SME
Measurements & Instrumentation · Problem type
Thermocouple with a reference junction
Use this for “A type K thermocouple (about 41 μV/°C) has its reference at 25 °C and its tip at 250 °C. What voltage does it make?”
Chart
E(250) = 9.225
This is an example. Type your own values to start a new problem.
E (mV)
=
S (μV/°C)
×
(
Tₕ (°C)
−
Tref (°C)
)
S in μV/°C times a temperature difference gives μV; the formulas’ ÷ 1000 gives mV.
Formulas
E
=
S
×
Tₕ − Tref
1,000
+9.225
=
41
×
250 − 25
1,000
Assumptions
•
S is the average slope over the range; for better than about 1 °C use the reference tables.
•
For type K, S ≈ 41 μV/°C holds from 0 to about 1000 °C.
•
The voltage comes from the difference between the tip and the reference junction.
•
The reference temperature is known (an ice bath, or a sensor at the terminals).
Step-by-step
WE KNOW
Tₕ = 250 °C, Tref = 25 °C, S = 41 μV/°C
FIND
voltage (E)
Step 1 · Find voltage (E)
Use E = S × (Tₕ − Tref) ÷ 1,000
Each
degree
between
the
tip
and
the
reference
adds
S
microvolts;
÷
1000
turns
μV
into
mV.
E
=
41
×
250 − 25
1,000
E
=
41
×
225
1,000
E
=
9,225
1,000
E
=
+9.225
mV
Check
Put the numbers back in. Both sides should match.
+9.225
=
41
×
250 − 25
1,000
✓
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