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RESISTANCE TEMPERATURE DETECTORS

A resistance temperature detector (commonly called RTD, resistance bulb, etc.) assembly consists of (1) an element (2) a support or bobbin for the element (3) a protection tube or sheath (4) connecting wires which extend from the element to the termination end (5) a means of securing the connecting wires to the termination end, and (6) a means of connecting it to the resistance-measuring equipment.

Resistance temperature detectors (R.T.D.) or resistance thermometry is based on a well-known principle that most metals increase in resistivity when their temperature is increased, and on cooling to the original temperature, will return to the original resistivity. The resistance-temperature curves of pure metals, e.g., platinum and nickel, over definite spans makes them ideal materials for the elements in resistance thermometers.

Laboratory resistance temperature detectors of pure platinum, fully annealed and strain free, have been chosen as the International Standard of Temperature Measurement from liquid oxygen [(LO2)-182.97 deg C] to the melting point of antimony [(sb) + 630.5 deg C]. Range -250 to +500 deg C for platinum. Temperature coefficient: .003915 ohms/ohm/ deg C and .00385 ohms/ohm/ deg C. (0.00385 or din standard has been adopted as the world and USA standard.)

Pure nickel has been widely used as a temperature-sensitive element over the range of -700 deg C to +3000 deg C principally because of its low cost and high temperature coefficient of resistivity.

Absolute Measurement - Resistance thermometers, unlike thermocouples, do not require a reference point. No ice baths or compensation circuits.

High Output - With an output of 50 to 200 times that of a thermocouple, resistance thermometers permit the use of simpler indication and control instruments. No amplifiers are needed and the resulting system is less expensive and more reliable.

Greatest Accuracy - The pre-eminent position of the resistance thermometer as a precision temperature measuring instrument is demonstrated by its selection to define the International Temperature scale from -260 deg C to +660 deg C. The main reasons for its selection are: 1) the exceptional stability and 2) the repeatability of the resistance thermometer.

Resistance Thermometers can be used for a wide variety of industrial applications. A high electrical output can be obtained by using the RTD with many types of simple resistance bridges. This high output can then be fed directly into recorders, temperature controllers, transmitters, or digital readouts which can be calibrated to read very precise increments of temperature over wide dynamic ranges. RTD's can also be read out on precision laboratory bridges and digital ohmmeters.

A simple Wheatstone bridge circuit with a reasonable high impedance detector is recommended for reading out RTD probes.

If the detector impedance is assumed infinite

		Eo     Rx         Rs
		-- = ------  -  ------
		E    Rx - R     Rs + R
		
	where R	 = Ratio Arms
	      Rx = Probe resistance (at temperature x)
	      Rs = Balancing arm (equal to Rx at lowest 
	           temperature which may be variable
			   for zero set.)

Such a bridge is non-linear, when the probe undergoes any reasonable temperature excursion. In the case of platinum wire, the ratio arms (R) should be as large as possible (at least 10 times Rs) to minimize bridge non-linearity. To protect the probe and minimize the errors due to self-heating, an operating current of 1 MA is recommended. This current can be controlled by choice of R or L.

To measure temperature difference, two identical probes can be used in adjacent arms of the bridge (second one replaces Rs). In this case, provisions for zero setting (if desired) should be moved to one of the R arms.

1. How the point of measurement can be made. Whether in a small area, which would necessitate a tip sensitive, or a large area which would make a stem sensitive more desirable.
2. The O.D. of the tube.
3. The temperature and/or temperature range of the media to be measured accuracy excellent at room temperature.
4. What length of immersion would be required for your application.
5. How the R.T.D. is to be inserted, and how best it can be supported or mounted.
6. If pressure or vacuum has to be maintained, then the R.T.D. has to be supplied with either a compression fitting, fixed fitting, head with connector, or a thermowell.

PRECISION PROCESS TEMPERATURE CONTROL

• Textile
• Chemical
• Food
• Brewing

• Test Chambers
• Oven Temperature
• Plastic Extruders
• Injection Molders
• Solder Pots
• Bearing Temperature

• Digital Temperature Indicators
• 12-Inch Round Chart Recorders

• Branom Steam Control Systems and Multipoint Rtd Indicators
• Crompton and Jewell Rtd Analog Meters and Setpoint Controllers
• Red Lion digital Indicators and Controllers
• R.I.S. Transmitters and Trips and 36 Point Alarm Monitors
• Rustrak Miniature Recorders
• West Rtd Controllers: On-Off, Hi-Lo Limit, or PID

RTD - Denotes resistance temperature detector, a device which provides a useable change in resistance to a specified temperature change.

SENSING ELEMENT - The electrical portion of an RTD (Resistance winding) in which the change originates.

CALIBRATION ACCURACY (INTERCHANGEABILITY) - The conformance of the RTD's measured output to a standard calibration curve calibrated by a governmental standards agency such as NBS or calibrated on equipment directly traceable to NBS.

REPEATABILITY - The ability of the RTD to reproduce consecutive readings when the same temperature is applied to it consecutively under the same conditions, and in the same direction.

STABILITY - The ability of an RTD to retain its repeatability (and other specified performance characteristics) for a relatively long period of time.

SELF-HEATING - internal heating resulting from electrical energy dissipated within the resistance sensor. This is usually specified in watts or millivolts/ deg C. This is determined by the amount of power it takes to raise the output of the sensor 10C under certain conditions such as air, water or oil flowing at a specified velocity.

RESPONSE TIME - The length of time required for the output of an RTD to respond to 63.2% of a step change in temperature. This is usually specified in air, oil or water flowing at a specified velocity.

MAXIMUM SAFE CURRENT - The maximum current recommended to be applied to a particular RTD to prevent burn out or open circuiting. This is determined by the sensor wire diameter and the configuration.

INSULATION RESISTANCE - The resistance measured between specified in-insulated portions of an RTD (such as between sensing element and outer case) when a specified DC voltage is applied.

1. Use shielding and twisted-pair wire, avoid stress and steep gradients, use large extension wire. Use 3 wire or 4 wire cable.
2. Due to its construction, the RTD is somewhat more fragile than a thermocouple and some care should be taken to protect it
3. A current is and must be passed througout the RTD to provide a voltage that can be measured. This current causes joule (I2R) heating within the RTD. This self-heating does appear as a temperature error. To reduce self heating errors, use the minimum current possible, and use the largest rtd you can that will still give you the response you need. A typical value for self-heating error is 1/2 deg c per milliwatt in free air. If you immerse the RTD in a liquid, or any other thermally conductive medium, you will dissipate the self-heating aspect to a negligible error.

1. 1/8 inch x 2 inch encapsulated 100 ohm platinum (.00385) RTD for surface mounting.
2. 1/4 inch od x any practical length ss sheath 100 ohm plt RTD (.00385)--available with 1/8 inch, 1/4 inch or 1/2 inch brass or ss fitting or with standard wells and with aluminum or cast iron heads, or high temperature plastic (450 or 850 deg f) heads.