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Temperature Measurement

Use of Temperature Transmitters instead of Direct Wiring

For temperature measurements, two ways have traditionally been employed to get process readings back to a monitoring and control system.

The two ways are

One method  is to use sensor extension  wires to carry  the low-level signals (ohm or mv) generated  by field mounted RTD or thermocouple  sensors directly to control room.

Another  is to install  temperature transmitters at or near the measurement point. The transmitter amplifies  and conditions  the sensor signal.  It also transmits  it over a twisted wire pair back  to the control  room.

Importance of Temperature Transmitters instead of Direct Wiring

Direct wiring strategies  have generally been considered  less expensive  and sometimes easier. Because of cost considerations, transmitters were often  reserved  for control and  applications where signal  and loop integrity  were a must. Today’s  highly  functional  microprocessor-based  field-mount temperature transmitters are comparable in price to direct  wiring strategies.  When the additional  advantages  of using  intelligent transmitters  are factored in, we  can save considerable  time and  avoid  maintenance headaches.  This is especially  true when  the measurement point  is located  a long  distance  from the readout  and control  system.

Cut Wiring  Costs

The direct wiring of thermocouples  to a control  system  requires  the use of thermocouple  extension  wires. Extension  wire can cost several  times  more than  common shielded  copper  wire used  for a temperature transmitter’s 4-20 mA signal.  The savings in wire cost can go a long  way toward  paying for the temperature transmitter,  The longer  the wire run, the greater  the potential savings.

In retrofit situations.  we  may want to switch  to  transmitters,  but you  may be reluctant  to do so because  you mistakenly believe  that  new copper  wires must be run to accommodate   the 4-20  mA. This is not necessarily  the case. Temperature  transmitters can be installed  at the sensor,  and  the in-place  RTD or thermocouple  extension wires can  be used to  transmit  the 4-20 mA back to the control  system. The qualifier  is resistance.

Because  thermocouple  wire has higher resistance  than  copper wire. we should  run the resistance  calculation  on your  extension wire to make sure you  do not over-burden the temperature   transmitter, If these calculations confirm  that you can  use the existing field wiring,  you will have  no additional installation   time or material  costs. And, you still get all the advantages  of using temperature   transmitters.

Reduce  Hardware   Costs 

With direct  wiring,  it is necessary  to provide temperature input cards (Thermocouple/RTD Cards) for the DCS and PLC. These cards  usually cost  more per point  than  does a 4-20  mA input  card.  If you  are using  temperature transmitters,   the lower cost 4-20  mA input  cards  help to pay for the transmitters.   Additionally.  you  are keeping in  inventory  one less board  type and  reducing  another  source  of maintenance  confusion. Temperature transmitters   usually  can  handle more sensor  types  than  can a temperature  input  board  for a DCS or PLC. That enables  the user to select  the sensor  type that  is ideal for the application   and  not be limited by what sensors can be handled by an input card in a direct wired scheme. Sensors that are likely acceptable to a temperature transmitter and not to a DCS/PLC input card are four-wire RTDs. 1000-ohm RTDs, and 10-ohm copper RTDs.

With an intelligent temperature transmitter, we can simply change out the sensor and reconfigure the transmitter to accommodate the different sensor type. The loop’s twisted pair wiring and existing 4-20 mA input boards don’t even have to be touched. Because you never know what sensor you’ll end up with. make sure you select a universal transmitter that configures to accept all common temperature sensor types and temperature ranges.

we gain more application flexibility if you use a temperature transmitter with adequate input/output isolation. Temperature sensors insulated with Mg0 will eventually go to ground. If the sensor is an RTD, you must discard it and buy new. But if that sensor is a TC and you have isolation elsewhere in the measurement loop, the grounded TC still provides a valid tempera­ture measurement. If you place the loop isolation in the transmitter, you gain the flexibility of being able to use non-isolated 4-20 mA input cards, which are generally the least expensive input cards. If all your TCs are grounded. no error-causing ground loops can occur when you use isolated TTs.

Enhance Accuracy and Stability

Using temperature transmitters can substantially enhance measurement accuracy. DCS and PLC systems measure readings over the entire (very wide) range of a sensor. It is well known that measuring a narrower range produces far more accurate measurements. Transmitters can be calibrated to any range within a sensor’s overall capabilities. Their measurements are more precise than is possible with most direct wiring strategies. Some transmitters deliver accuracy ratings of ±0.13’C (±0.23’F) when paired with a common Pt 100 RTD sensor over a 200-degree span.

If you need even better accuracy, you can trim a universal transmitter to precisely match a particular sensor. Even though sensors are designed to have a high degree of conformance to an established curve, each one — even precision sensors — will vary slightly from its stated specification. By using the n electronics to match the IT to the sensor. you eliminate this final bit of error.

It’s called sensor-to-transmitter trimming. The transmitter is connected to the sensor and then immersed in calibration baths maintained at stabilized temperatures. The transmitter then captures two readings from the sensor, representing the upper and lower range values . and stores them in non-volatile memory. The transmitter uses these values to compensate for deviations

between the sensor’s stated linearization curve and its actual measurements. When transmitters are paired with a 1,000 ohm RTD, this technique results in the amazing measurement accuracy of up to ±0.014t (+0.025T) over a 100-degree span.

