SME
Bioinstrumentation · Problem type
A strain gauge in a quarter bridge
Use this for “A quarter bridge at 5 V with GF = 2 sees 500 με. What is the output?”
Diagram
Vₒᵤₜ
=
Vₑₓ
×
GF
×
ε
4
Vₒᵤₜ
=
5
V
×
2
×
500
με
4
=
1.25
mV
One gauge stretched: its R rises by ΔR = GF × ε × R and the bridge leaves balance.
This is an example. Type your own values to start a new problem.
Vₒᵤₜ (mV)
=
Vₑₓ (V)
×
×
ε (με)
4
Formulas
Vₒᵤₜ
=
Vₑₓ
×
GF
×
ε
4,000
1.25
=
5
×
2
×
500
4,000
Assumptions
•
One active gauge sits in a balanced quarter bridge with three equal fixed resistors.
•
The strain is small, so the gauge changes by ΔR ÷ R = GF × ε and the output is linear.
•
Strain in με times volts gives μV; dividing by 1000 gives mV, so the 4 becomes 4000.
Step-by-step
WE KNOW
Vₑₓ = 5 V, GF = 2, ε = 500 με
FIND
output (Vₒᵤₜ)
Step 1 · Find output (Vₒᵤₜ)
Use Vₒᵤₜ = Vₑₓ × GF × ε ÷ 4,000
The
gauge
changes
by
GF
×
ε
;
a
quarter
bridge
passes
a
quarter
of
that
times
Vₑₓ.
Vₒᵤₜ
=
5
×
2
×
500
4,000
Vₒᵤₜ
=
10
×
500
4,000
Vₒᵤₜ
=
5,000
4,000
Vₒᵤₜ = 1.25 mV
Check
Put the numbers back in. Both sides should match.
1.25
=
5
×
2
×
500
4,000
✓
Topic check
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