Hiệu chuẩn Stop watch và Timer

Hiệu chuẩn Stop watch và Timer

Hiệu chuẩn Stop watch và Timer

CALIBRATION METHODS
There are three generally accepted methods for calibrating a stopwatch or
timer: the Direct Comparison Method, which compares the DUT’s display to
a traceable time interval standard; the Totalize Method, which requires a
synthesized signal generator, a counter, and a traceable frequency standard;
and the Time Base Method, which compares the frequency of the DUT’s time
base to a traceable frequency standard.
[8]
All three methods are summarized
in Table 4, and discussed in detail below.
Table 4:Comparison of Calibration Methods
Method
Direct Time Base
Area Comparison Totalize Measurement
Equipment Requirements Best Better Better
Speed Good Better Best
Uncertainty Good Good Best
Applicability Good Best Better
4.A. Direct Comparison Method
The Direct Comparison Method is the most common method used to
calibrate stopwatches and timers. It requires a minimal amount of
equipment, but has larger measurement uncertainties than the other
methods. This section describes the references used for this type of
calibration and the calibration procedure.
4.A.1.References for Direct Comparison Method
The Direct Comparison Method requires a traceable time-interval
reference. This reference is usually an audio time signal, but in some
cases, a traceable time display can be used. The audio time signals
are usually obtained with a shortwave radio or a telephone. Since time
interval is being measured and not absolute time, the fixed signal delay
from the source to the user is not important as long as it remains
relatively constant during the calibration process. A list of traceable
audio time sources is provided in Table 5.
Calibration Methods ◆
◆ Stopwatch and Timer Calibrations
22
Table 5: Traceable Audio Time Signals
National Radio
Metrology Telephone Call Broadcast
Institute (NMI) Location Numbers Letters Frequencies
National Institute Fort Collins, (303) 499-7111* WWV 2.5, 5, 10,
of Standards and Colorado, 15, 20 MHz
Technology (NIST) United States
National Institute Kauai, (808) 335-4363* WWVH 2.5, 5, 10,
of Standards and Hawaii, 15 MHz
Technology (NIST) United States
United States Washington, (202) 762-1401*
Naval Observatory DC, (202) 762-1069* —— (USNO) United States
United States Colorado Springs, (719) 567-6742*
Naval Observatory Colorado,
(USNO) United States
National Research Ottawa, (613) 745-1576** CHU 3.33, 7.335,
Council (NRC) Ontario, (English language) 14.67 MHz
Canada (613) 745-9426**
(French language)
Centro Nacional Querétaro, (442) 215-39-02* —— de Metrologia México (442) 211-05-06
† (CENAM) (442) 211-05-07
††
(442) 211-05-08

Time announcements are
in Spanish, a country
code must be dialed to
access these numbers
from the United States,
see www.cenem.mx
for more information.
* Coordinated Universal Time (UTC)
** Eastern Time

Central Time
††
Mountain Time

Pacific Time
23
Please note that the local “time and temperature” telephone services
are not considered traceable and should not be used. In all cases,
use only sources that originate from a national metrology institute, such
as those listed in Table 5 for the United States, Canada, and Mexico.
The following sections briefly describe the various radio and telephone
time signals and provide information about the types of clock displays
that can and cannot be used.
Audio Time Signals Obtained by Radio —The radio signals listed
in Table 5 include a voice announcement of UTC and audio ticks that
indicate individual seconds. WWV, the most widely used station,
features a voice announcement of UTC occurring about 7.5 s before
the start of each minute. The beginning of the minute is indicated by
a 1500 Hz tone that lasts for 800 ms. Each second is indicated by
1000 Hz tones that last for 5 ms. The best way to use these broadcasts
is to start and stop the stopwatch when the beginning of the minute
tone is heard.
The reception of WWV, WWVH, and CHU requires a shortwave
receiver. A typical general coverage shortwave receiver provides
continuous coverage of the spectrum from about 150 kHz, which is
below the commercial AM broadcast band, to 30 MHz. These
receivers allow the reception of WWV, WWVH, and CHU on all
Figure 10.Portable shortwave radio receiver for reception of audio time signals.
Calibration Methods ◆
◆ Stopwatch and Timer Calibrations
24
available frequencies. The best shortwave receivers are designed
to work with large outdoor antennas, with quarter-wavelength or
half-wavelength dipole antennas often providing the best results.
