Murphy RTD, RTDT User Manual

RTD Transmitter Installation Instructions
RTDT Series Models

RTDT-96091N
Revised 01-01

Section 10

00-02-0849

Description

The Murphy RTD transmitter is designed to accept a signal from an RTD sensing
element, and to provide a linearized 4 to 20 mA output current. The output cur­rent is directly proportional to the temperature of the RTD temperature sensor.
Since the temperature is converted to a current signal, voltage drops in the power
supply leads to the transmitter have no effect on the integrity of the signal. As long
as the transmitter has sufficient DC voltage at its terminals, it will produce a cur­rent linearly proportional to temperature. This means that the transmitter may be
remote mounted but should be near the sensor, reducing the effects of noise and
minimizing errors due to long leads on the RTD sensor. The RTD instruments
have screw terminal connections and accept 2 or 3 wire RTD's* . They include
high resolution, 24-turn zero and span potentiometers and each unit is calibrat­ed after a minimum of 48 hours burn-in and checkout time.

*Three wire RTD's compensate for leadwire resistance.

Specifications

Sensor Input: Platinum RTD 100 ohms
Output Span: 4 to 20 mA
Loop Power Supply Voltage: Typically 24 VDC; when using Loop Resistance

Graph 13 – 40 VDC.

Temperature Span: 50°F (28°C) minimum; 1000°F (556°C) maximum
Calibrated Accuracy: 0.1 %
Conformity: 0.1 % of Span
Ranges: -60 to 140°F; 0 to 400°F
Ambient Temperature Limits: -30 to 165°F (-34 to 74°C)
Power Supply Effect: 0.001% volt
Current: 3.4 mA minimum; 30 mA maximum.
Upscale Burnout: Standard
Reversed Polarity Protection: Standard
Nominal Zero and Span Adjustments: ±10%
Connections: Screw Terminal

Power Supply

Transmitters are designed for a nominal 24 VDC power supply. They will
operate with a 13 to 40 VDC supply at the transmitter power supply terminals.
Use the following formulas to determine maximum resistive loading (RL)
allowed for the power supply used, or to determine minimum supply voltage (V)
required for fixing resistive loads. Formulas assume a max. current of 20 mA.

NOTE: Devices such as Murphy’s EMS Series controllers, 400 and 500 Series PLC

controllers, and the RMU16 have 250 ohms resistors that add to the resistve loading.

Power Supply Voltage Resistive Load

(Minimum) (Maximum)

V= (0.02 A x RL) +13 V RL=(V - 13) /0.02

Example: Example:

550 ohm load 24 VDC power supply
V=0.02 x 550 +13 RL=(24 V - 13 V) /0.02
V= 24 VDC minimum power supply RL=550 ohm maxi

mum

Wiring

The terminal strip on the transmitter accepts 14 to 22 gauge wires. Copper wire is
to be used since special compensation wire is not required. Route all transmitter
wiring in separate conduit to isolate it from high voltage or high current carrying
lines. It is also recommended that shielded wire be used for the 4-20 mA signal.

Please read the following information before installing. A visual inspection of this product for damage during shipping is recommended
before mounting. It is your responsibility to have a qualified person install this unit and make sure it conforms to NEC and local codes.

GENERAL INFORMATION

BEFORE BEGINNING INSTALLATION OF THIS MURPHY PRODUCT

✔ Disconnect all electrical power to the machine.
✔ Make sure the machine cannot operate during installation.
✔ Follow all safety warnings of the machine manufacturer.
✔ Read and follow all installation instructions.

Dimensions

CAUTION:

Do Not connect power to the RTD input. Do Not

connect power to the RTD itself. Do Not use AC line power.

Loop Resistance Graph

Supply voltage for the RTDT must be within 13-40 VDC. The figure below shows
the minimum supply voltage (VDC) required for a given load resistance (RL).

Under

Voltage

Area

Operating

Area

18V, 250 Ω

23V, 500 Ω

33V,
1000 Ω

Over

Voltage

Area

Total Load Resistance (RL)*

Power Supply Voltage (VDC)

*NOTE: Cable resistance effect included in RL.

WARNING

0.138 in. (3.50 mm)

NOTE

0.14 in.

(3.55 mm)

NOTE: Slot accepts 6-32 screw

ZERO Rlin SPAN

2.20 in. (55.8 mm) Max.

1.080 in.

(27.43 mm)

1.50 in. (38.1 mm) Max.

0.67 in.
(17.1 mm)

Red

Red

ZERO Rlin SPAN

RTD

White

+

Indicator
Controller

DC Power Supply

--

--

+

1500

1000

10 20 30 40 50 13

18

500

250

0

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 dri­ving 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 recalibra­tion 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

CONTROL SYSTEMS & SERVICES DIVISION

P.O. Box 1819; Rosenberg, Texas 77471; USA
[phone] fax [phone]

e-mail [e-mail]

MURPHY DE MEXICO, S.A. DE C.V.

Blvd. Antonio Rocha Cordero 300, Fracción del Aguaje
San Luis Potosí, S.L.P.; México 78384
[phone] fax [phone]
Villahermosa Office [phone]
e-mail [e-mail]
www.murphymex.com.mx

FRANK W. MURPHY, LTD.

Church Rd.; Laverstock, Salisbury SP1 1QZ; U.K.
[phone] fax [phone]
e-mail [e-mail]
www.fwmurphy.co.uk

MURPHY SWITCH OF CALIFORNIA

41343 12th Street West
Palmdale, California 93551-1442; USA
[phone] fax [phone]
e-mail [e-mail]
www.murphyswitch.com

In order to consistently bring you the highest quality, full featured products, we reserve the right to change our specifications and designs at any time.

MACQUARRIE CORPORATION

1620 Hume Highway
Campbellfield, Vic 3061; Australia
[phone] fax [phone]

e-mail [e-mail]

FW Murphy

P.O. Box 470248
Tulsa, Oklahoma 74147 USA
[phone]

fax [phone]
e-mail [e-mail]

www.fwmurphy.com

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USA–ISO 9001:2000 FM 28221

UK–ISO 9001:2000 FM 29422

Printed in U.S.A.

Lead Compensation

--

+

RTDT

+

+ 24 VDC

--

4-20 mA End of

Line Input

250 OHMS
Sampling
Resistor