<s>
A	O
tachometer	B-Device
(	O
revolution-counter	B-Device
,	O
tach	B-Device
,	O
rev-counter	O
,	O
RPM	O
gauge	O
)	O
is	O
an	O
instrument	O
measuring	O
the	O
rotation	O
speed	O
of	O
a	O
shaft	O
or	O
disk	O
,	O
as	O
in	O
a	O
motor	O
or	O
other	O
machine	O
.	O
</s>
<s>
Essentially	O
the	O
words	O
tachometer	B-Device
and	O
speedometer	O
have	O
identical	O
meaning	O
:	O
a	O
device	O
that	O
measures	O
speed	O
.	O
</s>
<s>
The	O
first	O
tachometer	B-Device
was	O
described	O
by	O
Bryan	O
Donkin	O
in	O
a	O
paper	O
to	O
the	O
Royal	O
Society	O
of	O
Arts	O
in	O
1810	O
for	O
which	O
he	O
was	O
awarded	O
the	O
Gold	O
medal	O
of	O
the	O
society	O
.	O
</s>
<s>
The	O
first	O
mechanical	O
tachometers	B-Device
were	O
based	O
on	O
measuring	O
the	O
centrifugal	O
force	O
,	O
similar	O
to	O
the	O
operation	O
of	O
a	O
centrifugal	O
governor	O
.	O
</s>
<s>
Tachometers	B-Device
or	O
revolution	B-Device
counters	I-Device
on	O
cars	O
,	O
aircraft	O
,	O
and	O
other	O
vehicles	O
show	O
the	O
rate	O
of	O
rotation	O
of	O
the	O
engine	O
's	O
crankshaft	B-Language
,	O
and	O
typically	O
have	O
markings	O
indicating	O
a	O
safe	O
range	O
of	O
rotation	O
speeds	O
.	O
</s>
<s>
On	O
analogue	O
tachometers	B-Device
,	O
speeds	O
above	O
maximum	O
safe	O
operating	O
speed	O
are	O
typically	O
indicated	O
by	O
an	O
area	O
of	O
the	O
gauge	O
marked	O
in	O
red	O
,	O
giving	O
rise	O
to	O
the	O
expression	O
of	O
"	O
redlining	B-Operating_System
"	O
an	O
engine	O
—	O
revving	O
the	O
engine	O
up	O
to	O
the	O
maximum	O
safe	O
limit	O
.	O
</s>
<s>
Diesel	O
engines	O
with	O
traditional	O
mechanical	O
injector	O
systems	O
have	O
an	O
integral	O
governor	O
which	O
prevents	O
over-speeding	O
the	O
engine	O
,	O
so	O
the	O
tachometers	B-Device
in	O
vehicles	O
and	O
machinery	O
fitted	O
with	O
such	O
engines	O
sometimes	O
lack	O
a	O
redline	B-Operating_System
.	O
</s>
<s>
In	O
vehicles	O
such	O
as	O
tractors	O
and	O
trucks	O
,	O
the	O
tachometer	B-Device
often	O
has	O
other	O
markings	O
,	O
usually	O
a	O
green	O
arc	O
showing	O
the	O
speed	O
range	O
in	O
which	O
the	O
engine	O
produces	O
maximum	O
torque	B-General_Concept
,	O
which	O
is	O
of	O
prime	O
interest	O
to	O
operators	O
of	O
such	O
vehicles	O
.	O
</s>
<s>
Tractors	O
fitted	O
with	O
a	O
power	O
take-off	O
(	O
PTO	O
)	O
system	O
have	O
tachometers	B-Device
showing	O
the	O
engine	O
speed	O
needed	O
to	O
rotate	O
the	O
PTO	O
at	O
the	O
standardized	O
speed	O
required	O
by	O
most	O
PTO-driven	O
implements	O
.	O
</s>
<s>
To	O
save	O
fitting	O
a	O
second	O
dial	O
,	O
the	O
vehicle	O
's	O
tachometer	B-Device
is	O
often	O
marked	O
with	O
a	O
second	O
scale	O
in	O
units	O
of	O
speed	O
.	O
</s>
<s>
Tractors	O
with	O
multiple	O
