Transmitter Selection Criteria

This section presents a number of considerations that should be viewed in selecting a transmitter. They include functional specifications, performance specifications, material selection and desirable features. Also included are the definitions of these specifications and the relationship to functional and performance requirements.

Functional Specifications
  1. Temperatures
  2. Pressure
  3. Environment
  4. Hazardous Locations
  5. Damping
  6. Zero Elevation or Suppression
  7. Power Supply and Load Limits

1. Temperatures  
             
Both the maximum process and ambient temperatures need to be considered. Often the process temperature will exceed the limits of the sensing element. The sensing element of most electronic pressure transmitters will not operate properly above 225° F(107° C). This will require the use of good impulse piping practices to get the transmitter temperature back within operating limits. High ambient temperatures on solid state electronics adversely affect component life. Most electronics are not rated for service above 200° F (90°C) and there are many components with a 185°F (85°C) rating. High temperatures tend to cause more electronic failures. Again, it is good engineering practice to keep the electronics package as cool as possible.

Winterizing, either by steam tracing, electrical heaters, or heater controlled enclosures may also be a consideration.

2. Pressure

Both the operating pressure range and the maximum pressure should be considered. Gauge pressure transmitters should have an over-pressure rating of at least 150 percent of the maximum rating operating pressure with no other ill effect than having to re-calibrate.

The minimum pressure should be also be considered. As part of the normal operation, a vacuum may be applied. Many transmitters have to be ordered special to obtain this capability.On differential pressure transmitters, over-pressure may be accidentally applied to either the high or low side of the unit when a three-value manifold is not sequenced properly. High over-pressure capability eliminates a possible shut-down while the unit is being re-calibrated or repaired. The static line pressure for differential transmitters should also be called out. Units are available on the market with standard line pressure capability from 500 to 6,000 psi.



3. Environment  

The transmitter should be capable of operating in environments with 0 to 100% relative humidity.The working fluid and the ambient environment should be considered for corrosiveness. For instance, transmitters used on offshore oil rigs are subject to corrosion from salt water. Another example is a transmitter in a steam or cooling water system in the vicinity of acids or bases that tend to get into the atmosphere. The above applications have a non-corrosive working fluid, and a very corrosive ambient environment.

4. Hazardous Locations    

Use of Instruments in Hazardous Locations:

The Williams-Steiger Occupational Safety and Health Act of 1970 (OSHA), Subpart S, Electrical Considerations, has been in effect since 15 February, 1972. The purpose of OSHA is to accelerate the adoption of national standards for occupational safety. The Act given the Secretary of Labor two years to promulgate the adoption of such standards.

All electrical instruments or electrical equipment used in hazardous locations must now be approved. Equipment or an installation is acceptable to the Assistant Secretary of Labor, and approved within the meaning of Sub-part S if it is accepted, or certified, or listed, or labeled, or otherwise determined to be safe by a nationally recognized testing laboratory, such as, but not limited to, Underwriters Laboratories Inc. and Factory Mutual Engineering Corp.

Definition of Hazardous Locations:

Class I, Division I
Locations in which hazardous concentrations of flammable gases or vapors exist continuously, intermittently, or periodically under normal operating conditions.

Class I, Division II
Locations in which volatile flammable gases are hazardous liquids, vapors or gases will normally be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown of such systems or containers, or in case of abnormal operation of equipment.

Class II Locations
Locations which are hazardous because of the pressure of combustible dust.

Class III Locations
Location in which easily ignitable fibers or materials producing combustible flyings are present.

Group A
Atmosphere containing acetylene.

Group B
Atmospheres containing hydrogen or gases or vapors of equivalent hazards such as manufactured gas.

Group C
Atmospheres containing ethyl ether vapors, ethylene, or cyclopropane.

Group D
Atmospheres containing gasoline, hexane, naptha, benzine, butane, alcohol, benzol, lacquer solvent vapors, or natural gases.

Group E
Atmospheres containing metal dust, including aluminum, magnesium, and their commercial alloys, and other metals of similar hazardous characteristics.

Group F
Atmospheres containing carbon black coal or coke dust.

Group G
Atmospheres containing flour, starch, or grain dusts.

