Dytran Instruments 3312A2T User manual

Dynamic Transducers and Systems
21592 Marilla St. • Chatsworth, CA 91311 • Phone [phone]
www.dytran.com • e-mail: [email protected]
OG3312A2T.docx
REV A, ECN 14100, 04/23/18
OPERATING GUIDE
MODEL 3312A2T
IEPE ACCELEROMETER
WITH TEDS ELECTRONIC DATA SHEET FUNCTION,
HERMETICALLY SEALED AND CASE ISOLATED
NOTE:
Model 3312A2T is an IEPE accelerometer with
100mV/g sensitivity and TEDS electronic data sheet
function. A built-in charge amplifier amplifies the
signal generated by the piezoceramic shear mode
seismic element to obtain the 100 mV/g sensitivity.
Model 3312A2T features hermetically sealed
construction and electrically isolated case for "off-
ground" performance. Hermeticity is obtained by
all-welded construction and glass-to-metal sealed
connector. Case ground isolation is by an
electrically isolated insert located in the base of the
instrument. Signal ground return is electrically
isolated from the mounting surface.
This guide contains:
1) Outline/installation drawing 127-3312A2T
2) Specifications, Model 3312A2T
NOTE: IEPE is an acronym for Integrated Electronics Piezoelectric types of low impedance voltage
mode sensors with built-in amplifiers operating from constant current sources over two wires. IEPE
instruments are compatible with other comparable systems labeled LIVM
TM
.

REV A ECN 14100 04/23/18
2
OPERATING INSTRUCTIONS
MODEL 3312A2T IEPE ACCELEROMETER
INTRODUCTION
The Dytran Model 3312A2T is a ceramic
shear element accelerometer featuring IEPE
operation and including the IEEE TEDS data sheet
function. The self-generating seismic element,
converts acceleration to an analogous voltage signal.
This very high impedance signal is fed to the gate of a
tiny on-board low-noise charge amplifier which
converts the signal to a low impedance voltage signal
allowing this instrument to drive long cables without
appreciable effect on sensitivity and frequency
response.
Simple constant current IEPE type power
units supply power to operate the integral IC and
separate the signal from the DC bias of the internal
amplifier. Power and signal are conducted over the
same two-wire coaxial cable from power unit to
sensor. Model 3312A2T also features signal ground
isolation from the mounting surface to avoid annoying
ground loops and is hermetically sealed for normal
operation in moist and dirty environments.
DESCRIPTION
The seismic mass, made from a very dense
tungsten alloy, is tightly preloaded against the ceramic
shear mode crystals by means of a special preload
screw under hundreds of pounds of force. This is so
there is absolutely no relative motion between mass,
crystals and base keeping the non-linearity low and
the natural frequency high.
The force from acceleration (vibration or
shock) acting upon the mounting base, is transferred
to the seismic mass through the crystals, stressing the
crystals in shear and producing a voltage exactly
analogous to the input acceleration. This voltage is
impressed across the gate of the MOSFET IC.
Refer to Figure 1 below. Figure 1 is a
simplified schematic of the typical
accelerometer/power unit system for Model 3312A2T.
Figure 1 - Electro-mechanical schematic,
accelerometer and power unit system.
When constant current from the IEPE power
unit is applied to the accelerometer amplifier source
terminal, the amplifier "turns on" at approx. +8 Volts
DC quiescent bias level. When the accelerometer
senses acceleration, the resultant signal is
superimposed upon this bias voltage.
In the power unit, in its simplest form, a
capacitor blocks the DC bias and allows the dynamic
signal voltage to be separated and brought out to an
"output" jack on the power unit. At this point the signal
may be connected directly to almost any type of
readout instrument such as DVM's, oscilloscopes,
data collectors, spectrum analyzers, etc. The
approximate 100 Ohm output impedance of the signal
allows the driving of long cables without adverse
effects on sensitivity or frequency response.
Referring to figure 1, the gate resistor R
serves to bias the gate of the IC to its proper
operating point and it also, in conjunction with total
crystal shunt capacitance C, forms a first order high-
pass filter which sets the low frequency response of
the accelerometer in accordance with the following
equation:
.16
f
-3db = ------------------ (eq.1)
RC
where:
f
-3db = lower -3db frequency (Hz)
R = resistance value R (Ohms)
C = total shunt capacitance C (Farads)
RC = discharge time constant TC (Seconds)
3312A2T

