Thermometers and elements: detailed specifications

Thermometer specification
Here, we will see the detailed specifications of the thermometer.
In general applications, bimetallic instruments are better than filled instruments. These instruments shall have the following characteristics
● Manufactured according to SAMA grade.
● Case compensation must be provided
● The temperature bulb material shall be at least SS 316

The instrument dialing shall have the following features
● 150 mm nominal size
● Black numbers on a white background
● Negative temperature shall be marked with red and white background
● Provide adjustable head
● It shall be specified that the pointer can be adjusted without removing it from the shaft
● The instrument shall be of weatherproof construction conforming to IP66.
Under vibration conditions, the filling type with capillary extension shall be used. Capillary tubes shall be at least SS 316. In addition, it should have flexible armor and PVC covering on the armor.
The accuracy of the thermometer shall be ± 1% FSD (full range deviation).
Bimetal thermometers can only be used when the acceptable error exceeds 1% of the measuring range. Bimetal dial thermometer shall be of each angle and can be adjusted by external pointer. The shell material shall be SS304. The stem diameter shall be manufacturer's standard. In places where excessive vibration is encountered, such as compressors, bimetallic dial thermometers should be avoided.
Thermometer stem adjustable gland with union and bushing shall be suitable for 1/2 "NPTF connection.
Thermometer and element specification drawing
Specification of temperature element
Here, we will see the detailed specifications of the temperature element.
For remote sensing of temperature, use thermocouple or RTD temperature sensor. Elements shall be spring loaded, mineral insulated and shall have a minimum 316 SS sheath.
Thermocouple
Thermocouple assemblies shall be equipped with weatherproof screw in heads. Terminals shall be colour coded according to the component colour code.
Thermocouples shall conform to ANSI MC 96.1 and IEC-60584-2/IS-7358. In general, for most applications, they are 18 AWG for single wire and 20 AWG for duplex thermocouples.
Unless otherwise specified, thermocouples shall be ungrounded and type K filled magnesium oxide (MgO). The thermocouple wires shall be connected together by welding to form a hot junction.
The thermocouple assembly shall be designed for online replacement.
The thermocouple sheath material shall be at least 316 stainless steel and shall be made of seamless tube with a wall thickness of at least 1 mm. Inconel 600 sheath shall be selected for temperatures above 600 ° C.
It shall be ensured that the minimum radius of curvature that the thermocouples can bend is equal to three times the outer diameter of the sheath, and also at the tip of the swage. This bending operation shall not cause cracking or rupture of the sheath. Thermocouples shall be bright annealed to meet this requirement.
The calibration of shall be carried out on all thermocouples according to IEC 584.2 class 1 degree tolerance. Electromotive force measurements shall be made on these thermocouples in accordance with ITS – 90 at six fixed temperature points from 0 to 1000 degrees Celsius.
The calibration result report shall be provided to the Buyer in a convenient form. This can be a table of electromotive force values measured at corresponding temperatures.
Thermoelectric characteristics of thermocouples shall comply with IEC 584.1. The electromotive force generated by thermocouples shall be of the order of 41 microvolts per degree Celsius. The change rule between electromotive force and temperature should be linear. In addition, the accuracy of the measured values shall be better than ± 1.0 ° C to 350 ° C and ± 0.4% (350 to 1000 ° C).
Thermal resistance
RTD temperature sensors should be preferred when the process use temperature is between - 200 ° C and 650 ° C.
The connection on the RTD temperature sensor thermowell shall be ½”  NPTM。
The cable inlet of RTD temperature sensor shall be ½”  NPTF。
In general, RTD temperature sensors shall be 3-wire duplex type with two single wire wound elements compatible with 99.99% pure platinum 100 ohm conductor characteristics.
RTD temperature sensors should generally be selected for applications requiring low temperature measurement. The temperature span is relatively short, requiring 0.25% or better accuracy.
If dual element sensors are used, there should be two separate cable entries. The multipoint temperature element shall be equipped with a complete junction box.
RTDs/thermocouples shall be assigned Class "A"/Class "1" tolerances for temperature measurement in all open and closed loop, interlock and shutdown applications in accordance with IEC 60751/60584-2. In addition, all such circuits shall be equipped with remote mounted 2-wire temperature transmitters.