not only is the display digital, but the measurement process is automatic.

Mercury sphygmomanometer
image The image shows the components of a mercury sphygmomanometer.
The user inflates the cuff using the rubber bulb. For this reason, the bulb has an inlet valve at the end. When the bulb is squeezed, the air from the bulb passes by the closed release valve and the flexible rubber tubing into the cuff. There is also a second tube attached to the cuff which is connected to the mercury reservoir and the riser tube. The higher the pressure, the more mercury is pumped up the riser tube from the reservoir.
Once the cuff has been inflated to the desired pressure, it must be slowly deflated. To do this, the user carefully opens the release valve screw. Air is released from the system, the pressure decreases, the mercury level in the riser tube drops and the mercury returns into the reservoir.
In order to allow the air above the mercury column to escape during inflation, the end of the riser tube is closed with an air-permeable plug, usually made of felt. Another similar plug is in the inlet of the mercury reservoir to prevent mercury escaping into the cuff.

Aneroid sphygmomanometer
Aneroid sphygmomanometers are smaller, lighter, more portable and therefore more convenient than mercury sphygmomanometers.
The hose connection at the bottom of the meter is connected to a closed brass pressure can. This pressure can is made out of corrugated brass sheet which expands when the pressure inside increases. The bellow is coupled to a gear mechanism which converts the up and down movement of the bellow into rotation of the pointer.

Parts of an aneroid sphygmomanometer. Despite its delicate mechanics, the instrument is surprisingly robust.

NIBP monitor
While the sphygmomanometer blood pressure method is based on listening to Korotkoff sounds, electronic blood pressure monitors work differently. They use the oscillometric method.
A pressure transducer (pressure sensor) in the device detects small pressure changes in the cuff pressure caused by oscillations in blood flow. A filter and amplifier then separates the oscillation signal, an A/D (Analogue/Digital) converter converts the signal into digital data, and a microprocessor interprets the signals and displays the result.
Additionally the inflation and deflation of the cuff work automatically. Hospital NIPB monitors are also be timer controlled so that a long term monitoring can be carried out.

Home blood pressure monitor
The following is the construction of an automated blood pressure monitor for home use. These devices are also called digital blood pressure devices. The word digital refers only to the display, the blood pressure measurement itself is analogue. As the accuracy of the device depends on the conversion of the measurement signal from analogue to digital, or on its calibration, these devices are not fundamentally more accurate than an aneroid sphygmomanometer. They are just easier to use.
The air pump consists of a small DC motor with attached diaphragm pump. The pump pumps air through the connector into the cuff. At this moment the solenoid valve is closed. When the device releases the pressure from the cuff, the pump stops and the solenoid valve opens.
Also connected to the tubing is the pressure sensor on the electronic board. As the pressure in the system is everywhere the same, the sensor must not be close to the cuff. That is why the sensor can be mounted directly on the PCB. A microcontroller on the board analyses the pressure in the cuff as well as the oscillations during blood flow through the artery, drives the display and controls pump motor and solenoid valve.

The inside of a typical automated blood pressure monitor for home use.

In principle, more expensive clinical blood pressure monitors do not work much differently. However, there are two things that are different, apart from additional display options. For more accurate measurement, these devices usually have a second tube. Inflating and deflating the cuff is separate from measuring the pressure. The pressure sensor has its own tube. Also the pump is bigger and more robust, because the clinical monitors need to be able to take measurements regularly and automatically.

Clinical blood pressure monitor (NIBP monitor)
The working principle of an automated blood pressure monitor for hospital use is similar to that of the small portable device for home use. The components, however, are designed for continuous use and are therefore of a higher quality and more robust. The following photo shows the inside of such a monitor.
Let's take a look at the tubing. The pump is at the top right. A one-way valve (red) is connected to it. A pressure sensor is connected to the first T-piece. This is followed by another T-piece to which a solenoid valve (yellow) is connected. This gradually releases the pressure during the measurement. From there, a tube leads to another pressure sensor and to the patient connector.