Explosion- Proof Enclosure

Explosion-proof enclosure means an enclosure for electrical apparatus which is capable of withstanding, without damage, an explosion which may occur within it, of specified gas or vapor, and capable of preventing ignition of specified gas or vapor surrounding the enclosure from sparks or flames from explosion of specified gas or vapor within the enclosure.
To make a system explosion-proof, the enclosure must be capable of withstanding an explosion, and the system must be installed per national electrical code for hazardous locations.

Intrinsically Safe Equipment

Intrinsically safe equipment and wiring are incapable of releasing sufficient electrical energy under normal or abnormal conditions to cause ignition of specific hazardous atmospheric mixture. Abnormal condition will include accidental damage to any part of the equipment or wiring, insulation, or other failure of electrical components, application of over-voltage, adjustment and maintenance operations, and other similar conditions.

Equipment built for this requirement is designed with low energy storage components as outlined in ISA procedure RP12-2
Several advantages to the intrinsic safety approach are listed below. These advantages have to be weighed against the initial higher purchase price. Today it is estimated that 60 percent of these types of installations are classified as intrinsically safe.
· Lower installation cost.
· Less operator-dependent to maintain a safe system.
· Easier to maintain and repair.
· Accessible to repair without special precautions before opening the unit.

5. Damping                  

In some applications, pump or other process noise pulses must be dumped out to get good control or indication. The more unit is damped , as specified by the corner frequency, the slower the response time. In other cases where the system dynamics require fast transmitter response for best performance. A review of the specific application is necessary to determine the requirements for adequate performance. However, in most cases this is not a serious problem.


Bode plot

Damping is defined in terms of corner frequency or "time constant." Corner frequency is the junction of two confluent straight line segments of a plotted curve (see Figure 7). 
In a first-order system, the frequency at which the magnitude ratio is down 3 db

 Db = 20 log Eo / Ein 

For the output of a first order system forced by a step or an impulse, the time constant is the time required to complete 63.2 percent of the total rise or decay.
 In a Bode diagram, the break-point or corner frequency occurs where:

 Fc = 1 / 2p T 

Where T = Time constant,
           Fc = Corner Frequency.
Output                  


The standard output for two-wire transmitters is 4 - 20 mA dc or 10 - 50 mA dc. There are also four-wire transmitters that can provide zero-based voltage signals. The most common is 0 - 5 V dc.Three-wire transmitters are also available, which can provide a 4 - 20 mA dc, 10 -50 mA dc, or zero-based signal.


6. Zero Elevation or Suppression 
          
Zero Elevation-for an elevated zero range, the amount the measure variable zero is above the lower range value. It may be expressed either in units of measured variables or in percent of span(see Table 1).

Zero Suppression-for a suppressed zero range, the amount the measured variable zero is below the lower range value. It may be expressed either in units of the measured variable, or in percent of span (see Table 1).
Gage pressure ranges are usually expressed in pounds per square inch gauge (i.e., 0-100 psig). The range may have a suppressed zero (i.e., 50 to 100 psig), or it may be a compound range (i.e., 20 in Hg vacuum to 50 psig).
Absolute ranges are usually expressed in inches of mercury absolute or psia (i.e., 0-30 HgA or 0-100 psia). The most common range with a suppressed zero is the barometric range (i.e., 28 to 32 in HgA).

7. Power Supply and Load Limits          

The choice of a power supply for two-wire transmitters will depend on the load. Most transmitters are capable of operating over a wide range of load limits (see Figure 9).

Wiring should consist of twisted pairs. Most transmitters do not require shielding, but it is recommended to eliminate noise pickup from electric motors, inverters, or other noise generating electrical equipment in the area getting into the receiver.
The size of the wire is usually not critical; 18-gauge is usually sufficient. The resistance of the wire adds to the total load, and in most cases simply requires a power supply of sufficient voltage to handle the entire voltage drop across the system.
Special considerations should be given when using computers, especially where sampling times are short. Electronic transmitters often have internally generates driving frequencies, which may show up as high frequency noise on the output.As an example, assume the sample time in 85 microseconds, and assume some 50 kHz frequency noise on the line, which would have an equivalent period of approximately 6.4 microseconds. The integration time to average out this noise is only 13.4 cycles. This does not allow enough time to completely integrate a high level noise. If it is significantly high, a filter should be considered.