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How to Calibrate a Thermocouple Transmitter

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To calibrate a thermocouple transmitter will require a thermocouple simulator with an accuracy of at least four times greater than the thermocouple sensor we desire to calibrate. 

Equipment and Materials Required
The following equipment/materials are required to successfully calibrate a thermocouple transmitter:
  1. Thermocouple Simulator (of at least four times the accuracy of sensor)
  2. Two Digital Voltmeters (Five-digit readout) with accuracy of at least ±0.01% with resolution 1mV
  3. 24 VDC Power Supply of at least 35 – 40mA current output
  4. Thermocouple wire of the same type of wire the thermocouple transmitter is constructed of.

Equipment Setup
Below is the equipment set up for the calibration


Calibration Procedure
  1. Remove the thermocouple transmitter terminal housing cover
  2. If the transmitter is already connected, remove all the thermocouple lead connections.
  3. Determine the base and full scale temperatures. Read: How to convert thermocouple millivolt to temperature.
  4. Turn power supply on.
  5. Consult the thermocouple simulator manual for instructions on setting the thermocouple type and engineering units.
  6. Set the simulator to the base (zero) temperature and adjust the zero pot until the output is 4mA or 40mV at the test terminals
  7. Set the simulator to the full scale temperature and adjust the span pot until the output is 20mA
  8. Repeat steps (1-7) above until both the 4 and 20mA readings are obtained without re-adjusting the span and zero pots.






How to Specify a Digital Pressure Gauge

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Digital Pressure Gauge (Photo Credit: ASHCROFT)
With the advancement in gauge technology, digital pressure gauges are becoming more popular. With their  superior accuracy and functionality, most new plants are increasingly employing digital gauges especially in industrial processes where accuracy is paramount.

Although we probably all know the key parameters required to purchase a regular dial gauge, we may be at a loss when it comes to digital gauges. This is because in addition to the regular features of a typical pressure gauge, digital gauges are equipped with other functionalities that need to be specified. 

Digital pressure gauges are specified in much the same way a typical pressure gauge will be specified except for few specification items that are specific to digital pressure gauges. The table below shows the key parameters that must be taken into consideration for a general purpose digital pressure gauge to be accurately specified.
Parameters to Specify
Details of Specification
AccuracyTypical accuracies include ±0.1% FS, ±0.25% FS, ±0.5% FS etc 
Case Size  Standard case sizes include 3’’, 41/2’’,6’’ etc. 
Case MaterialDepending on your application, typical ones include 300 series stainless steel, fiberglass reinforced thermoplastic, black painted aluminium etc
Wetted Materials Specify materials that will resist corrosion. Typical materials include 17 – 4 PH stainless steel sensor, stainless steel socket etc
Socket Size Typical socket sizes include 1/8’’ , ¼’’ , ½’’ etc. 
Connection Specify whether process connection will be lower (6 o’clock), top or side
Measuring Range Specify the measuring range of gauge in the desired unit. For example 15 psi through 20,000 psi, 0 – 100 bar etc
Power Source For digital pressure gauges, they could be powered through:
1. Alkaline batteries (two pieces of size AA typically)
2. C Alkaline batteries (two pieces)
3. Loop powered by a 4 – 20mA source
4. Line powered (specify voltage and current)
Specify the desired power source you desire for the digital gauge
Battery Life Battery life is specified in hours. Typical battery life include 500hrs, 1000hrs, 2000hrs etc
Battery Indicator Specify the battery indicator levels
Cycle Life Specify the cyle life of the digital pressure gauge. e.g 5million cycles, 10 million cycles etc
Operating Temperature Operating temperature of gauge is critical for accurate performance. Specify the operating temperature of gauge depending on temperature regime of the environment where the gauge is to be used.
Storage Temperature  Specify storage temperature of gauge
LCD Display Specify the type of LCD display
Character Height Specify the character height of the LCD display - specify height of upper character and that of lower character
Engineering Units Specify the engineering units you want your digital gauge to display - psi, bar, mmHg etc
Backlight Specify whether the LCD display should have a backlight especially in applications where illumination could be a problem especially at night
Enclosure Rating Specify the enclosure rating. Common enclosure rating for gauges is IP 67
Keypad Functions Specify the keypad functions of the gauge. Three key with multi press functionality are typical. 




