Robertshaw 158A Uživatelský manuál stáhnout pdf (Strana 2)

PRINCIPLE OF OPERATION

The Model 158A makes use of a unique circuit design

concept developed by Robertshaw to provide a true DC

current output signal as a function of input capacitance

change. Referring to the block diagram, Figure 1, two free

running saw tooth oscillators are utilized; one being used as

a reference and consisting of a reference capacitor and

reference current generator, the second being the zero

generator and probe capacitor connected through the Triaxial

cable. Both capacitors, reference and probe, charge up

linearly until a pre set voltage is reached causing the "set-

reset" solid-state switch to momentarily close "shorting" out

both capacitors - thus starting the charging cycle once again

and developing a "saw tooth" waveform across both

capacitors.

Connected directly across both capacitors is the "charge rate

discriminator" amplifier which produces a voltage output

when the charging rate of the probe capacitor differs from

that of the reference capacitor. When a change in probe

capacitance occurs (due to a change in process level), the

discriminator amplifier causes an output signal change,

which, with voltage feedback, readjusts the probe charging

rate through the span generator until the charging rates are

again equal.

Since the sensing probe is remotely mounted from the Model

158A, any change in the connecting cable capacitance

between the probe and the transmitter would normally affect

the measurement accuracy. This error is eliminated,

however, in the Model 158A by using a Triaxial cable with

the inner shield of the cable connected to the reference

capacitor. This causes the inner shield to be "driven" in

phase with the center conductor of the cable, keeping the

voltage difference near zero, thereby canceling out the

effects of the inherent cable capacitance. This permits long

lengths of cable to be used between the Model 158A and the

sensing probe − without sacrifice of performance.

SPECIFICATIONS

ENVIRONMENTAL:

Intrinsic Safety: Models 158A − (A,B) (1,2) − (A,B,C,D)1 have

CSA Certified intrinsically safe probe input circuit for

Class I, Div. 1, Group A, B, C & D; Class II, Div. 1,

Group E, F & G hazardous areas when connected as

shown on Robertshaw drawing no. 907GA518. Safety

barrier required.

Storage ................................................................-55° F to 225° F

(-48° C to 107° C)

Operating Temperature Limits.........................-40° F to 160° F

(-40° C to 70° C)

Operating Vibration Limits...........................2 g's, 10 to 200 Hz

Operating Humidity Limits...........................95% RH @ 100° F

Mechanical Shock................................. 75 g's for 11 ms duration

without permanent damage

PERFORMANCE:

Supply Variation Effect ...................... 0.5%/10% supply change

Temperature Coefficient............................1.0%/100° F ambient

(including connecting cable) change, or 1 pF/100° F

whichever is greater

Load Resistance Effect...............0.25% from no load to full load

Output Signal Ripple ....................0.2% PP maximum @ supply

voltage frequency

Terminal Non-linearity................0.5% maximum for spans less

than 1000 pF, 1.0% maximum

for spans greater than 1000 pF

ELECTRICAL:

Supply Voltage:

Standard ................................ 120 VAC ± 10%, 50/60 Hz

Optional.................................................26.5 VDC ± 10%

Optional................................. 240 VAC ± 10%, 50/60 Hz

Supply Power:

AC ...............................................5 watts, 7 VA maximum

DC ..........................................................85 mA maximum

Output Signal:

Standard ...............................4-20 mA into 0 to 650 ohms

Optional................................1-5 mA into 0 to 2500 ohms

Optional..............................10-50 mA into 0 to 250 ohms

Optional..............................0-10 mA into 0 to 1200 ohms

Input Signal (Capacitance):

Terminal Adjustment ...................................0 to 1000 pF

Span Adjustment ...........10 pF plus 1 pF per each 10 feet

of connecting cable minimum,

2000 pF maximum

Maximum Zero Suppression.................................10 times span

Connecting Cable Length..............................150 feet maximum

ENCLOSURE:

Standard ............... Weather tight, cast aluminum, painted

with blue polyurethane enamel,

CSA Enclosure 5 & NEMA 4

Optional............ Explosion proof, cast aluminum, painted

with blue polyurethane enamel, suitable

for Class I, Div. 1, Group C & D; Class II,

Div. 1, Group E, F & G hazardous areas

Optional.......................Large weather tight, steel, painted

with blue polyurethane enamel,

CSA Enclosure 3, NEMA 12 & 13

1 2 3 4 5 6 7 8

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