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an701: SCM7B Thermocouple Modules and CJC
Application Note
A. VOLTAGE-TO-TEMPERATURE CONVERSION
When SCM7B37 thermocouple modules are used to measure temperature, the measured output voltage must often be converted back to temperature. This is readily done with the SCM7B37 series because Cold Junction Compensation (CJC) is incorporated into the module and SCM7B backpanels.The conversion method is illustrated with an example:
A type K thermocouple (TC) is to be used with the SCM7B37K-02:
SCM7B37 Module values used in Equation 1 are shown in the following table. ITS-90 characteristics were used to calculate.
- Identify the TC and Module Voltage ranges:
Temperature Input Module Voltage Output -100°C +1V +1350°C +5V
The Minus Full Scale Module Output (known as the "Pedestal") = +1V
The Module VOUT Range is 5 1 = 4V - From the type K TC tables determine the full scale voltages:
V(100°C) = 3.5531mV (TC Neg. F.S. Voltage)
V(+1350°C) = +54.138mV (TC Pos. F.S. Voltage)TC VIN Range = TC Pos. F.S. Voltage TC Neg. F.S. Voltage
The SCM7B37K module gain (G), is given by:
G = Module VOUT Range / TC VIN Range ∴G = (4) / [0.054138 (0.0035531)] = 69.335 V/V 3. C - Calculate the effective TC voltage (VT) from the Measured Module Output Voltage (VOUT) using the following formula:
SCM7B37 Module Output Voltage-to-Thermoelectric Voltage
Equation 1:VT = [(VOUT Pedestal) / G] + (TC Neg. F.S. Voltage)
∴VT = (Module Measured VOUT 1V) / 69.335) + (0.003553)
- Find the value of the field temperature being measured by crossing VT to thermocouple temperature in your application programs thermocouple looktable (referenced to a 0°C CJC temperature, which is the case as long as the thermocouple-backpanel junction is within the specified CJC ambient range).
| Module type | G (V/V) | PEDESTAL, V | Thermocouple Neg.Full Scale (mV) |
|---|---|---|---|
| SCM7B37J-01 | 84.120 | +1 | -4.6325 |
| SCM7B37J-01A | 105.150 | 0 | -4.6325 |
| SCM7B37J-01D | 210.300 | 0 | -4.6325 |
| SCM7B37J-10 | 371.101 | +1 | 0.0 |
| SCM7B37J-10A | 463.876 | 0 | 0.0 |
| SCM7B37J-10D | 927.752 | 0 | 0.0 |
| SCM7B37J-11 | 183.083 | +1 | 0.0 |
| SCM7B37J-11A | 228.853 | 0 | 0.0 |
| SCM7B37J-11D | 457.707 | 0 | 0.0 |
| SCM7B37J-12 | 120.837 | +1 | 0.0 |
AN701: Continued
| Module type | G (V/V) | PEDESTAL, V | Thermocouple Neg.Full Scale (mV) |
|---|---|---|---|
| SCM7B37J-12A | 151.046 | 0 | 0.0 |
| SCM7B37J-12D | 302.093 | 0 | 0.0 |
| SCM7B37J-13 | 238.447 | +1 | +16.3272 |
| SCM7B37J-13A | 298.059 | 0 | +16.3272 |
| SCM7B37J-13D | 596.118 | 0 | +16.3272 |
| SCM7B37K-02 | 69.335 | +1 | -3.5531 |
| SCM7B37K-02A | 86.669 | 0 | -3.5531 |
| SCM7B37K-02D | 173.338 | 0 | -3.5531 |
| SCM7B37K-20 | 327.639 | +1 | 0.0 |
| SCM7B37K-20A | 409.549 | 0 | 0.0 |
| SCM7B37K-20D | 819.097 | 0 | 0.0 |
| SCM7B37K-21 | 160.607 | +1 | 0.0 |
| SCM7B37K-21A | 200.759 | 0 | 0.0 |
| SCM7B37K-21D | 401.518 | 0 | 0.0 |
| SCM7B37K-22 | 81.903 | +1 | 0.0 |
| SCM7B37K-22A | 102.379 | 0 | 0.0 |
| SCM7B37K-22D | 204.758 | 0 | 0.0 |
| SCM7B37K-23 | 167.135 | +1 | +24.9055 |
| SCM7B37K-23A | 208.919 | 0 | +24.9055 |
| SCM7B37K-23D | 417.837 | 0 | +24.9055 |
| SCM7B37T-03 | 164.945 | +1 | -3.3786 |
| SCM7B37T-03A | 206.181 | 0 | -3.3786 |
| SCM7B37T-03D | 412.362 | 0 | -3.3786 |
| SCM7B37E-04 | 58.151 | +1 | 0.0 |
| SCM7B37E-04A | 72.689 | 0 | 0.0 |
| SCM7B37E-04D | 145.377 | 0 | 0.0 |
| SCM7B37R-05 | 191.598 | +1 | 0.0 |
| SCM7B37R-05A | 239.497 | 0 | 0.0 |
| SCM7B37R-05D | 478.994 | 0 | 0.0 |
| SCM7B37S-06 | 216.177 | +1 | 0.0 |
| SCM7B37S-06A | 270.221 | 0 | 0.0 |
| SCM7B37S-06D | 540.441 | 0 | 0.0 |
| SCM7B37B-07 | 294.331 | +1 | 0.0 |
| SCM7B37B-07A | 367.914 | 0 | 0.0 |
| SCM7B37B-07D | 735.828 | 0 | 0.0 |
When the SCM7B47 thermocouple modules are used to measure temperature, the measured output voltage is also often converted back to temperature. This is readily done with the SCM7B47 series because, like the SCM7B37 Modules, Cold Junction Compensation (CJC) is incorporated into the module and SCM7B backpanels. However, unlike the SCM7B37 Modules, the module output voltage is a linear representation (Y = M X + B) of the input temperature.