An open blood pressure monitor for clinical use.
The two sensors are used because blood pressure and control pressure are measured separately. Some monitors even have two separate tubes leading to the cuff. One sensor is closer to the patient connector. This is the sensor that measures blood pressure. Both pressure sensors and their input amplifiers are located under a metal shield to protect them from external interference. The output from the first amplifier then goes to the measurement electronics with the display. The other output goes to the control electronics, which controls the pump and the solenoid valve.

Blood pressure monitors should always be delivered for repair with the cuff and connecting tube, as this is often where the fault lies.

Before you start with the repair:

  Note accessories such as cuff and power adapter on the job card to avoid confusion later.
  Put the cuff on yourself and carry out a test run in order to confirm the fault. Make sure that
    the problem is really a fault and not an operating error.
  If you are not sure how to operate the unit yourself, consult the user manual.

TIP!     Flexible hoses made of rubber or silicone should not be pulled from their connectors but pushed off, possibly using a screwdriver. Pulling will make the flexible hose longer and the diameter smaller. The connection becomes tighter and the tube or connector may tear off.

Special measuring and test devices
For calibration a reference pressure gauge is needed. The instrument is connected with a Y-piece to the sphygmomanometer on test. For creating a stable pressure in the cuff, the cuff should be wrapped around a solid object with the approximate diameter of an upper arm. A glass bottle, for example an infusion bottle, is perfect.

Tips & Tricks: Cheap workshop reference pressure gauge
A mercury sphygmomanometer is the perfect reference instrument for doing calibrations and adjustments because it does not have to be calibrated itself. Get an old one that is no longer in use and clean the mercury if necessary. Just add a tube and a Y-piece and you are ready to calibrate all types of sphygmomanometers.

Leakage test
Leaks in the pressure cuff or connecting tubing can be detected with a simple test set-up. Wrap the cuff around a glass bottle approximately the diameter of an arm. Apply pressure to the cuff with the rubber bulb until 200 mmHg is reached. The pressure should remain stable and should not fall by more than 2 mmHg in 10 seconds.
If you are using an automatic blood pressure monitor, disconnect the connecting tube and connect it to a rubber bulb and follow the same procedure.

Performance test
Calibration is performed by comparing the device under test with a reference instrument. This reference instrument, for example a mercury sphygmomanometer, is attached to the device under test by a Y-piece. The cuff is wrapped around a glass bottle, such as an infusion bottle, and calibration can be performed.
However, before calibrating, perform a leakage test as described above. Then calibrate and adjust the '0' position of the sphygmomanometer. Aneroid sphygmomanometers are set to '0' as described above.
Aneroid sphygmomanometers can now be calibrated at different pressures. Mercury sphygmomanometers do not require calibration as the specific weight of mercury does not change. It is only necessary to ensure that the level indicates "0" when no pressure is applied. Aneroid manometers are different. The mechanics can wear out or get damaged over time. They need a regular calibration.

Set-up for calibration of sphygmomanometers.

  1. Inflate the cuff to 200 mmHg and compare the readings. The result should not differ by
      more than ± 3 mmHg.
  2. Repeat the procedure at 150 mmHg, 100 mmHg and 50 mmHg.
  3. Inflate the cuff again to 200 mmHg and check that the pressure remains stable for
      10 seconds. The system should not lose more than 2 mmHg during this time.

Common problems
Apart from dropped aneroid sphygmomanometers that need to be readjusted, most problems are related to leaks. Leaks can be caused by holes in the bladder, worn tube connections, porous tubes and rubber bulbs, dirty release valves or damaged pressure gauges.
Leaks in the bladder of the cuff can usually be repaired. This involves taking the bladder out of the fabric. The fabric must be opened on one side and sewn back together later. A rubber bladder can then be repaired in the same way as a bicycle inner tube. The first step is to locate the leak. Inflate the bladder and watch and listen for air escaping. Major leaks will be easy to hear or see. A quicker way is to put the inflated bladder in a bucket of water. The escaping air will create bubbles.
Cheap cuffs without inner bladders can often not be repaired because rubber glue does not work on synthetic or plastic materials. Instead of using rubber glue and bicycle repair patches, try gluing a piece of similar synthetic material, for example from an old cuff, with elastic contact adhesive or silicone.

