6.4 The errors produced in temperature and pressure measurement will arise from a number of factors. Some are inherent in the design, age and condition of the measuring equipment, and others are due to loose terminals, imperfect plug and socket connections, and the change of environmental temperature around the instrument. Variations in
thermocouple alloys, preparation of thermocouple hot junctions, the method of introducing sensors into the chamber, and their location within the load will add to the error in temperature measurement. Temperature fluctuations within pressure-sensing elements will lead to errors in pressure
measurement.
6.5 Every effort should be made to eliminate or minimise these errors by
attention to detail, location of instruments, effective maintenance, and skill in the application, handling and use of the instruments. Systematic errors can be reduced by careful calibration.
6.6 Instruments should be maintained and calibrated as recommended by the manufacturer as part of a planned maintenance programme. Each
instrument should be labelled with the calibration date and a reference to its certificate. The calibration of all test instruments should be verified yearly by using reference instruments with a valid certificate of calibration traceable to a national standard. A history record should be kept for each instrument. 6.7 All electronic test instruments should be allowed a period of time to stabilise
within the test site environment. They should be located in a position protected from draughts, and should not be subjected to rapid temperature variations. The manufacturer’s instructions should be followed.
Recorders
6.8 Test recorders are required to measure temperature and pressure in all types of sterilizer, and humidity in EO sterilizers. They should be designed for use with the appropriate sensors, independent of those fitted to the sterilizer, as described later in this chapter. Most of the tests in this SHTM may be conducted with a single recorder combining temperature and pressure functions, preferably showing both records on the same chart or print-out. For EO sterilizers, a third function, for humidity, is desirable but not essential.
6.9 Twelve temperature channels are sufficient for all the tests on each type of sterilizer in this SHTM, though more may be convenient for determining chamber temperature profiles (see paragraph 7.21). One pressure channel is required for all sterilizers except fluid sterilizers which require up to three. The pressure channel for a dry-heat sterilizer is required to measure the small differential pressure (no more than 10 mbar) across the air filter. Two relative-humidity channels are desirable for EO sterilizers.
6.10 Analogue recorders (conventional pen and chart recorders) should comply with the display requirements of BS 3693. If they use potentiometric
techniques, they should comply with BS 5164.
6.11 Digital recorders (data loggers) are rapidly coming into use and have many advantages over traditional pen recorders. They measure the variables electronically and store the values in digital form suitable for computer
processing. Data may be presented graphically or as a numerical list, or as a combination of both. Parts of the operating cycle, such as the plateau
period, can be expanded and replotted for closer examination. The record should quantify all turning points in the data, and distinguish by colour, print format or separate list, measurements which are within the sterilization temperature band for the operating cycle under test. The recorder should
6.12 The detailed specification for a test recorder will depend upon the range of sterilizers with which it is to be used. In all cases the recorder and its sensors should be capable of measuring cycle variables to considerably greater accuracy than the instruments fitted to the sterilizer.
6.13 The accuracy with which a variable can be read from the recorder will be affected not only by the sources of error discussed above (see paragraph 6.4), but also by the precision of the calibration, the scale range selected, the integration time, the sampling interval and the intrinsic accuracy of the recorder itself. Digital recorders will invariably register measurements to a precision greater than the accuracy of the system as a whole, and care should be taken in interpreting such measurements.
6.14 The intrinsic accuracies quoted by recorder manufacturers are measured under controlled reference conditions and do not include errors from temperature, pressure or humidity sensors. Temperature measurement errors due to ambient temperature changes should not exceed 0.04°C per °C rise.
6.15 The scale ranges should include the expected maximum and minimum values of the cycle variables throughout the operating cycle, with sufficient leeway to accommodate any deviations resulting from a malfunctioning sterilizer. (Note that in some sterilizers the temperature in the chamber free space will considerably exceed the upper limit of the sterilization
temperature band for a short time at the start of the plateau period.) 6.16 The most critical stage of the operating cycle is the plateau period (the
equilibration time plus the holding time, see paragraph 7.11) during which the load becomes exposed to the sterilization conditions. It is during this period that the values of the cycle variables are at their most critical and the recorder should be capable of measuring them to sufficient accuracy to confirm that the sterilization conditions have been attained. The criteria are as follows:
a. for digital recorders, the sampling interval should be short enough for the holding time to contain at least 180 independent measurements in each recording channel. This corresponds to a sampling interval of one second for the shortest holding time (3 minute, high-temperature steam sterilizers) and 40 seconds for the longest (120 minute, dry-heat
sterilizers). For pen recorders, the chart speed should be fast enough to allow fluctuations on that scale to be clearly resolved. The duration of the holding time should be measurable to within 1%;
b. the integration time of the recorder (the response time) should be short enough to enable the output to follow significant fluctuations in the cycle variables and to ensure that successive measurements are independent of each other, It should not be longer than the sampling interval;
c. the width of the sterilization temperature band (see paragraph 7.14) varies from 3°C (high-temperature steam sterilizers) to 10°C (dry-heat sterilizers). The recorder must be accurate enough to show clearly whether the measured temperatures are within the band or not. For all
the types of sterilizer covered by this SHTM, the repeatability of the recorder should be ± 0.25°C or better, and the limit of error of the complete measurement system (including sensors) should be no more than 0.5°C;
d. for pressure measurement, the limit of error should be no more than 0.5% of the absolute pressure during the plateau period;
e. for humidity measurement, the limiting factor is likely to be the performance of the sensor (see paragraph 6.47).
6.17 A recorder chosen to meet these criteria for the plateau period will have more than enough performance for the preceding and following stages of the operating cycle.
6.18 If a fluid sterilizer is fitted with an F0 integrating system (see Part 4 for a
discussion of the use of F0 in controlling operating cycles), then the recorder
should be capable of computing and printing values of F0 for each channel
with integration times no greater than 2 s (see BS 3970: Part 2).