However, in the United States, adequate reception of at least one
station can usually be obtained with a portable receiver with a whip
antenna, such as the one shown in Figure 10. This type of receiver
typically costs a few hundred dollars or less.
The reason that WWV, WWVH, and CHU broadcast on multiple
frequencies is due to changing atmospheric conditions, not all of
the frequencies will be available at all times. In many cases, only one
frequency will be receivable, so you might have to tune the receiver
to several different frequencies before finding a usable signal. In the
case of WWV, 10 MHz and 15 MHz are probably the best choices for
daytime reception, unless you are within 1000 km of the Fort Collins,
Colorado station, in which case 2.5 MHz might also suffice. Unless
your receiver is near the station, the 5 MHz signal will probably be
easiest to receive at night.
[9]
Audio Time Signals Obtained by Telephone — The telephone time
signals for NIST radio stations WWV and WWVH are simulcasts of
the radio broadcasts, and time announcements are made in UTC once
per minute. The length of the phone call is typically limited to 3 min.
The format of the other broadcasts varies. The USNO phone numbers
broadcast UTC at 5 s or 10 s intervals. The NRC phone number
broadcasts Eastern Time at 10 s intervals, and CENAM offers
separate phone numbers for UTC and the local time zones of Mexico.
Time Displays — It might be tempting to use a time display from a
radio controlled clock or a web site as a reference for stopwatch or
timer calibration. As a general rule, however, these displays are not
acceptable for establishing traceability. The reason is that the displays
are only synchronized periodically, and in between synchronizations,
they use a free running local oscillator whose frequency offset is usually
unknown. An unknown uncertainty during any comparison breaks the
traceability chain. For example, a low-cost, radio-controlled clock that
receives a 60 kHz signal from NIST radio station WWVB is usually
synchronized only once per day. In between synchronizations, each
“tick” of the clock originates from a local quartz oscillator whose
uncertainty is unknown and is probably of similar or lesser quality than
the oscillator inside the device under test. The NIST web clock located
at http://nist.time.govpresents similar problems. It synchronizes to
25
UTC(NIST) every 10 min if the web browser is left open. However,
between synchronizations it keeps time using the computer’s clock,
which is usually of very poor quality, and whose uncertainty is
generally not known. In contrast, each “tick” of an audio broadcast
from WWV originates from NIST and is synchronized to UTC.
Therefore, WWV audio keeps the traceability chain intact.
There are a few instances where a time display can be used to
establish traceability. One example would be a display updated each
second by a 1 Hz signal, such as a WWVB receiver or a pulse from
a Global Positioning System (GPS) satellite receiver. In this case,
if the traceable input signal were not available, the display would stop
updating. Therefore, if the display is updating, then it is clear that
each “tick” is originating from a traceable source. However, nearly all
receivers have the capability to “coast” and keep updating their display
even when no GPS signal is available. There must be an indicator on
the unit to tell whether it is locked to the GPS signal or is in “coast”
mode. If the receiver is in “coast” mode, it should not be used as a
calibration reference.
Another example of a usable time display would be a digital time
signal obtained from a telephone line, such as signals from the NIST
Automated Computer Time Service (ACTS).[9]
With an analog
modem and simple terminal software (configured for 9600 baud,
8 data bits, 1 stop bit, and no parity), you can view time codes on a
computer screen by dialing (303) 494-4774, and you can use these
codes as a reference in the same way that you would use an audio time
announcement from WWV. However, the length of a single telephone
call is limited to just 48 s. In theory, Internet time codes could be used
the same way, although the transmission delays through the network
can vary by many milliseconds from second to second. For this reason,
the currently available Internet signals should not be used as
measurement references.
4.A.2.Calibration Procedure for Direct Comparison Method
Near the top of the hour, dial the phone number (or listen to the radio
broadcast) of a traceable source of precise time. Start the stopwatch
at the signal denoting the hour, and write down the exact time. After
a suitable time period (depending on the accuracy of the stopwatch),
listen to the time signal again, stop the stopwatch at the sound of
the tone, and write down the exact stopping time. Subtract the start
Calibration Methods ◆
◆ Stopwatch and Timer Calibrations
26
time from the stop time to get the time interval, and compare this time
interval to the time interval displayed by the stopwatch. The two time
intervals must agree to within the uncertainty specifications of the
stopwatch for a successful calibration. Otherwise, the stopwatch
needs to be adjusted or rejected.