Process Safety – Basics of UEL & LEL of Hazardous Gases

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In today’s highly complex industrial environment, process safety is a key consideration in the maintenance and sustenance of very expensive and complex process facilities. Furthermore, the classification of a plant environment into various classes – Class I, II, III – according to the degree or probability of occurrence of hazards has greatly simplified the management of process safety. However, despite these classifications and the abundance of a lot of knowledge on hazardous gases, accidents still occur in process plants in the most bizarre manner. These are largely due to a lack of basic knowledge about the nature of the hazardous gases themselves and what constitute an explosive atmosphere for example.

The objective of this piece is to explain the concept of explosive limits in hazardous atmospheres in a plant.

Fire Triangle and Explosive Limits
To have combustion in a hazardous atmosphere, there must be sufficient fuel, sufficient oxidizer (commonly Oxygen) and sufficient energy for ignition. These three elements make up what we call the FIRE TRIANGLE. Please read: Hazardous Area Classification for more detail.

The fire triangle does not give sufficient information to tell us if the necessary     conditions exist to support a fire or explosion. For a fire or explosion to occur, we need an adequate mixture of fuel and oxidizer in the correct proportions, and a source of ignition energy exceeding a certain minimum threshold. The thresholds required for combustion to take place with fuels are referred to as the Lower Explosive Limits (LEL) and the Upper Explosive Limits (UEL). While the threshold for energy is referred to as the Minimum Ignition Energy (MIE)

Ignition Curve of a Typical Combustible Fuel
The ignition curve for a gas or flammable vapor shows all the conditions -fuel, air(oxidizer), minimum energy – required for an explosion or combustion to take place in a flammable atmosphere. It is specific and typical for any fuel and oxidizer combination. Most ignition curves are published with the assumed conditions of air as the oxidizer, at room temperature and at atmospheric pressure.. Below is a typical ignition curve for a combustible gas.
Typical Ignition Curve of a Gas

There are three critical values on the ignition curve above:
(a) The Lower Explosive Limit (LEL) of a gas
(b) The Upper Explosive Limit (UEL) of a gas
(c) The Minimum Ignition Energy (MIE) of a gas

Lower Explosive Limit (LEL)
The LEL of a gas is the lowest concentration (percentage) of a gas or vapor in air capable of producing combustion in the presence of an ignition source (flame, heat etc). It can also be referred to as the Lower Flammable Limit (LFL).

Upper Explosive Limit (UEL)
The UEL of a gas is the maximum concentration(percentage) of gas or vapor that will burn in air in the presence of an ignition source. Above the UEL, the mixture is too “rich” to burn. The range between the LEL and the UEL as shown in the graph above is known as the flammable range of the gas. The larger the flammability range, the greater the potential for an explosive mixture of the gas with air.

Minimum Ignition Energy (MIE) of a Gas
As the name implies, the MIE of a gas is the minimum energy required for the gas – air mixture to burn in air in the presence of an ignition source. It is specific for each type of gas.

Variation of LEL, UEL and MIE
The critical values of LEL, UEL and MIE differ for every type of fuel and oxidizer combination and they change with ambient temperature and pressure. They may be rendered irrelevant in the presence of a catalyst.

LEL and UEL of Some Common Combustible Substances
Substance
LEL (% Volume)
UEL (% Volume)
Acetylene
2.5
100
Acetone
2.5
12.8
Butane
1.58.5
Carbon Disulfide
1.350
Carbon Monoxide
12.5
74
Ether 
1.936
Gasoline
1.4
7.6
Kerosene
0.75
Hydrazine
2.9
98
Hydrogen
4.0
75
Methane
4.4
17
Propane
2.1
9.5