Timers General Information
There are many types of laboratory timers on the market today. For
purposes of convenience we have classified them into the following groups: Bench Top
Timers, Pocket Timers, Stopwatches, and Time Controllers. Although several configurations
exist, all timers share some of the same functions.
BENCH TOP TIMERS
Bench timers should have the minimum following characteristics: one,
a loud alarm (to be heard over normal laboratory noise); and two, a display that is easily
viewable (for quick spot checks or elapsed time). In the past, large analog timers were
common on the bench. Recently, digital timers have become more accepted. The older analog
timers had the advantage of a large dial that could be read from far away, and were easily
set by manually turning the 'hands' to the desired start time. The analogs, though, have
very important drawbacks—one is repeatability of the set time, and the other is
general accuracy.
POCKET TIMERS
Pocket timers are the most popular timers in the laboratory. They
are small, portable, and accurate. There are configurations to fit almost every test
requirement. Each of Sper Scientific's certified pocket timers has a 5-year warranty and
comes complete with batteries.
STOPWATCHES
Today's digital stopwatches are far more accurate than the old
spring wound models. They are highly versatile pieces of test equipment that can count up,
count down, do split and lap timing, even calculate beats, strokes, or breaths per minute.
Some have memory functions that will log up to 100 data points. Others are made to
function in high humidity or harsh field conditions.
TIME CONTROLLERS
A "Time Controller" is a device used to turn on and off
other equipment without an operator or technician present. In the past, most time
controllers were mechanical, with arms that would move around a dial on a motor. The arm
would rotate and physically contact a switch turning the equipment on or off. Today, you
can find controllers which are electronic and programmable, giving you many on and off
settings per day, or even selected days of the week.
If you have a time sensitive process, knowing the exact deviation
from 0 sec/day can be very important. For example, if you have a process that requires a
pump to pump a chemical for exactly 10 hours and then shut off, you can accurately
calculate the exact amount of time the pump should run, if you know the deviation from '0
sec/day' of the time controller you are using, thus increasing your accuracy and
repeatability.
QUARTZ VS. MECHANICAL
Normally timers with quartz movements are more accurate than ones
with mechanical movements. The oscillation of the quartz crystal is more consistant than
the tension in a spring. All Sper Scientific Timers have quartz movements, are traceable
to the Prangins Atomic clock, and NIST. All Sper Timers, certified and non-certified, meet
a minimum requirement for accuracy of ±3 seconds per day. (See the table below.)
RESOLUTION
Resolution is the smallest division to which the timer will record.
For example, 1.0, 0.1, or 0.01 seconds, etc. In timers today, 0.01 second resolution is
found primarily in count up stopwatches. It can sometimes be found in count down, count up
timers but resolutions of 1.0 and 0.1 seconds are more common.
ACCURACY
Accuracy is usually reported as the %, or actual gain or loss from
"0" seconds per day.
For example, a timer's accuracy can be reported as
"+0.15 second/day." What this means is that this particular timer will gain 0.15
seconds in a 24 hour period. Knowing the deviation means the exact time can then be
calculated and a correction factor applied to the reading. If a timer has the accuracy
stated above and a timed process takes a period of 27 hours to complete, the actual, or
real, elapsed time is calculated by the following formula. T = ET + ((ET x C
seconds)/24hrs)
Where T=Actual time, ET=Elapsed time on timer, and C=the
deviation in seconds from "0."
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Therefore:
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T = 27 hrs + ((27hrs x 0.15 seconds)/24hrs)
T = 27 hours 0.16875 second
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Time Deviation
Chart
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± Deviation
From "0"
(sec/day)
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Deviation
In minutes
at One Day
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Deviation
In Minutes
at One Week
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Deviation
In Minutes
at One Month
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Deviation
In Minutes
at Six Months
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7.00
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0.13
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0.89
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3.55
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21.29
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|
6.50
|
0.12
|
0.82
|
3.30
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19.77
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|
6.00
|
0.11
|
0.76
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3.04
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18.25
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|
5.50
|
0.10
|
0.70
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2.79
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16.73
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5.00
|
0.09
|
0.63
|
2.53
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15.21
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|
4.50
|
0.08
|
0.57
|
2.28
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16.39
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|
4.00
|
0.07
|
0.51
|
2.03
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12.17
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|
3.50
|
0.06
|
0.44
|
1.77
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10.65
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3.00
|
0.05
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0.38
|
1.52
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9.13
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|
2.50
|
0.05
|
0.32
|
1.27
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7.60
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|
2.00
|
0.04
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0.25
|
1.01
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6.06
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|
1.50
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0.03
|
0.19
|
0.76
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4.58
|
|
1.00
|
0.02
|
0.13
|
0.51
|
3.04
|
|
0.50
|
0.01
|
0.06
|
0.25
|
4.52
|
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
|
-0.50
|
-0.01
|
-0.06
|
-0.25
|
-1.52
|
|
-1.00
|
-0.02
|
-0.13
|
-0.51
|
-3.04
|
|
-1.50
|
-0.03
|
-0.19
|
-0.76
|
-4.56
|
|
-2.00
|
-0.04
|
-0.25
|
-1.01
|
-5.08
|
|
-2.50
|
-0.05
|
-0.32
|
-1.07
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-7.60
|
|
-3.00
|
-0.05
|
-0.38
|
-1.52
|
-9.13
|
|
-3.50
|
-0.06
|
-0.44
|
-1.77
|
-10.65
|
|
-4.00
|
-0.07
|
-0.51
|
-2.03
|
-12.17
|
|
-4.50
|
-0.08
|
-0.57
|
-2.28
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-13.69
|
|
-5.00
|
-0.09
|
-0.63
|
-2.53
|
-15.21
|
|
-5.50
|
-0.10
|
-0.70
|
-2.79
|
-16.73
|
|
-6.00
|
-0.11
|
-0.76
|
-3.04
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-18.25
|
|
-6.50
|
-0.12
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-0.82
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-3.30
|
-19.77
|
|
-7.00
|
-0.13
|
-0.89
|
-3.55
|
-21.29
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