FW MURPHY RTDT Series Resistance Temperature Detector Instruction Manual
- June 3, 2024
- FW MURPHY
Table of Contents
Installation of RTD Transmitter
Models: RTDT Series
sect. 10 00-02-0849
RTDT Series Resistance Temperature Detector
Please read the following information before installing. Visually inspect this
product for any shipping damage. It is your responsibility to have a qualified
person install this unit and make sure it conforms to NEC and local codes.
The FW 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 cur rent 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 cur rent 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 calibrat ed after a
minimum of 48 hours burn-in and checkout time.
BEFORE BEGINNING
INSTALLATION OF THIS 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
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; l000° 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 M required for
fixing resistive loads.
Formulas assume a maximum current of 20 mA.
NOTE: Devices such as FW Murphy’s EMS Series controllers, 400 and 500
Series PLC controllers, and the RMUJ 6 have 250 ohms resistors that add to the
resistive loading.
Power Supply Voltage (Min)
V = (0.02 A x RL) + 13 V
Example:
550 ohm load
V = 0.02 x 550 + 13
V = 24 VDC minimum power supply| Resistive Load (Max)
RL = (V – 13) /0.02
Example:
24 VDC power supply
RL = (24 V – 13 V) /0.02
RL = 550 ohm maximum
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Dimensions
NOTE: Slot accepts
6-32 screw
**Wiring
**
CAUTION: Do Not connect power to the RTD input. Do Not connect power to
the RTD itself. Do Not use AC line power.
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.
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).
* NOTE: Cable resistance
effect included in RL.
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.
FW MURPHY product names and the FW MURPHY logo are proprietary trademarks.
This document, including textual matter and illustrations, is copyright
protected with all rights reserved. (c) 2021 FW MURPHY. A copy of our typical
warranty may be viewed or printed by going to
www.fwmurphy.com/warranty.
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 FW 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 voles (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 voles. 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 multiplied by the operating
current. A transmitter with 24 voles 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
FW 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.
- With a resistance value corresponding with the temperature at 4.0 mA, adjust the zero potentiometer for 4.0 mA output.
- With a resistance value corresponding with temperature at 12.0 mA, adjust the Rlin potentiometer for 12.0 mA output.
- With a resistance value corresponding with the temperature at 20.0 mA, adjust the span potentiometer for 20.0 mA output.
- The zero, Rlin and span adjustments are interactive. Repeat steps 1, 2, and 3 as necessary.
Example 1:
- If range equals 50 – 250°, then span equals 200°
- Divide 200° by 2: 200° + 2 = 100°
- Add 100° to lower end of range: 50° + 100° = 150°
- 50° equals the Zero point (4 mA)
- 150° equals 50% of the Span (12 mA) (Rlin)
- 250° equals the Span point (20 mA)
Example 2:
- Set zero at 4.0 mA for -100° F (70.95 Q)
- Set Rlin at 12.0 mA for 0° F (93.01 Q)
- Set span at 20.0 mA for l00° F (114.68 Q)
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
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FM 668576 (San Antonio, TX- USA)
FM 668933 (Rosenberg, TX – USA)
FM 523851 (China)
TS 589322 (China)