Murphy RTD Transmitter RTDT Series - Manual (Page 2)

INSTALLATION
Open Sensor Indication
When an RTD has failed due to an open sensor (broken path in the element),
the transmitter will indicate an error. The way the error is produced is by dri-
ving the current high, above 20 mA (upscale burnout). However, the burnout
indication does not apply to breaks in the lead compensation loop.
Application Hints
The calibrated output of the Murphy Transmitter is 4.0 to 20.0 mA. However,
the lower and upper limit of output current is approximately 3.4 and 30 mA
respectively. This means that for the system using a 250 ohm resistor (1 to 5 V),
the maximum voltage could be as high as 7.5 volts (30 mA x 250 ohms) in the
case of an open RTD. Some computer-based systems will not tolerate input
voltages greater than approximately 5.5 volts. Instead of selecting a different
scaling resistor (167 ohms for example gives 5 V for an open RTD), a 5.1 V
zener diode in parallel with the scaling resistor will clamp the voltage across the
scaling resistor to 5.1 V while still allowing the calibrated range to remain 1 to 5
volts. See figure below.
The transmitter dissipates power according to the formula: Power = Volts x Amps.
The dissipated power is equal to the voltage at the transmitter’s terminals multi-
plied by the operating current. A transmitter with 24 volts at its terminal and
with maximum current (30 mA for an open RTD) will dissipate 0.7 watts of
power. This can be significant if the RTD sensor can be affected by thermal
gradients from the transmitter.
RTDT Calibration
Murphy RTD transmitter modules are factory calibrated. Please DO NOT
attempt to recalibrate unless the proper equipment is available. If recalibra-
tion is necessary, a Zero, Rlin, and Span adjustment can be made from the
top of the unit. A precision RTD simulator with a traceable calibration
must be used. Consult a book of tables for resistance values.
1. With a resistance value corresponding with the temperature at 4.0 mA,
adjust the zero potentiometer for 4.0 mA output.
2. With a resistance value corresponding with temperature at 12.0 mA,
adjust the Rlin potentiometer for 12.0 mA output.
3. With a resistance value corresponding with the temperature at 20.0 mA,
adjust the span potentiometer for 20.0 mA output.
4. The zero, Rlin and span adjustments are interactive. Repeat steps 1, 2,
and 3 as necessary.
Example 1:
1. If range equals 50 -250°, then span equals 200°
2. Divide 200° by 2: 200° ÷ 2 = 100°
3. Add 100° to lower end of range: 50° + 100° = 150°
4. 50° equals the Zero point (4 mA)
5. 150° equals 50% of the Span (12 mA) (Rlin)
6. 250° equals the Span point (20 mA)
Example 2:
1. Set zero at 4.0 mA for -100°F (70.95 Ω)
2. Set Rlin at 12.0 mA for 0°F (93.01 Ω)
3. Set span at 20.0 mA for 100°F (114.68 Ω)
Troubleshooting
Problem Possible causes
No current flow on signal loop Current loop open at some point.
No voltage out at power supply.
Wrong polarity on loop supply.
Current over 20 mA RTD is open.
Current loop connections shorted.
Erratic readings Loose connection in RTD or signal loop.
Damaged RTD.
AC noise on loop connections.
Exceeds loop resistance R.
Warranty
A limited warranty on materials and workmanship is given with this
FW Murphy product. A copy of the warranty may be viewed or printed by going to
www.fwmurphy.com/support/warranty.htm
Lead Compensation
250 OHMS
Sampling
Resistor
4-20 mA End of
Line Input
+ 24 VDC
RTDT
+
+
- -
- -
CONTROL SYSTEMS & SERVICES DIVISION
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MACQUARRIE CORPORATION
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Tulsa, Oklahoma 74147 USA
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USA–ISO 9001:2000 FM 28221
UK–ISO 9001:2000 FM 29422
Printed in U.S.A.