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Basic measurement concepts
PHYSICS
Basic measurement concepts
Characteristic thermistor curve
John Curnow
600
500
Resistance (Ω)
Anaesthetists commonly record data from patients, intermittently or continuously. Data collection can be as simple as measuring the heart rate by feeling the pulse and counting it over a period of time, through regular intermittent monitoring of blood pressure and heart rate, to continuous monitoring of variables using electronic data collection, analysis and storage systems. The general concepts discussed here mainly relate to continuous monitoring, though some of them also apply to intermittent methods.
400 T 300
200
100 n
Measurement is the interpretation of a feature or function, in a way that is repeatable and calibrated, so that it can be understood and compared with other measurements taken at different times or by different people and on different patients. In many basic measurement techniques, such as measuring length, the device used to make the measurement is calibrated to a national or international standard. This use of a standard to calibrate a measurement is the basis of making it repeatable and accurate so that measurements made by anyone with a calibrated instrument can be compared. For medical monitoring, data are collected directly from the patient using two different systems. If the feature to be monitored is an electrical activity in the body, electrodes are used to collect the data directly. If the feature to be monitored does not produce electrical activity then a transducer is used to collect the data. A transducer measures the feature required and produces an electrical signal proportional to it. For example, a standard method of temperature measurement is to use a thermistor as the sensor. A thermistor is an electronic device that has been specifically designed so that its electrical resistance changes with temperature in a regular and defined way (Figure 1). This characteristic curve shows several important features. Within the temperature range A to B the change in temperature is directly related to change in resistance and therefore the curve for this part can be represented by a straight line. This is called a linear relationship because it can be represented by the straight line characteristic, and therefore the changes in resistance mirror the changes in temperature. Many systems are said to be linear, and the linearity of a measurement system is a measure of how well the measured variable and the measurement variable can be represented by a straight line. In the temperature ranges zero to A and B to C, the characteristic is a curve that follows a fixed mathematical function. In this case, the relationship can be modelled within the electronics of the measurement system to produce a linear relationship and allow monitoring. In the range above C, the curve approximates to a straight horizontal line because there is no change of resistance with change of temperature. The measuring system is said to have reached saturation at point C and no readings can be made above this temperature. Saturation can be a problem for the unwary
0
A
30
D
E
40
B
50
C
Temperature (°C) See text for explanation 1
because the measurement system is apparently giving an acceptable reading although it is wrong. It is always important to identify the maximum value a system should read before using it. Offset is a change within the transducer that can happen in real systems. In Figure 1, the calibrated results are shown by the solid line and the error caused by offset in the transducer is shown by the dotted line. Thus, a particular result may be a resistance of T as shown. The system will assume this is correctly calibrated and give a result of E degrees from the solid line, however as this reading has a resistance n ohms high the real result is found by taking the result from the dotted line instead of the solid line which is a true temperature of D ohms, which means the instrument will overestimate the result. Drift – most transducers used with monitoring equipment have active electronic devices such as a thermistor. These devices are designed to work accurately over a known environmental temperature range. If they are used at environmental temperatures outside this range the results may be incorrect because of drift. Drift can be seen as a change of the characteristic curve and can consist of offsets and/or changes in slope of the characteristic curve. The latter will reproduce as an overestimate or underestimate of temperature that changes across the measurement range of the system. If one or other of these occurs the final result will be an error in the reading of the measurement that will be difficult to identify. One method of identifying drift is to monitor the expected baseline continuously. If it shows a consistent rise or fall that is not expected, then drift may be the cause. Hysteresis – some transducers (e.g. pressure measurement transducers) use the mechanical properties of a material as the basic measurement technique. In the case of some pressure transducers a thin metal or plastic membrane is fitted over the end of a small chamber connected to the fluid having its pressure measured. The membrane flexes out as the pressure increases and goes back towards its original position as the pressure falls. The flexing of the membrane is measured using an electrical method. Many solid materials demonstrate a feature called hysteresis when
John Curnow is Biomedical Engineer at Derriford Hospital, Plymouth, Devon, UK. His specialist interest is in patient monitoring and the application of computing techniques in medicine.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:9