Advantages of the Direct Comparison Method —This method is
relatively easy to perform and, if a telephone is used, does not require
any special test equipment or standards. It can be used to calibrate
all types of stopwatches and many types of timers, both electronic
and mechanical.
Disadvantages of the Direct Comparison Method —
The operator’s start/stop reaction time is a significant part of the
total uncertainty, especially for short time intervals. Table 6 shows the
contribution of a 300 ms variation in human reaction time to the overall
measurement uncertainty, for measurement periods ranging from 10 s
to 1 day.
Table 6: The Contribution of 300 ms Variation in Reaction Time to
the Measurement Uncertainty
Hours Minutes Seconds Uncertainty (%)
10 3
1 60 0.5
10 600 0.05
30 1800 0.005 6
1 60 3600 0.001 67
2 120 7200 0.004 2
6 360 21 600 0.001 4
12 720 43 200 0.000 69
24 1440 86 400 0.000 35
27
As Table 6 illustrates, the longer the time interval measured, the less
impact the operator’s start/stop uncertainty has on the total uncertainty
of the measurement. Therefore, it is better to measure for as long a
time period as practical to reduce the uncertainty introduced by the
operator and to meet the overall measurement requirement.
To get a better understanding of the numbers in Table 6, consider a
typical stopwatch calibration where the acceptable measurement
uncertainty is 0.02 % (2 ×10
–4
). If the variation in human reaction
time is known to be 300 ms for the Direct Comparison Method, a
time interval of at least 1500 s is needed to reduce the uncertainty
contributed by human reaction time to 0.02 %. However, if a 1500 s
interval were used, we would be measuring the variation in human
reaction time, and nothing else. Our goal is to measure the performance
of the DUT, and to make human reaction time an insignificant part of
the measurement. Therefore, at the very least, we should extend the
time interval by at least a factor of 10, to 15 000 s. To be “safe,”
NIST Handbook 105-5[3]
and other references refer to an acceptable
time offset of 2 s in 3 h (10 800 s) for a stopwatch to be declared
within tolerance. This is a long enough time interval to exceed the
0.02 % requirement, and to ensure that the uncertainty of human
reaction time is insignificant. Keep in mind that the actual length of
the time interval can vary according to each laboratory’s procedures.
However, it must be long enough to meet the uncertainty requirements
for the device being tested. If your uncertainty requirement is 0.01 %
or lower, the Direct Comparison Method might not be practical.
4.B. Totalize Method
The Totalize Method partially eliminates the measurement uncertainty
from human reaction time, but it requires a calibrated signal generator
and a universal counter. The counter is set to TOTALIZE, with a
manual gate. A signal from a calibrated synthesized signal generator is
connected to the counter’s input, and the laboratory’s primary frequency
standard is used as the external time base for the synthesizer (Figure 11).
An external reference is not needed for the counter because the operator
is controlling the counter’s gate time. The frequency should have a
period at least one order of magnitude smaller than the resolution of the
stopwatch. For example, if the stopwatch has a resolution of 0.01 s
(10 ms), use a 1 kHz frequency (1 ms period). This provides the counter
with one more digit of resolution than the stopwatch.
Calibration Methods ◆
◆ Stopwatch and Timer Calibrations
28
Figure 11.Block diagram of the Totalize Method.
To begin the measurement, start the stopwatch and manually open the
gate of the counter at the same time. One way to do this is by rapidly
pressing the start-stop button of the stopwatch against the start button on
the counter (Figure 12). Another method is to press the start/stop button
of the stopwatch with one hand and simultaneously press the start/stop
button of the counter. After a suitable period of time (determined by the
calibration requirements of the stopwatch or timer being calibrated), use
the same method to stop the stopwatch, and simultaneously close the gate
of the counter.
Once the counter and stopwatch are stopped, compare the two readings.
Use the equation ∆t/T to get the results, where ∆t is the difference
between the counter and stopwatch displays, and T is the length of the
measurement run. For example, if ∆t = 100 ms and T = 1 h, the time
uncertainty is 0.1 s /3600 s or roughly 2.8 ×10
–5
(0.0028 %).