The conversion method is illustrated with an example.
A type T thermocouple (TC) is to be used with the SCM7B47T-06.
- Determine the Module Transfer Function:
Temperature Input Module Voltage Output -100°C +1V +200°C +5V
Let TLOW ≡ Neg. Full Scale Temperature = 100°C
Let M ≡ (Output Voltage Span)/(Input Temp Span) = +13.333mV / °C - Find the Temperature corresponding to Module Output Voltage: Since Y = MX + B, with X = (Field Temperature TLOW)
VOUT = M x (Field Temperature TLOW) + Pedestal
Solving for the Field Temperature gives:
SCM7B47 Voltage-to-Temperature Conversion
Equation 2:
Temperature = (VOUT Pedestal) / M + TLOW
with
M = (Module Output Volt Span) / (Input Temp Span)
M = (Module Output Volt Span) / (Input Temp Span)
In this case, Temperature (°C) = (VOUT 1V) / 13.333mV + (100°C)
AN701: Continued
SCM7B47 Module values used in Equation 2 are shown in the following table:| Module type | M (mV/°C) | Pedestal, V | TLOW, °C |
|---|---|---|---|
| SCM7B47J-01 | 5.2632 | +1 | 0 |
| SCM7B47J-01A | 6.5790 | 0 | 0 |
| SCM7B47J-01D | 13.1579 | 0 | 0 |
| SCM7B47J-02 | 10.0000 | +1 | -100 |
| SCM7B47J-02A | 12.5000 | 0 | -100 |
| SCM7B47J-02D | 25.0000 | 0 | -100 |
| SCM7B47K-03 | 3.0769 | +1 | 0 |
| SCM7B47K-03A | 3.8462 | 0 | 0 |
| SCM7B47K-03D | 7.6923 | 0 | 0 |
| SCM7B47K-04 | 6.6667 | +1 | 0 |
| SCM7B47K-04A | 8.3333 | 0 | 0 |
| SCM7B47K-04D | 16.6667 | 0 | 0 |
| SCM7B47T-05 | 10.0000 | +1 | 0 |
| SCM7B47T-05A | 12.5000 | 0 | 0 |
| SCM7B47T-05D | 25.0000 | 0 | 0 |
| SCM7B47T-06 | 13.3333 | +1 | -100 |
| SCM7B47T-06A | 16.6667 | 0 | -100 |
| SCM7B37J-12A | 33.3333 | 0 | -100 |
| SCM7B47E-07 | 4.4444 | +1 | 0 |
| SCM7B47E-07A | 5.5555 | 0 | 0 |
| SCM7B47E-07D | 11.1111 | 0 | 0 |
| SCM7B47R-08 | 3.2000 | +1 | +500 |
| SCM7B47R-08A | 4.0000 | 0 | +500 |
| SCM7B47R-08D | 8.0000 | 0 | +500 |
| SCM7B47S-09 | 3.8095 | +1 | +700 |
| SCM7B47S-09A | 4.7619 | 0 | +700 |
| SCM7B47S-09D | 9.5238 | 0 | +700 |
| SCM7B47B-10 | 4.0000 | +1 | +800 |
| SCM7B47B-10A | 5.0000 | 0 | +800 |
| SCM7B47B-10D | 10.0000 | 0 | +800 |
| SCM7B47N-11 | 3.6364 | +1 | +200 |
| SCM7B47N-11A | 4.5455 | 0 | +200 |
| SCM7B47N-11D | 9.0909 | 0 | +200 |
B. COLD JUNCTION COMPENSATION (CJC)
A negative temperature coefficient Thermistor is used as the SCM7B CJC sense element in a voltage divider configuration. It is mounted underneath each field side terminal block. A nonlinear current is used to develop a linear voltage potential which is input to the modules X+ input pin. This potential changes over temperature.Inside the module, this slope is modified to match the thermocouple type's Seebeck Coefficient (at+25°C) which offsets the effect of the thermocouple to backplane junction potential. Thus, the module high-level output potential is the field Thermocouple temperature and NOT the difference between the field temperature and backpanel temperature.
This Thermistor is manufactured by BetaTHERM, P/N 100K6A1 (other manufacturers of acceptable replacements are listed in the Other Part Numbers of Interest at end of the SCM7B catalog section). The Thermistor is rated at 100kΩ at +25°C, ±0.2°C from 0°C to +70°C. For temperatures other than +25°C, the Steinhart-Hart Equation can be used with coefficients provided by the Thermistor manufacturer.
STEINHART-HART EQUATION
Equation 3:
1/T = A + Bln(R) + C[ln(R)]3
Where T is in Kelvin, Thermistor Resistance (R) in Ohms, and coefficients A,B,C are given by:
A = 8.27153E-04
B = 2.08796E-04 (per BetaTHERM for model 100K6A1)
C = 8.060985E-08
B = 2.08796E-04 (per BetaTHERM for model 100K6A1)
C = 8.060985E-08
To convert to °C, simply subtract 273.15 from the Kelvin temperature result.
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