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Analytical Instruments Cheat Sheet

Process analyser formulas and reference values on one page: pH, ORP, conductivity, dissolved oxygen, gas concentration units, LEL and combustion oxygen.

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pH

Definition	pH = −log₁₀ a_H+
Nernst slope	S = 0.198416 × T(K) mV/pH
pH from mV	pH = 7 − E ÷ S
Slope %	S_meas ÷ S × 100 (often 90 to 105 %)
Offset	mV at pH 7 (often within ± 30 mV)

Nernst slope vs temperature

°C	mV/pH
0	54.20
25	59.16
50	64.12
75	69.08

Standard buffers (25 °C)

pH 4.01, 7.00, 10.01 (10 drifts fastest, absorbs CO₂).
Calibrate with pH 7 plus one buffer near the process value.

Conductivity

Conductivity	κ = G × K (cell constant)
To 25 °C	κ₂₅ = κ_T ÷ [1 + α (T − 25)], α ≈ 0.02
TDS	TDS ≈ 0.5 to 0.7 × κ₂₅ (mg/L per µS/cm)
Ultrapure water	0.055 µS/cm at 25 °C

DO saturation (fresh water, sea level)

°C	mg/L
0	14.62
10	11.29
20	9.09
25	8.26
30	7.56

Gas units

ppm to mg/m³	mg/m³ = ppm × M ÷ V_m
V_m	24.45 L/mol at 25 °C, 22.41 at 0 °C
% vol	1 % = 10,000 ppm
Hardness	2.497 Ca + 4.118 Mg (mg/L as CaCO₃)

LEL (IEC 60079-20-1, % vol)

Gas	LEL
Methane	4.4
Propane	1.7
Hydrogen	4.0

Mixture: LEL = 100 ÷ Σ(C_i ÷ LEL_i). 20 % LEL methane = 0.88 % vol.

Combustion

Excess air	EA % ≈ O₂ ÷ (20.9 − O₂) × 100
O₂ correction	C_ref = C × (20.9 − O₂_ref) ÷ (20.9 − O₂)
Zirconia cell	E = 0.0496 T log₁₀(20.95 ÷ O₂) mV

Sample systems

Lag time	t = V ÷ Q = π d² L ÷ 4Q
Example	4 mm ID × 30 m at 1 L/min ≈ 23 s

Related Calculators and Tools

pH CalculatorFree on instrumentationblog.in LEL CalculatorFree on instrumentationblog.in Analytical GlossaryFree on instrumentationblog.in

Frequently Asked Questions

Why does pH slope change with temperature?

The Nernst slope is proportional to absolute temperature.

Why reference conductivity to 25 °C?

Conductivity changes about 2 % per °C, so values at different temperatures are not comparable otherwise.

Which LEL for methane?

IEC 60079-20-1 gives 4.4 % vol; older sources use 5 %.

Learn more on the blog

Electrical conductivity explained

A practical guide on instrumentationblog.in that explains the theory behind this cheat sheet.

Read Full Article

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