To further enhance measurement accuracy. some transmitters can be trimmed to respond to two data points within the selected zero and span measurement range. This advantage allows a complete range to be monitored, while placing measurement emphasis on a specific segment of the range most critical to the process.

Simplify Engineering

In place of numerous sensor lead-wire and DCS/PLC input board combinations, engineering designs and drawings will need to show only one wire type (twisted wire pair) and one input board type (4-20 mA). This one wire and one input board system means maintenance is greatly simplified, and the chances of loop mis-wiring” are virtually eliminated.

Throughout the lifetime of a process. enhancements are routinely made to accommodate the manufacture of upgraded or even completely new products. Process changes may require different measurement ranges or greater temperature accuracy than was previously required. Either of these conditions may necessitate a change in the type of sensors that are used.

Lower Maintenance Expenses

Temperature transmitters have come a long way since the days of fixed-range. inflexible instruments. Some transmitters are not only universal in regards to input type and range. but they also incorporate powerful sensor diagnostics that save considerable time and money.

Temperature transmitters with intelligent diagnostic capabilities help you keep track of sensor operation and quickly find and diagnose sensor failures. Capable of continually monitoring the sensor. if a wire breaks or otherwise stops sending a signal during operation. the transmitter sends the output upscale or downscale to warn of sensor burnout and other unwanted conditions.

Furthermore, the transmitters can tell you which wire has broken via an error message either on an integral digital display or using PC configuration software. Specific fault messages eliminate the work of removing the sensor or checking all the lead wins to diagnose a problem. During startups. in the middle of the night, or in the middle of winter. this can be a huge time-saving advantage.

Avoid Lead Wire Imbalances

Where feasible. use four-wire RIDs and specify a temperature transmitter that is able to accept a “true” four-wire RTD input. The advantage is that four-wire RID measuring circuits effectively cancel out errors caused by resistance imbalances in the current-carrying leads. Every ohm of imbalance in the RTD sensor’s lead wires can produce as much as a 2.5*C error in the measurement. Serious imbalances usually occur gradually over time and are caused by corrosion of the lead wire. Occasionally, improper installation techniques also cause resistance imbalances from lead length differences, wire gauge mismatches, loose connections. terminal block corrosion. and work hardening from bending and other stresses.

Intelligent temperature transmitters are capable of accepting me four-wire RID inputs and provide a constant current source to the outer leads of the RTD. The voltage drop is measured across the inner leads, which is a high impedance loop. Because there is essentially no current flow in the voltage loop. voltage is directly proportional to RTD resistance. Lead resistance is ignored. You will get a very accurate measurement providing the resistance value of the Rn) — plus corrosion. plus wire resistance — is less than 2.000 ohms (typically’. A four-wire RID costs about the same as a three-wire and can be used with less expensive. smaller gauge wire without concern for added resistance.

Protect Against Plant Noise

Common  in nearly  every  industrial  environment,  RFI (radio frequency  interference) and  EMI (electromagnetic interference)  can negatively  affect  process signals.  Before you eliminate  RFI and  EMI as the possible  culprits of erratic  signals,  you  should  consider some  of their  common  sources:  mobile and stationary   radios, TVs, hand-held   walkie- talkies,  radio-controlled   overhead  cranes. radar,  induction  heating  systems.  static  dis- charge, high-speed  power-switching   elements,  high ac current  conductors,   large solenoids  and  relays,  transformers. AC and DC motors,  welders,  and  even  fluorescent lighting.  If you  have one  or more of these in your  plant,  you may  have  an RFI/EMI problem. The result  is sometimes  Just   a minor inconvenience.   Other  times, it can be as serious  as a nuisance  plant  trip,

In a direct  wiring  scheme.  the low-level signals  generated   by an  RTD (ohm) or  thermocouple  (mV) arc particularly   susceptible to the signal-degrading    effects  or RFI/EMI. Compounding   the  problem,  sensor extension  wires can  behave  much  like an RFI/EMI antenna   by actually  drawing  plant noise  (0  the wires  and affecting  weak,  low- level signals.

Conversely, a properly  designed  tempera- ture transmitter  effectively  negates  the effects of RFI in two ways.  If the RFI/EMI is on the temperature  sensor,  the transmitter’s RFI filtering  will prevent  the noise  from propagating   to the transmitter’s   output.  If the noise  is being  picked up on the 4-20 mA side of the transmitter,   the 4-20 mA signal  strength  will be less distorted  by the RFI than  would  a mV level signal.  The transmitter’s   output  has a higher  signal-to-noise  ratio. When specifying  your  transmitter, always  check  for RFI/EMl protection.   If there  is no specification   given,  it’s usually because  the instrument   is not designed  to resist noise.  It probably  will not perform’ very  well in a noisy  plant  environment.

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1 comment

Shaju September 9, 2015 at 11:01 am

Dear sir,
The difference between mv and 4-20mA transmission explanation is very effective.The point that we can use extension wires for carrying transmitter 4-20mA signal is informative.
Thanks for the information.

Reply

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