Use AD8494 Thermocouple Amplifier for temperature

We are no longer using a Type K thermocouple.

src/psc_datalogger/connection.py

@@ -12,7 +12,7 @@
 from pyvisa.resources import Resource, SerialInstrument
 
 from .statusbar import StatusBar
-from .thermocouple.thermocouple import volts_to_celcius
+from .temperature.converter import volts_to_celcius
 
 
 class ConnectionManager:

src/psc_datalogger/temperature/converter.py

@@ -0,0 +1,25 @@
+# Handles converting millivolt readings into degrees Celcius for an Analog Devices
+# AD8494 Thermocouple Amplifier
+"""
+The equation governing this is specified as:
+V_OUT = (T_MJ * 5 mV/°C) + V_REF
+where
+- T_MJ is the thermocouple measurement junction temperature.
+- V_OUT is the measured output voltage
+- V_REF is the reference voltage, which has been configured to 0 on the hardware.
+Substituting V_REF=0 and rearranging this equation gives:
+
+T_MJ = V_OUT / 5 mV
+
+i.e. every 5 mV gives 1 degree celcius increase
+
+"""
+
+
+def volts_to_celcius(volts: str) -> float:
+    # Convert to millivolts
+    mv = float(volts) * 1000
+    temp = mv / 5.0
+    # Check the result is at least vaguely sensible
+    assert -10.0 <= temp <= 60.0
+    return temp

tests/test_connection.py

@@ -461,7 +461,7 @@ def test_query_instruments_temperature(
         address = 22
         worker.instrument_addresses[1] = InstrumentConfig(address, convert_to_temp=True)
         voltage_str = "1E-03"
-        temperature = "24.993219514361623"  # degrees Celcius, calculated
+        temperature = "0.2"  # degrees Celcius, calculated from voltage_str
         mocked_write = MagicMock()
         mocked_query = MagicMock(return_value=voltage_str)
         worker.connection = MagicMock()

tests/test_thermocouple.py → tests/test_temperature.py

@@ -2,7 +2,7 @@
 
 import pytest
 
-from psc_datalogger.thermocouple.thermocouple import volts_to_celcius
+from psc_datalogger.temperature.converter import volts_to_celcius
 
 # Tests for the thermocouple voltage -> degrees Celcius conversion.
 
@@ -11,24 +11,24 @@
 
 
 @pytest.mark.parametrize(
-    "input_millivolts, expected_temp",
+    "input_volts, expected_temp",
     [
-        (-0.392 * 10**-3, -10),
-        (0.0 * 10**-3, 0.0),
-        (1.203 * 10**-3, 30),
-        (2.395 * 10**-3, 59),
+        (0.2, 40.0),
+        (-0.01, -2.0),
+        (0.0, 0.0),
+        (0.11, 22.0),
     ],
 )
-def test_volts_to_celcius_valid(input_millivolts, expected_temp):
+def test_volts_to_celcius_valid(input_volts, expected_temp):
     """Test that a handful of millivolt readings produce the correct output temperature
     to within TOLERANCE."""
-    calculated_temp = volts_to_celcius(input_millivolts)
+    calculated_temp = volts_to_celcius(input_volts)
     assert math.isclose(expected_temp, calculated_temp, abs_tol=TOLERANCE)
 
 
 @pytest.mark.parametrize(
     "invalid_value",
-    [-1 * 10**-3, -0.393 * 10**-3, 2.396 * 10**-3, 3 * 10**-3, 10 * 10**-3],
+    [-5.1 * 10**-2, 3.1 * 10**-1],
 )
 def test_volts_to_celcius_invalid_values(invalid_value):
     """Test that various values are all invalid i.e. outside modelled range"""