'	O
road	O
gears	O
 '	O
often	O
have	O
tachometers	B-Device
with	O
more	O
than	O
one	O
speed	O
scale	O
.	O
</s>
<s>
Aircraft	O
tachometers	B-Device
have	O
a	O
green	O
arc	O
showing	O
the	O
engine	O
's	O
designed	O
cruising	O
speed	O
range	O
.	O
</s>
<s>
In	O
older	O
vehicles	O
,	O
the	O
tachometer	B-Device
is	O
driven	O
by	O
the	O
RMS	O
voltage	O
waves	O
from	O
the	O
low	O
tension	O
(	O
LT	O
contact	O
breaker	O
)	O
side	O
of	O
the	O
ignition	O
coil	O
,	O
while	O
on	O
others	O
(	O
and	O
nearly	O
all	O
diesel	O
engines	O
,	O
which	O
have	O
no	O
ignition	O
system	O
)	O
engine	O
speed	O
is	O
determined	O
by	O
the	O
frequency	O
from	O
the	O
alternator	B-Algorithm
tachometer	B-Device
output	O
.	O
</s>
<s>
Tachometers	B-Device
driven	O
by	O
a	O
rotating	O
cable	O
from	O
a	O
drive	O
unit	O
fitted	O
to	O
the	O
engine	O
(	O
usually	O
on	O
the	O
camshaft	B-Application
)	O
exist	O
-	O
usually	O
on	O
simple	O
diesel-engined	O
machinery	O
with	O
basic	O
or	O
no	O
electrical	O
systems	O
.	O
</s>
<s>
On	O
recent	O
EMS	B-Device
found	O
on	O
modern	O
vehicles	O
,	O
the	O
signal	O
for	O
the	O
tachometer	B-Device
is	O
usually	O
generated	O
from	O
an	O
ECU	B-Device
which	O
derives	O
the	O
information	O
from	O
either	O
the	O
crankshaft	B-Language
or	O
camshaft	B-Application
speed	O
sensor	O
.	O
</s>
<s>
Tachometers	B-Device
are	O
used	O
to	O
estimate	O
traffic	O
speed	O
and	O
volume	O
(	O
flow	O
)	O
.	O
</s>
<s>
A	O
vehicle	O
is	O
equipped	O
with	O
the	O
sensor	O
and	O
conducts	O
"	O
tach	B-Device
runs	O
"	O
which	O
record	O
the	O
traffic	O
data	O
.	O
</s>
<s>
However	O
,	O
because	O
of	O
the	O
expense	O
,	O
spacing	O
(	O
a	O
lower	O
density	O
of	O
loop	O
detectors	O
diminishes	O
data	O
accuracy	O
)	O
,	O
and	O
relatively	O
low	O
reliability	O
of	O
loop	O
detectors	O
(	O
often	O
30%	O
or	O
more	O
are	O
out	O
of	O
service	O
at	O
any	O
given	O
time	O
)	O
,	O
tach	B-Device
runs	O
remain	O
a	O
common	O
practice	O
.	O
</s>
<s>
Speed	O
sensing	O
devices	O
,	O
termed	O
variously	O
"	O
wheel	O
impulse	O
generators	O
"	O
(	O
WIG	O
)	O
,	O
pulse	O
generators	O
,	O
speed	O
probes	O
,	O
or	O
tachometers	B-Device
are	O
used	O
extensively	O
in	O
rail	O
vehicles	O
.	O
</s>
<s>
Common	O
types	O
include	O
opto-isolator	O
slotted	O
disk	O
sensors	O
and	O
Hall	B-Algorithm
effect	I-Algorithm
sensors	I-Algorithm
.	O
</s>
<s>
Hall	B-Algorithm
effect	I-Algorithm
sensors	I-Algorithm
typically	O
use	O
a	O
rotating	O
target	O
attached	O
to	O
a	O
wheel	O
,	O
gearbox	O
or	O
motor	O
.	O
</s>
<s>
The	O
slotted	O
disk	O
is	O
typically	O
sandwiched	O
between	O
two	O
circuit	O
boards	O
containing	O
a	O
photo-diode	O
,	O
photo-transistor	O
,	O
amplifier	O
,	O
and	O
filtering	O
