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Temp TempF C1652° 9001472° 81292° 701112° 60932° 500°752° 400°572° 300°392° 200°212° 100°32°-148° -100°ArFA MilPIMEIMOFM25°C (Reference) WAIIpiMilIl& 0° C (ReferencePAMNMFAIIIINEllEN I IIIlillI■0 5 10 15 20 25 30 35 40Signal in MillivoltsFigure 43. Iron-Constantan Thermocouple Chart■model 2) is selected, the reference voltage is convertedby the ADC to a digital value.RBT Operating CharacteristicsThe RBT circuit supplied with the 1851 model 2 hasthe following characteristics.Range Reference Ou put RBT Bridge Output10 mV MaximumMinimum20 mV MaximumMinimum50 mV MaximumMinimum65°C 25°C 5°C 65°C 25°C 5°C4.90 mV3.78 mV9.58 mV7.47 mV4.63 mV3.70mV9.10 mV7.33 mVVrbtVrV rbtVr4.58 mV3.64mV9.01 mV7.21 mV+ 41.0+5.010.00mV7.91 mV20.00 mV16.05mV3.37mV2.48 mV6.63 mV4.90mV0.12mV-O.IOmV0.24mV-0.19mV24.03 mV 23.16 mV 23.02 mV 50.01 mV 16.87 mV 0.61 mV18.94 mV 18.71 mV 18.79mV 39.58 mV 12.52 mV -0.50mV129175AThe preceding millivolt values are converted to adigital value by the ADC , and they are converted tothe corresponding temperature by one of the followingformulas.Where:Trbt (°F) = 51.876Trbt (°C) = 28.82Trbt= RBT TemperatureVrbt = ADC reading (Q value) for the RBT bridge output45 N Vr = = ADC reading (Q value) for the reference outputI17603AThe values 51.876 and 41.0 (Fahrenheit) and 28.82 and 5.0(Celsius) are constants for the RBT supplied with the thermocoupleblock. Trbt can be computed in degrees Celsius and converted todegrees Fahrenheit with the following formula.produced by this imbalance is called the RBT bridgeoutput. This output and the reference voltage aremade available to the 1800 program, which theninterprets the two values. Thus, thermocouplesignals are compensated for reference junctiontemperature changes.The RBT circuit outputs are made available atthe first and second address terminals within the1851 model 2. (See Figure 37 for the Mpx/R addressthat will select points 00 and 01 within the1851 model 2 being used). When point 01 (withinan 1851 model 2) is selected by the multiplexer,the RBT bridge output is converted by the ADC toa digital value. When point 00 (within an 1851Trbt (°F) =i Trbt in °C) +3229165AThermocouple signals of up to ±50 mV can be terminatedin the first Mpx/R group of an 1851 model2. However, when the multiplexer (under control ofthe 1800 program) connects a thermocouple signal tothe ADC, the millivolt signal is converted to a digitalvalue between 00000 and ±16383. The 1800 programmust compute a temperature that correspondsto the ADC value.Thermocouple Conversion AccuracyAccuracy of thermocouple signal conversion dependson many factors. Some of these factors are:110