Analog transmitters are adjusted and set for sensor type and for one measuring range. If a sensor type or measuring range is changed, the transmitter must also be exchanged. A programmable transmitter on the other hand, can be reprogrammed by entering the new parameters for the changed application. When designing a system, it is possible to select transmitters in which the required measuring range can be set at start-up. This simplifies and reduces the planning and design time and reduces replacement part inventory costs. Programmable transmitters also clearly reduce ser- vice and maintenance expenditures thus reducing the cost of ownership.
Circuit Block Diagram
The following circuit block diagram shows a typical design for a programmable temper- ature transmitter. The transmitter contains two microcontrollers. In the primary circuit as well as in the secondary circuit the controller operates using the software (Firmware) designed for that circuit. In the primary circuit the multiplexer is controlled, which trans- fers the values from the sensors, the reference and the reference junction. The signals reach the analog-digital-converter and are read by the microcontroller. Filter functions and sensor failure monitoring is also carried out by the this controller. The digitized sig- nal is fed by a transducer to the microcontroller in the secondary circuit. The transducer also provides the electrical isolation between the primary and secondary circuits.
Fig. 5-13: Circuit block diagram of a digital temperature transmitter
The second microcontroller in the secondary circuit controls the digital-analog-con- verter and is responsible for the data exchange between the communication and the programming software. The required software (Firmware) is stored in an EEPROM. An I/U-converter powers the transmitter from the 4...20 mA signal. This same 4...20 mA signal is used to provide communication with a supervisory system (PC) using a FSK- interface. µC µC U FSK EEPROM I 4...20 mA D A A D MUX Ref. junc- tionPt100 Reference Filter Sensor break monitor
As can be recognized in the following figure, two sensors can be connected to the transmitter. The averages and differences between the two sensor signals can be cal- culated and also transmitted as an output signal.
Fig. 5-14: Software structure of a digital temperature transmitter
Programmed Curves
In a programmable transmitter are all the curves for the most common measuring applications stored. They include the basic values for the appropriate measurement resistors and thermocouples, which can simply be selected when programming the transmitter.
A Pt100-resistance thermometer in accuracy class Type B has a temperature depen- dent measuring error at 400 °C (752 °F) of several K (seeFig. 3-5). For measurements with resistance thermometers the achievable accuracy after selecting the standard curve can never be better than the allowable measuring deviations of the sensor. Programmable transmitters, such as the TTH300, offer the possibility to use the exact curve of a previously measured temperature sensor by entering the coefficients for the Callendar Vandusen equation (polynomial see chapter 3.1.5).
+ lin T 1 2 3 4 5 6 Reference junction Pt100 Input 1 Input 2 Measure- ment val. conditio- ning, wire compen- sation Refer- ence junction correc- tion Lineari- zation average differ- ence Damping Scaling FSK Status Analog output
1 = Sensor signal input
2 = Reference junction temperature 3 = Linearized measured value 4 = Percent of the output span 5 = Output value in mA 6 = FSK programming
For curves with a monotomic curve shape it is possible to enter a free style curve with using as many as 64 points. In this way a digital transmitter can be matched to any sen- sor or to the calibration or adjustment of the entire measuring chain. To accomplish this, the sensor to be calibrated, together with the transmitter and its power supply instru- ment, are calibrated against a “Standard“. The deviations of the output signal are cor- rected in the transmitter. Deviations from the curve for the entire measuring chain as low as < ± 0.05 K are possible.
Diagnosis
Programmable transmitters include extensive capabilities to detect and signal error conditions. In order to provide the user with an effective trouble shooting strategy, the error types where classified and prioritized by NAMUR based on their cause and importance to operation. A distinction is made between sensor, transmitter, configura- tion/calibration and measuring range errors. Based on the priority assigned to each error, the transmitter selects and signals the error with the highest priority. Process control systems utilize a classification system for display and diagnosis based on their operating phase, start-up, operation, monitoring or asset management. In this way the user is provided with the most important information at the correct location at the correct time.
Tbl. 5-1: Diagnosis and error classifications for transmitter TTH300
Standard
• Sensor error (break or short circuit)
• Instrument error
• Over/under measuring range
• Simulation active Expanded
• Over/under alarm value
• Sensor backup active (Sensor 1 or Sensor 2 failure)
• Zero or span adjustment active
• Low power supply
• High transmitter ambient temperature (> 85 °C (185 °F))
• Memory
• Indicator
• Writing protection
Drift Warning and Redundancy Circuit
Recalibration and recertification are normal procedures for measuring locations which are subject to measuring instrument inspections. Two channel transmitters, such as the TTH300, can provide some relief, by increasing the required recalibration interval. To check for drift, a temperature sensor with two integrated measuring locations can be used. In addition to its actual measuring function, the transmitter continuously com- pares the difference between the two measuring locations. If the deviation exceeds a specified value, an alarm is signalled. Using this signal, the user is advised by the trans- mitter that a recalibration is required. The number of manual inspections are apprecia- bly reduced, because a recalibration will only be conducted when it is really necessary.
To increase the operational availability, two redundant temperature sensors are installed. For single channel transmitters the connections can be manual switched to the other sensor if one fails. Two independent Pt100 measuring locations can be connected to a two channel transmitter. Using the integrated “hot swap“ function, if a malfunction in one of the measuring locations is recognized by the transmitter, an error is signalled and the input is immediately switched to the redundant element. The on- time of the measuring location is significantly increased, since the repair of the defec- tive element can made during the next, scheduled service shut down. In summary, two channel transmitters appreciably reduce service and maintenance expenditures.