Advantages of the Totalize Method —When using the stopwatch’s
start/stop button to open and close the counter’s gate, this method
partially eliminates human reaction time, and, therefore, has a lower
measurement uncertainty than the direct comparison method.
Disadvantages of the Totalize Method —This method requires
more equipment than the direct comparison method, including a
calibrated signal generator and counter.
Laboratory Frequency
Reference Standard
Frequency Synthesizer
Universal Counter
Connect to
Synthesizer
External
Timebase Input
29
Figure 12.Using the start-stop button of the stopwatch to start the counter.
4.C.Time Base Method
The Time Base Method is the preferred measurement method for
stopwatch and timer calibrations, since it introduces the least amount of
measurement uncertainty. Because the DUT’s time base is measured
directly, the calibrating technician’s response time is not a factor.
The exact method of measuring the stopwatch’s time base depends upon
the type of stopwatch or timer being calibrated. If the unit has a quartz
crystal time base, an inductive or acoustic pickup is used to monitor the
stopwatch’s 32 768 Hz time base frequency on a calibrated frequency
counter (the pickup is fed into an amplifier to boost the signal strength).
If the unit is an older LED-type stopwatch, the frequency is usually
4.19 MHz. An inductive pickup can even be used to sense the stepping
motor frequency of analog mechanical stopwatches, or the “blink rate”
of a digital stopwatch display. Or an acoustic pickup can be used to
measure the “tick” of a mechanical stopwatch.
References for the Time Base Method —The reference for a time
base calibration is the time base oscillator of the measuring instrument.
Calibration Methods ◆
◆ Stopwatch and Timer Calibrations
30
For example, if a frequency counter is used, the measurement reference
is the time base oscillator of the frequency counter. In order to establish
traceability, the frequency counter time base must have been recently
calibrated and certified. However, a better solution is to have the
laboratory maintain a traceable 5 MHz or 10 MHz signal that can be
used as an external time base for the frequency counter and all other
test equipment. If an external time base is used and its measurement
uncertainty is known, it is unnecessary to calibrate the internal time
base oscillator.
Calibration Procedure for the Time Base Method —Two methods
of calibrating a stopwatch time base are described below. One uses a
commercially available measurement system; the other uses a frequency
counter with an acoustic pickup. Note that neither calibration method
requires opening the case of the stopwatch or timer. Keep in mind
that you should never disassemble a stopwatch or timer and attempt to
measure the time base frequency by making a direct electrical connection.
The crystal oscillators in these units are very small, low-power devices.
Their frequency can dramatically change if they are disturbed or loaded
down by the impedance of a frequency counter, and, in some cases, they
can even be destroyed by incorrect electrical connections.
1) Using a Commercial Time Base Measurement System —
One example of a commercially available time base measurement
system (Figure 13) is described here for the purposes of illustration.
This unit measures the frequency offset of the time base oscillator,
and displays seconds per day, or seconds per month. This same
function could be performed with a sensor (acoustic or inductive
pickup), a frequency counter, and the conversion formula described
in the next section.
Figure 13.Time base measurement system for stopwatches and timers.
31
The commercial unit uses a 4.32 MHz time base oscillator as a
measurement reference. In a 2 s measurement period (the shortest
period used by the instrument), the oscillator produces 8 640 000 cycles,
a number equal to the number of 0.01 s intervals in one day. Therefore,
the instrument resolution is 0.01 s per day. The time base oscillator feeds
a programmable divider chain that allows increasing the measurement
period to intervals as long as 960 s. However, since the time base
frequency is divided to support longer intervals, the number of cycles
per interval remains the same, and the resolution is still limited to 0.01 s.
The time base measurement system shown in Figure 13 has several
different averaging times available, from 2 s up to 960 s. It is important
to select an averaging time long enough to get an accurate, stable reading.
When testing a 32 768 Hz quartz-crystal stopwatch, a 10 s to 12 s
averaging time is normally sufficient to obtain a reading stable to ±1 count.
Table 7 shows the effect averaging time has on the stability of the stopwatch
calibrator’s readings. When testing an older mechanical (Type II)
stopwatch, a longer averaging time of 120 s or more may be required.
