Temperature Sensors & Indicators

Thermocouples are temperature sensors, which work on principles for flow of electron in closed  circuit as described below. Whenever two dissimilar material are joined at ends and the two ends  are subjected to temperature differential, electrons flow in such closed circuit and there is potential  difference between cold and hot junction. This potential difference is proportional to the temperature  differential and is dependent on the metals joined. Any two dissimilar metals can be used for this  purpose, however there are many standard combinations on metals, which are internationally used  and designated for use as thermocouple elements. Most commonly used thermocouple elements  are broadly divided in two categories:

 A) Base metal thermocouples ( ) and
 B) Precious metal thermocouples ( ). J, K, T, and E type R, S and B type


 Thermocouple Types:

 Given below is general description of commonly used thermocouples.


 IEC 584-1, PART1: S:

 This thermocouples can be used in oxidising or inert  atmosphere at temperatures described above. Recrystallised Alumina beads and protective sheaths are generally used. However, for lower temperatures upto 1100 Deg C metal sheaths with porcelain beads can be used. Please note that continuous use at elevated temperature may cause fusing of Rhodium in pure Platinum causing reduction in output emf.


 IEC 584-1, PART2: R:

 This thermocouples can be used in applications similar to S type thermocouples. zhe advantage being slightly higher output at same temperature and better stability. This type is preferred over S type thermocouples.


 IEC 584-1, PART3: J:

 This thermocouples are commonly known as iron/constantant thermocouples and can be used in reducing or inert atmosphere at temperatures described above. Restrict use in oxidizing atmosphere to temperatures up to 400 deg C, otherwise degradation will be rapid. Avoid using for application where condensation temp is often reached otherwise Fe {+leg} will rust rapidly.


 IEC 584-1, PART4: K:

 This is most commonly used thermocouple ind is are commonly known as chromel/Alumel thermocouple These are designed primarily for use in Oxidising atmosphere at temperatures described above.

 Great care should be taken when using for any other atmosphere. These thermocouple will get oxidised when used continuously at temp above 1000 Deg C and cause drift.

 For temp range from 250 to 600 Deg C stability is suspect. Other base metal thermocouples are better bet at  this temp range.


 Cyclic

 temperature changes can cause error of several degrees at these temperatures.


 IEC 584-1, PART5: T:

 This is commonly used in laboratory applications where sub zero temperatures are often maintained. Repeatability at -200 + 200 Deg C is excellent. However at temp above this Copper arm rapidly oxidizes causing drift. Note that the negative arm of T and J type are not same and are not interchangeable.


 IEC 584-1, PART6: E and IEC 584-1, PART7: B

 These thermocouples are not in much use presently. E type is more stable in -250 + 200 Deg Range than K type and can be preferred over K in this range. Type B thermocouple can be used at temperature upto 1600 Deg c. Interesting point is that in this type cold junction compensation is not normally required as emf change from 0 to 50 is negligible.

Resistance temperature detectors (RTD) are  temperature sensors, which work on principles for resistance offered by metallic element and its variation based on temperature. Metals which provide predictable, stable and linear resistance gradient are generally used for this purpose.

 Many metals exhibit these properties. Of which copper, Platinum, Silver and Gold are commonly
 used.

 Copper is used for cryogenic and sub-zero applications. Commonly available element is Cu53 : 53 Ohm resistance at Zero Deg Temperature.

 Where as Platinum is used for most industrial applications. Commonly available elements are Pt50, Pt100, Pt200, Pt500 and Pt 1000.

 Pt100: 100 Ohm resistance @ 0 Deg C is by far the most popular and easily available element.
 
 RTD elements are classified in various categories based on its accuracy such as: Class - A, Class - B, Band1 to Band 5.

 
RTD ELEMENT CONSTRUCTION :

 For laboratory applications Platinum wire embedded in Glass capsules are available. However for industrial purpose fragility and price restrict their use.
 
 For industrial applications two most popular constructions are : Wire wound ceramic bulbs and thin film type elements.
 
 Wire wound elements are more stable and offer wider temperature range where as film type are economical and are restricted to low temperature applications.


 RTD STANDARDS:

 IEC - 751:1983, DIN 43760, BS EN 60751:1969 are commonly used calibration and tolerance standards for RTD elements.
 
 Typical Class A element Error is 0.15 Deg C @ 0 Deg C and
 Typical Class B element Error is 0.3 Deg C @ 0 Deg C.
 Tolerance values of all stadard RTD elements are plotted on the following page of this catalogue.

 Standard Pt100 RTD elements are constructed from Pt having “Alpha” coefficient at 0.00385 / Deg C and resistance of 100 ohms at 0 Deg C.


Satiable for general application up to 200 Deg C. Film and Ceramic type element in Seamless SS316 sheath with extension lead wire in two / three wire construction.

 Hand held RTD assemblies are suitable for liquid, Gas and semi  solid, Available in Pt 100 film and Ceramic element with Mior rigid SS Sheath and Ebonite handle with PVC extension cable Suitable for Temp up to 180 Deg C.


 Bayonate type suitable for plastic machinery, bearing chokes etc. Available in Film and Ceramic type element with SS 316 sheath and extension lead wire. Suitable for Temp up to 250 Deg C.


 Heavy duty metal sheathed industrial RTD assemblies are suitable for most arduous industrial environments. Available in Ceramic Element in Accuracies such as Class B,A, Band 1, 2 and 3 with variety of sheaths such as SS 304, SS 316 and SS 310.