Tips & Tricks: Using bicycle repair patches
image Rubber bladders with holes can be repaired with bicycle repair patches. Sets of patches and rubber cement for gluing the patch can be found in supermarkets or bicycle or motorcycle shops.
Use sandpaper to roughen the surface around the hole. Apply the rubber cement to the bladder and spread it out with your finger. Allow the glue to dry for 10 minutes. When the glue is dried, press the patch firmly onto the repair area. It is the pressure, not the time, that is important. The repaired tube can be used immediately.

Blood pressure tubes are very flexible and are made of rubber or soft plastic. They have an internal diameter of 4 mm. The connectors and adapters have a larger diameter, usually 5 mm.
Deformed, cracked or lost stethoscope diaphragms are easy to replace. The only problem is finding a suitable replacement. The diaphragm must be thin but also stiff. Old X-ray images are perfect. The foil can easily be cut into shape with scissors.
To remove the membrane, the chrome ring must be unscrewed. If the ring cannot be turned, you are pressing too hard on the ring. The ring will become deformed and will not come off. Loosen your grip and turn gently.
Mercury sphygmomanometers should no longer be used in hospitals for environmental and health reasons. However, as they make a good reference instrument, it is worth refurbishing an old one.
At the end of the riser pipe and at the inlet of the mercury reservoir there are felt discs. Their task is to ventilate the system with air and keep the mercury inside the sphygmomanometer. If dust and dirt accumulate on the felt, the air cannot escape or enter and the mercury column moves slowly or not at all. After cleaning the felt, the sphygmomanometer will respond quickly again.
Clean mercury has a shiny silver appearance, but over time it can become dirty. Either dirt from outside gets into the system or the mercury oxidises. Mercury oxide is a black powder that floats on the surface of the mercury. A small amount of mercury oxide is not a problem, but if there is too much, the mercury will not flow. It can also block the filter at the end of the riser or at the inlet to the mercury reservoir.
If this happens, the mercury must be cleaned. This means that all the mercury must be removed from the sphygmomanometer and then filtered. The procedure is not difficult, but it must be done very carefully.

Hint! There is no health concern from acute short-term exposure to mercury under the conditions of a broken mercury-containing thermometer or energy-saving lamp at home. (Wikipedia: Mercury poisoning)

It is advisable to work over a plastic tray so that spilt mercury cannot disappear. Get two syringes, one should be a larger one (35 or 50 ml), the other can be smaller. Pull the plunger completely out of the large syringe and place one or two layers of cotton gauze in the syringe. Put the plunger back in and push it down so that the cotton is in front of the outlet of the syringe. Now remove the plunger again.
Tilt the sphygmomanometer so that all the mercury flows into the reservoir. Close the outlet if possible and open the reservoir. Now use the small syringe and draw up all the dirty mercury into the syringe. Inject the mercury into the prepared syringe. Replace the plunger and push the mercury through the cotton gauze into the reservoir. The mercury oxide will remain in the gauze.
Before returning the cleaned mercury to the reservoir, clean the riser tube and reservoir. The riser tube can be cleaned by pulling a small piece of cotton gauze with a long wire through the tube.
When filling the reservoir, make sure that the mercury level is correct. The level is correct when the mercury level shows '0'.

After the mercury is pressed through the gauze, the black oxide remains.

image It often happens that the pressure gauge of an aneroid sphygmomanometer does not show zero in a pressure-less state. The pressure gauge must then be set to '0'. In fact, '0' is often not marked on the scale, but you will find a rectangle instead. The pointer should be inside this square.
Most aneroid sphygmomanometers can be adjusted by turning the tube connector. To do this, the locknut must be loosened slightly. Usually this locknut is a round type with two holes. Loosening should be done with circlip pliers, but small long nose pliers will also work. Once the locknut has been loosened, the tube connector can be turned. When this is done, the pointer will also rotate as the tube connector and dial are mechanically connected. When the pointer is in the rectangle, tighten the locknut.