Table 7:The Effect of Averaging Time on Stability
Variations Due to Averaging Time, 25 Readings
Averaging Time 2 s 10 s 12 s 20 s
Mean –0.03000 – 0.06000 –0.06000 – 0.0600
Standard Deviation of the Mean 0.0050 0.0012 0.0011 0.0006
Maximum 0.0000 – 0.05000 –0.05000 – 0.0600
Minimum –0.09000 – 0.07000 –0.07000 – 0.0700
Range 0.0900 0.0200 0.0200 0.0100
To support 0.01 s resolution, the instrument’s 4.32 MHz time base
oscillator must be calibrated to within 1.16 ×10
–7
. If the instrument is
calibrated to within specifications, the display uncertainty is ±0.05 s per
day (maximum time base frequency offset of about 6 ×10
–7
). In all
cases, the uncertainty of the time base oscillator relative to UTC must be
known in order to establish traceability.
Calibration Methods ◆
◆ Stopwatch and Timer Calibrations
32
The device under test can be a Type 1 stopwatch (both 32 768 Hz and
4.19 MHz devices can be measured), or a Type 2 mechanical stopwatch.
The 32 768 Hz signal is picked up acoustically with an ultrasonic sensor,
amplified, and then compared to the time base oscillator. A 1 Hz offset
in the 32 768 Hz signal translates to a time offset of about 2.6 s per day.
A capacitive sensor is used to detect the 4.19 MHz frequency of quartz
time base oscillators, an acoustic or inductive pickup is used to sense the
stepping motor frequency of analog mechanical stopwatches, and an
inductive pickup is used to sense the “blink rate” of digital stopwatches.
Front panel switches allow the operator to select the type of device
being tested, the measurement interval, and whether the time offset
should be displayed as seconds per day or seconds per month. Once
these parameters have been chosen, the device is measured by simply
positioning it on top of the sensor until a usable signal is obtained, waiting
for the measurement interval to be completed, and then recording the
number from the display. It is always a good idea to allow the stopwatch
calibrator to complete at least two complete measurement cycles before
recording a reading.
2) Using a Frequency Counter and an Acoustic Pickup — If an
acoustic pickup and amplifier are available, you can measure the
frequency of a stopwatch time base directly with a frequency counter.
The reading on the counter display can be used to calculate the
frequency offset using this equation:
where fmeasuredis the reading displayed by the frequency counter, and
f
nominalis the frequency labeled on the oscillator (the nominal frequency
it is supposed to produce).
If f
nominal
is 32 768 Hz, and f
measuredis 32 767.5 Hz, the frequency
offset is –0.5 / 32 768 or –1.5 ×10
–5
or –0.0015 %. To get time offset in
seconds per day, multiply the number of seconds per day (86 400) by the
frequency offset:
86 400 ×(–1.5 ×10
–5
) = –1.3 s per day
nominal
f
nominal
f
measured
f
f(offset)

=
33
which means the stopwatch can be expected to lose 1.3 s per day. You
might find it easier to note that a 1 Hz error in a 32 768 Hz device equates
to a time offset of about 2.64 s, since 86 400 / 32 768 = 2.64. Therefore,
a 2 Hz offset is about 5.3 s /day, a 3 Hz offset is about 7.9 s /day, and so on.
If the acceptable tolerance is 10 s /day, then you’ll know that 3 Hz is well
within tolerance.
As you can see from these results, even a low cost 8-digit frequency
counter will provide more measurement resolution than necessary when
measuring 32 768 Hz devices. The last digit on an 8-digit counter will
represent .001 Hz (1 mHz), and a 1 mHz frequency offset represents a
time offset of just 2.6 ms per day. Very few stopwatches or timers can
perform at this level.
Advantages of the Time Base Method —The Time Base Method
completely eliminates the uncertainty introduced by human reaction time.
The measurement uncertainty can be reduced by at least two orders of
magnitude when compared to the Direct Comparison Method, to 1 ×10
–6
or less. This method is also much faster. The measurement can often
be performed in a few seconds, as opposed to the several hours often
required for the Direct Comparison Method.
Disadvantages of the Time Base Method —This method requires
more equipment than the Direct Comparison Method, and it does not
easily work on some electrical, mechanical, or electro-mechanical units.
It also does not test the functionality of the stopwatch or timer, only the
time base. Function tests need to be performed separately by starting the
unit, letting it run for a while (a few minutes to a few hours, depending on
how the unit is used), and stopping the unit. If the unit appears to be
counting correctly, the displayed time interval will be accurate.
Calibration Methods ◆
◆ Stopwatch and Timer Calibrations

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