   
 Mineral insulated metal sheathed RTD Assemblies are suitable for spaces which are difficult to reach. Available in Ceramic element with variety of sheaths such as SS 304, SS 316 and SS 310.

For applications where inly indication is desired it is recommended to used dial type thermometers.These instruments contains hermetically sealed Mercury filled system.The thermometer utilizes natural phenomenon of mercury to expand when heated. The advantage of use of mercury over any other filling medium is close to linear coefficient of expansion from -40 Deg C to 600 Deg c.The instruments are housed in weatherproof die cast Alluminium case or bayonet locking type
SS 304 cases. Die Cast gauges are available in 65 mm, 100 mm, 150 mm and 200 mm dial sizes.Where as SS 304 case are available in 100 mm and 150 mm dial size. All stainless steel temperature gauges are best suited for use in application involving corrosive process fluid coupled with hostile and corrosive environment. These gauges are manufactured in Rigid Stem type for local reading or flexible capillary type for distant reading. Fall the instruments are provided with case temperature compensation. Gauges with long capillary are provided with capillary compensation for better accuracy.


SPECIFICATIONS :

Dial Size :	50, 65, 100 and 150 mm
Case :	Bayonate lock SS 304 Die Cast Aluminium
Bezel Ring :	Bayonate lock SS 304 ABS Plastic.
Window :	Plain, Toughened Glass Acrylic Sheet Glass
Dial :	Aluminium, White Base, Black Markings
Pointer :	Balanced , Micro Zero Adjustable for 100 & 150 mm.
Bourdon :	SS 304
Stem /Bulb :	8, 10, 12 and 16 mm OD in SS 304 / SS 316
Stem length :	50 mm to upto 3000 mm
Capillary :	Cold Drawn M.S / SS 316 with copper SS 304/ SS 316 protection sleeve.
Cap. length :	500 mm to upto 10000 mm
Movement :	Non Ferrous / SS 304
Joints :	Argon Arc Welded
Mounting :	Direct Bottom Center or offset back with Front panel or clamp.
Conn. Size :	3/8", 1/2” & 3/4” BSP/NPT
Ranges :	-35 Deg C to upto 600 Deg C.
Accuracy :
65 mm :	±2.0% of FSD
100, 150mm :	±1.0% of FSD

Thermocouples are connected to indicators/controllers using special cable which are know as compensating cable. In cases where accuracy of reading is of prime importance cables with original conductor is used such cable is known as extension cable.
 
 The cables are manufactured in various configurations. We are giving below various types of cable constructions available.


 Compensating Cables:

Compensating cables are manufactured from conductors having composition different from corresponding thermocouples. They are designated by letter “C” following the designation of the thermocouple. Eg: TC



Extension Cables:


 Extension cables are manufactured from conductors having composition same as that of from corresponding thermocouples. They are designated by letter “X” following the designation of the thermcouple. Eg: TX

User selectable for all thermocouple types, mV, and Pt-100 RTD.

The Mescon TH-11U is a low cost, non-isolated temperature transmitter designed to fit in a standard thermal head. A new design features a unique circuitry that enables the use of a single transmitter for the measurements of either Pt-100 RTD, Thermocouple input or mV. When set to Thermocouple mode, the TH-11U accepts all known Thermocouple types. The advantages of using the TH-11U transmitter are:
 

• Reduced inventory levels for both in-plant users and distributors.
• Ease of maintenance through product standardization .
• Better measurement stability through improved circuitry and enhanced RFI-EMI rejection .

The TH-11U is constructed of an upper and lower section. The lower section contains the signal conditioning electronics, while the upper half contains the "personality" components that define the input sensor type and the measurement range. The two halves snap into each other for a perfect fit, yet enable easy disassembly for the purpose of input and range changes.

Specifications :
Output Span:	4-20mA, limiting @ <28mA
Input:	RTD - Pt-100, 2 or 3 wire connection 20°C minimum, 500°C maximum. TC - all known types. 10mV min. span.
Input Span:	RTD - 20°C min. 500°C max. (36°F min. 900°F max.)
Burnout Detection:	Upscale - standard.
Supply Voltage:	8-38 VDC polarity protected.
Maximum Load:	Rmax = (Vsupply - 8V)/20mA
Ambient Temp.:	-20°C to +70°C
Humidity:	0 - 95% RH, Non-condensing
Linearity:	RTD: better than ±0.05% of span referred to sensor temperature. TC/mV: better than ±0.03% of span referred to mV input level.
Stability: (for both zero and span.)	Pt-100 (100°C span)0.03% of span/°C TC/mV (25mV input): 0.04% of span/ °C
CJ Compensation:	For TC - 0.05°C/°C of ambient temp

Calibration and adjustments:

It is assumed that the unit undergoing calibration has been properly ranged at the factory or the workshop.

1. Pt-100 RTD: Connect the Pt-100 sensor simulator to the TH11 input terminals according to the wiring diagram. Turn the power on OR T/C: Connect the Thermocouple sensor simulator to the TH-11 input terminals according to the wiring diagram. Turn the power on. For optimum performance allow 15 minutes for temperature gradients to equalize.
2. Set the input to and adjust the ZERO pot until the current indicator reads 4.00 mA.
3. Set the input to and adjust the SPAN pot until the current indicator reads 20.00 mA.
4. Repeat steps 2 and 3 until no further adjustment is needed.

Different type of sensor heads, Thermo wells, Ceramic and Alumina Protection Sheaths, Terminal Blocks.