20
10
288
© 2005 The Medicine Publishing Company Ltd
PHYSICS
Typical hysteresis curve
Input
B
See text for explanation
A Output 2
stretched and relaxed repeatedly. Figure 2 shows a typical hysteresis curve. If the system starts at point A and the input rises, the characteristic will follow the full line to point B where saturation occurs. If the input is then reduced the characteristic will follow the dotted line back to the lower saturation level at A. Therefore, the output value for any input value depends on the direction in which the input is changing at the time, and means accurate calibration of the output values is impossible. If the input signal is continuously changing, the output will also be a continuously changing value but will appear to read above the expected value during increasing input, and below the expected value during reducing input. This will produce a distorted signal shape. Measurement equipment electronics Most medical measurement equipment depends on electronic systems to process and display the data collected. These electronic systems therefore become part of the overall measurement system. The electronics are often intended to make correction for known consistent errors in the signal produced by the transducer or to make non-linear responses linear. The bulk of the electronics of a measurement system usually means that electrodes and transducers need long cables to connect them. These cables lie in the general environment and act as an aerial to airborne electrical signals. These signals consist mainly of power line frequency signals and radiofrequency signals. The signal is added to the signal from the electrode or transducer and both signals are presented to the electronics for processing. The unwanted signals are called noise. Other sources of noise are unwanted body potentials such as the muscle activity added to an ECG signal. The level of noise is important when considering the quality of a signal. The signal-to-noise ratio (SNR) is a ratio of the amplitude of the required signal to the amplitude of the noise signal and is used to define the quality of the signal. SNR = 20 log10 signal amplitude noise amplitude A value of 6 dB means the signal is twice the size of the noise, and 60 dB means the signal is 1000 times larger than the noise. The electronic system can also affect the signals, for example saturation occurs if the signal requires a voltage higher than that available from the electronics. Also the electronics can have thermal drift, which can produce the same results as transducer drift. The usual cause is the electronics becoming too hot and it is essential that all vents are kept clear to minimize this effect.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:9
289
© 2005 The Medicine Publishing Company Ltd
Basic measurement concepts
Characteristic thermistor curve
John Curnow
600
500
Resistance (Ω)
Anaesthetists commonly record data from patients, intermittently or continuously. Data collection can be as simple as measuring the heart rate by feeling the pulse and counting it over a period of time, through regular intermittent monitoring of blood pressure and heart rate, to continuous monitoring of variables using electronic data collection, analysis and storage systems. The general concepts discussed here mainly relate to continuous monitoring, though some of them also apply to intermittent methods.
400 T 300
200
100 n
Measurement is the interpretation of a feature or function, in a way that is repeatable and calibrated, so that it can be understood and compared with other measurements taken at different times or by different people and on different patients. In many basic measurement techniques, such as measuring length, the device used to make the measurement is calibrated to a national or international standard. This use of a standard to calibrate a measurement is the basis of making it repeatable and accurate so that measurements made by anyone with a calibrated instrument can be compared. For medical monitoring, data are collected directly from the patient using two different systems. If the feature to be monitored is an electrical activity in the body, electrodes are used to collect the data directly. If the feature to be monitored does not produce electrical activity then a transducer is used to collect the data. A transducer measures the feature required and produces an electrical signal proportional to it. For example, a standard method of temperature measurement is to use a thermistor as the sensor. A thermistor is an electronic device that has been specifically designed so that its electrical resistance changes with temperature in a regular and defined way (Figure 1). This characteristic curve shows several important features. Within the temperature range A to B the change in temperature is directly related to change in resistance and therefore the curve for this part can be represented by a straight line. This is called a linear relationship because it can be represented by the straight line characteristic, and therefore the changes in resistance mirror the changes in temperature. Many systems are said to be linear, and the linearity of a measurement system is a measure of how well the measured variable and the measurement variable can be represented by a straight line. In the temperature ranges zero to A and B to C, the characteristic is a curve that follows a fixed mathematical function. In this case, the relationship can be modelled within the electronics of the measurement system to produce a linear relationship and allow monitoring. In the range above C, the curve approximates to a straight horizontal line because there is no change of resistance with change of temperature. The measuring system is said to have reached saturation at point C and no readings can be made above this temperature. Saturation can be a problem for the unwary