REV A ECN 14100 04/23/18 3
Equation 1 above, defines the low frequency
at which the accelerometer sensitivity will be 3db
down when compared to the reference sensitivity
measured at 100 Hz. The discharge time constant for
Model 3306A is 6.2 Sec. yielding a lower -3db
frequency of 0.03 Hz, from equation 1. Due to the
extremely long dischange time constant the sensor
might exhibit spiking during fast thermal transients
As rule of thumb, the lower -5% frequency is
three times the -3db frequency or 3 x .03 = 0.09 Hz.
INSTALLATION
(Refer to Outline/Installation drawing 127-
3312A2T) To install Model 3312A2T, is necessary to
prepare (or find) a flat mounting area of approximately
1 inch diameter. Ideally, the mounting surface should
be flat to .001 in. TIR. The flat mounting surface
ensures intimate contact between accelerometer base
and mounting surface for best high frequency
transmissibility, thus accuracy.
At the center of the mounting area, drill and
tap a 10-32 mounting port in accordance with
instructions on drawing 127-3312A2T. Clean the area
to remove all traces of machining chips, burrs, etc.
Next, thread the Mod. 6200 mounting stud
into the base of the 3312A2T. The stud should enter
easily and thread in up to the raised collar of the stud
by hand. This collar prevents the stud from bottoming
inside the tapped hole in the 3312A2T where it could
possibly cause stresses in the base structure which
could, in turn, cause anomalous behavior of the
accelerometer at higher frequencies.
After seating the stud, spread a light coating
of silicone grease, or other lubricant, on either of the
mating surfaces and thread the accelerometer/stud
combination into the tapped hole by hand, until the
accelerometer base seats against the mounting
surface. Check to see that the mating surfaces are
meeting properly, i.e., that they are meeting flush and
that there is not an angle formed between the two
surfaces indicating that they are not co-planar. If this
condition is observed, torquing the accelerometer
down will strain the base causing possible poor
frequency response and even erroneous reference
sensitivity. Inspect the perpendicularity of the tapped
hole.
If the hand-tight meeting between the two
surfaces is satisfactory, torque the 3312A2T to the
mating surface with 20 to 25 lb-inches of torque,
preferably measuring the torque with a torque wrench
torquing on the hex surface only.
Proper torque will ensure the best high
frequency performance from the instrument as well as
repeatability of sensitivity when mounting and
remounting.
Connect the cable (typically Model 6019AXX
to the accelerometer snugging up the threaded lock
ring tightly by hand.
NOTE: Do not use a pliers or vise grips on
the knurled lock ring. This could damage the
connector of the 3312A2T and/or the cable
connector.
To avoid stressing the cables which could
lead to early failure, especially under larger
excursions of the test object, it is good practice to tie
the cable down to a fixed surface near the mounting
area at a point approximately one inch from the
accelerometer.
If there is excessive motion between the
accelerometer and the nearest tie point, allow a strain
loop of cable to let relative motion occur without
stressing the cable.
Connect the other end of the cable to the
"Sensor" jack of the Dytran power unit (Models 4102,
4103, 4110, 4114, etc.) and switch the power on.
Observe the monitor voltmeter located at the
front panel of each of the power units. If the meter
reads in the mid-scale region, (labeled "Normal"), this
tells you that the cables, accelerometer and power
unit are functioning normally and you should be able
to proceed with the measurement.
Check for shorts in the cables and connectors
if the meter reads in the "Short" region. Check for
open cables or connections if the meter reads in the
"Open" area. In this manner, the meter becomes a
trouble shooting tool for the measurement system.
HIGH FREQUENCY RESPONSE
All piezoelectric accelerometers are basically
rigid spring mass systems, i.e., second order systems
with essentially zero damping. As a result, these
instruments will exhibit a rising characteristic as the
resonance is approached. A filter incorporated into
Model 3312A2T compensates for this rise.
The frequency at which the sensitivity may
increases or decrease by 5% is approximately 4000
Hz, the frequency to which the 3312A2T is calibrated.
The accelerometer is usable above this frequency but

REV A ECN 14100 04/23/18
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to use it above 4000 Hz, it must be calibrated at the
specific frequencies of intended use because
sensitivity deviations will increase drastically as you
greatly exceed this high frequency calibration limit.
Consult the factory for special calibrations required
above 4000 Hz.
CAUTIONS
1) Do not store or use the 3312A2T above
250°F. To do so can damage the IC amplifier.
2) Do not allow cables to vibrate unrestrained.
This will eventually destroy the cable and could lead
to system inaccuracies.
3) If the 3312A2T is to be used in rapidly
changing thermal environments, call the factory to ask
about our thermal insulating boot, Model 6215.
4) Avoid dropping or striking the
accelerometer, especially against rigid materials such
as concrete and metals. While Model 3312A2T is
protected against shock induced overloads, the very
high overloads induced by dropping can do
permanent damage to the JFET amplifier or to the
mechanical structure of the accelerometer. This type
of damage is not covered by the warranty.
MAINTENANCE AND REPAIR
The welded construction of the Model
3312A2T precludes field repair.
Should the mounting surface become
distorted, nicked and otherwise distressed, it can be
redressed by CAREFULLY wiping on a new sheet of
400 grit emery paper on top of a clean surface plate.
We stress "carefully" because if not done properly,
this procedure can do more harm than good. Press
the surface firmly against the paper and draw directly
toward you in several short precise strokes making
sure that the surface remains in full contact with the
paper and does not "rock". Rotate the accelerometer
90 degrees and repeat the procedure. When you
observe the bottom surface it should appear perfectly
flat with straight marks across it. If you cannot achieve
flatness with several attempts, return the instrument to
the factory for repair.
Should the electrical connector become
contaminated with moisture, oil, grease, etc., the
entire instrument may be immersed in degreasing
solvents to remove the contaminants. After
degreasing, place the instrument in a 200 to 250°F
oven for one hour to remove all traces of the solvent.
Should a problem be encountered with the
operation of the instrument, contact the factory for
trouble shooting advice. Often our service engineers
may point out something which may have been
overlooked and which may save the expense and time
of returning the 3312A2T to the factory.
If the instrument must be returned, the service
department will issue you a Returned Materials
Authorization (RMA) number to aid in tracking the
repair through the system.
Do not send the instrument back without first
obtaining an RMA number. At this time you will be
advised of the preferred shipping method.
A short note describing the problem, included
with the returned instrument, will aid in trouble
shooting at the factory and will be appreciated.
We will not proceed with a non-warranty
repair without first calling to notify you of the expected
charges. There is no charge for evaluation of the unit.