Murphy RTDT Series specifications Open Sensor Indication, Application Hints, Rtdt Calibration

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.

Lead Compensation

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-

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)

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RTDT

+24 VDC

4-20 mA End of

Line Input

250 OHMS

Sampling

Resistor

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.

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.

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

FW Murphy

P.O. Box 470248

Tulsa, Oklahoma 74147 USA +1 918 317 4100

fax +1 918 317 4266 e-mailsales@fwmurphy.com www.fwmurphy.com

CONTROL SYSTEMS & SERVICES DIVISION P.O. Box 1819; Rosenberg, Texas 77471; USA +1 281 633 4500 fax +1 281 633 4588 e-mail sales@fwmurphy.com

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

Blvd. Antonio Rocha Cordero 300, Fracción del Aguaje San Luis Potosí, S.L.P.; México 78384

+52 444 8206264 fax +52 444 8206336 Villahermosa Office +52 993 3162117 e-mailventas@murphymex.com.mx www.murphymex.com.mx

FRANK W. MURPHY, LTD.

Church Rd.; Laverstock, Salisbury SP1 1QZ; U.K. +44 1722 410055 fax +44 1722 410088 e-mailsales@fwmurphy.co.uk www.fwmurphy.co.uk

MURPHY SWITCH OF CALIFORNIA

41343 12th Street West

Palmdale, California 93551-1442; USA

+1 661 272 4700 fax +1 661 947 7570 e-mailsales@murphyswitch.com www.murphyswitch.com

MACQUARRIE CORPORATION 1620 Hume Highway Campbellfield, Vic 3061; Australia

+61 3 9358 5555 fax +61 3 9358 5558 e-mailmurphy@macquarrie.com.au

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USA–ISO 9001:2000 FM 28221

UK–ISO 9001:2000 FM 29422

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