circuits	O
which	O
produce	O
a	O
square	O
wave	O
pulse	O
train	O
output	O
customized	O
to	O
the	O
customers	O
voltage	O
and	O
pulses	O
per	O
revolution	O
requirements	O
.	O
</s>
<s>
Strictly	O
,	O
such	O
devices	O
are	O
not	O
tachometers	B-Device
since	O
they	O
do	O
not	O
provide	O
a	O
direct	O
reading	O
of	O
the	O
rotational	O
speed	O
of	O
the	O
disk	O
.	O
</s>
<s>
A	O
weakness	O
of	O
systems	O
that	O
rely	O
on	O
wheel	O
rotation	O
for	O
tachometry	B-Device
and	O
odometry	O
is	O
that	O
the	O
train	O
wheels	O
and	O
the	O
rails	O
are	O
very	O
smooth	O
and	O
the	O
friction	O
between	O
them	O
is	O
low	O
,	O
leading	O
to	O
high	O
error	O
rates	O
if	O
the	O
wheels	O
slip	O
or	O
slide	O
.	O
</s>
<s>
To	O
compensate	O
for	O
this	O
,	O
secondary	O
odometry	O
inputs	O
employ	O
Doppler	B-Algorithm
radar	I-Algorithm
units	O
beneath	O
the	O
train	O
to	O
measure	O
speed	O
independently	O
.	O
</s>
<s>
In	O
analogue	O
audio	O
recording	O
,	O
a	O
tachometer	B-Device
is	O
a	O
device	O
that	O
measures	O
the	O
speed	O
of	O
audiotape	O
as	O
it	O
passes	O
across	O
the	O
head	O
.	O
</s>
<s>
On	O
most	O
audio	O
tape	O
recorders	O
the	O
tachometer	B-Device
(	O
or	O
simply	O
"	O
tach	B-Device
"	O
)	O
is	O
a	O
relatively	O
large	O
spindle	O
near	O
the	O
ERP	O
head	O
stack	O
,	O
isolated	O
from	O
the	O
feed	O
and	O
take-up	O
spindles	O
by	O
tension	O
idlers	O
.	O
</s>
<s>
On	O
many	O
recorders	O
the	O
tachometer	B-Device
spindle	O
is	O
connected	O
by	O
an	O
axle	O
to	O
a	O
rotating	O
magnet	O
that	O
induces	O
a	O
changing	O
magnetic	O
field	O
upon	O
a	O
Hall	O
effect	O
transistor	B-Application
.	O
</s>
<s>
The	O
tape	O
recorder	O
's	O
drive	O
electronics	O
use	O
signals	O
from	O
the	O
tachometer	B-Device
to	O
ensure	O
that	O
the	O
tape	O
is	O
played	O
at	O
the	O
proper	O
speed	O
.	O
</s>
<s>
When	O
the	O
tach	B-Device
signal	O
and	O
the	O
reference	O
signal	O
match	O
,	O
the	O
tape	O
transport	O
is	O
said	O
to	O
be	O
"	O
at	O
speed.	O
"	O
</s>
<s>
This	O
effect	O
is	O
called	O
a	O
wow-and-flutter	O
,	O
and	O
a	O
modern	O
,	O
tachometer-regulated	O
cassette	O
deck	O
has	O
a	O
wow-and-flutter	O
of	O
0.07	O
%	O
.	O
</s>
<s>
Tachometers	B-Device
are	O
acceptable	O
for	O
high-fidelity	O
sound	O
playback	O
,	O
but	O
not	O
for	O
recording	O
in	O
synchronization	O
with	O
a	O
movie	B-Application
camera	I-Application
.	O
</s>
<s>
Tachometer	B-Device
signals	O
can	O
be	O
used	O
to	O
synchronize	O
several	O
tape	O
machines	O
together	O
,	O
but	O
only	O
if	O
in	O
addition	O
to	O
the	O
tach	B-Device
signal	O
,	O
a	O
directional	O
signal	O
is	O
transmitted	O
,	O
to	O
tell	O
slave	O
machines	O
in	O
which	O
direction	O
the	O
master	O
is	O
moving	O
.	O
</s>