0
A
30
D
E
40
B
50
C
Temperature (°C) See text for explanation 1
because the measurement system is apparently giving an acceptable reading although it is wrong. It is always important to identify the maximum value a system should read before using it. Offset is a change within the transducer that can happen in real systems. In Figure 1, the calibrated results are shown by the solid line and the error caused by offset in the transducer is shown by the dotted line. Thus, a particular result may be a resistance of T as shown. The system will assume this is correctly calibrated and give a result of E degrees from the solid line, however as this reading has a resistance n ohms high the real result is found by taking the result from the dotted line instead of the solid line which is a true temperature of D ohms, which means the instrument will overestimate the result. Drift – most transducers used with monitoring equipment have active electronic devices such as a thermistor. These devices are designed to work accurately over a known environmental temperature range. If they are used at environmental temperatures outside this range the results may be incorrect because of drift. Drift can be seen as a change of the characteristic curve and can consist of offsets and/or changes in slope of the characteristic curve. The latter will reproduce as an overestimate or underestimate of temperature that changes across the measurement range of the system. If one or other of these occurs the final result will be an error in the reading of the measurement that will be difficult to identify. One method of identifying drift is to monitor the expected baseline continuously. If it shows a consistent rise or fall that is not expected, then drift may be the cause. Hysteresis – some transducers (e.g. pressure measurement transducers) use the mechanical properties of a material as the basic measurement technique. In the case of some pressure transducers a thin metal or plastic membrane is fitted over the end of a small chamber connected to the fluid having its pressure measured. The membrane flexes out as the pressure increases and goes back towards its original position as the pressure falls. The flexing of the membrane is measured using an electrical method. Many solid materials demonstrate a feature called hysteresis when
John Curnow is Biomedical Engineer at Derriford Hospital, Plymouth, Devon, UK. His specialist interest is in patient monitoring and the application of computing techniques in medicine.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:9
20
10
288
© 2005 The Medicine Publishing Company Ltd
PHYSICS
Typical hysteresis curve
Input
B
See text for explanation
A Output 2
stretched and relaxed repeatedly. Figure 2 shows a typical hysteresis curve. If the system starts at point A and the input rises, the characteristic will follow the full line to point B where saturation occurs. If the input is then reduced the characteristic will follow the dotted line back to the lower saturation level at A. Therefore, the output value for any input value depends on the direction in which the input is changing at the time, and means accurate calibration of the output values is impossible. If the input signal is continuously changing, the output will also be a continuously changing value but will appear to read above the expected value during increasing input, and below the expected value during reducing input. This will produce a distorted signal shape. Measurement equipment electronics Most medical measurement equipment depends on electronic systems to process and display the data collected. These electronic systems therefore become part of the overall measurement system. The electronics are often intended to make correction for known consistent errors in the signal produced by the transducer or to make non-linear responses linear. The bulk of the electronics of a measurement system usually means that electrodes and transducers need long cables to connect them. These cables lie in the general environment and act as an aerial to airborne electrical signals. These signals consist mainly of power line frequency signals and radiofrequency signals. The signal is added to the signal from the electrode or transducer and both signals are presented to the electronics for processing. The unwanted signals are called noise. Other sources of noise are unwanted body potentials such as the muscle activity added to an ECG signal. The level of noise is important when considering the quality of a signal. The signal-to-noise ratio (SNR) is a ratio of the amplitude of the required signal to the amplitude of the noise signal and is used to define the quality of the signal. SNR = 20 log10 signal amplitude noise amplitude A value of 6 dB means the signal is twice the size of the noise, and 60 dB means the signal is 1000 times larger than the noise. The electronic system can also affect the signals, for example saturation occurs if the signal requires a voltage higher than that available from the electronics. Also the electronics can have thermal drift, which can produce the same results as transducer drift. The usual cause is the electronics becoming too hot and it is essential that all vents are kept clear to minimize this effect.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:9
289
© 2005 The Medicine Publishing Company Ltd