In order to adjust the pointer, simply loosen the locknut, which is the flat washer with the two holes, and turn the tube connector a little.

If you have a sphygmomanometer where the bulb and pressure gauge are combined, this method will not work. In this case, the gauge housing must be opened and the position of the pointer itself corrected. To do this, unscrew the metal ring, remove the glass and carefully lift the pointer off the shaft. Sometimes a tool such as a small, flat screwdriver is needed. Do not use pliers or forceps to remove the pointer. It can be easily deformed. Once removed, the pointer can be carefully placed back in the correct position.

TIP! Tool for loosing locknut and for lifting up pointer
To make a tool for loosening the locknut, simply take a bicycle spoke, cut off the ends, file them flat and bend them as shown in the picture. The wire ends will then fit perfectly into the nut holes.
To lift the pointer without damaging it, you can cut a strip of epoxy from an old circuit board and drill and file a small slot.

Automated blood pressure device for home care use are usually cheaply made and often hardly repairable. They are also not designed for later calibration and adjustment. They are disposable products. Battery-powered devices suffer from high battery consumption. This is normal because the pump motor draws quite a lot of current. In developing countries, where these devices are often used in hospitals, this is a problem, especially because batteries are either of very poor quality or very expensive.
Replacing the batteries with rechargeable ones does not work either, because the resulting operating voltage will be too low. Typically, these devices run on 6 V, which is supplied by 6x 1.5 V AA batteries. But 6x 1.2 V AA rechargeable batteries will only give you 4.8 V, which is often too low to run the device, or not long enough.
The best solution is to use an external power supply. This should provide at least 500 mA. Universal power supplies are available in any city. If your blood pressure monitor works reliably on 5 V, you can also use a USB power supply, which is cheaper and easier to get.
Clinical blood pressure monitors are robust devices that do not break often. When a problem does occur, it is often related to the tubing. Leaks are possible due to ageing of the tubes. It is also possible that dirt has got into the system and is blocking a sensor or the solenoid valve. This should not actually happen, as there should be a filter before the pump.

Situation in developing countries
As equipment in developing countries tends to be much older, there are also problems with wear and tear. Blood pressure cuffs are often very old, which means that the Velcro fasteners do not hold well any more. However, if the bladder is removed, a tailor can easily sew on new ones.
Blood pressure cuffs can also be subject to operator error, for example when the wrong cuff is used. This is because departments often only have one blood pressure device. The cuff is then used for all patients, whether they are children, adults or obese adults. But one size does not fit all. Not only will the cuff not fit properly, but the measurement results will be incorrect.
There have been many cheap blood pressure monitors in hospitals for some time - mostly as defective devices in the workshop. Once again, electronic devices with digital displays are not necessarily better or more accurate than mechanical devices! For routine measurements, aneroid sphygmomanometers are simply unbeatable. They are accurate, not expensive and, if handled with care, will last for many years. In addition, there are no running costs in the form of batteries and the calibration process is quick and easy.

A collection of broken blood pressure monitors in a hospital workshop in Tanzania. Few of these cheap monitors worked for more than a year.

After the repair
After repairing the NIBP monitor, switch it on briefly before reassembly to see that it works and that nothing else is faulty. If this is the case, remove dust from the inside of the unit if necessary. Then screw the unit back together.
When the NIBP monitor is reassembled, clean the outside of the unit with a soap solution. Do not forget to clean the mains cable and the cuff as well. After drying with a dry cloth, the unit can be disinfected with a spray disinfectant.
Now a test run should be carried out. The test run should include a few blood pressure measurements.
Finally, the electrical safety test should be carried out as explained in the Maintenance chapter of the book, if it is a mains powered device. When the test has been passed, you only need to fill out the job card and the NIBP monitor can be returned to the ward.

Further literature
On Wikipedia you can find further articles about these topics:
  Blood pressure
  Blood pressure measurement
  Korotkoff sounds
  Pressure measurement
  Pressure sensor

image You can read the whole article in the book:

Introduction to Medical Equipment Repair

The book also contains a chapter on preventive maintenance, with recommendations on how to perform the various tests.
Click here for more information.