Fluke 51 52 Series II Repair

Fluke released the 50 Series II line in the late 1990s as a big upgrade over its first-generation thermometers. These models were designed to be:

• Tough (impact-resistant holster, splash and dust protection)
• Accurate (±0.05% + 0.3 °C)
• Flexible (compatible with several thermocouple types — J, K, T, E)
• Long-lasting (about 1,000 hours on 3 AA batteries)

They don’t have fancy data logging or Bluetooth like newer models, but their simplicity and reliability have kept them in use for over 20 years.

If you’re trying to tell which version you have:

• Fluke 51 II: One input for a single thermocouple
• Fluke 52 II: Two inputs (can measure T1, T2, and T1–T2 differences)
• Thermocouple support: J, K, T, and E types by default
• Some versions (53 II and 54 II) add R, S, and N thermocouple support.
• You’ll see the model printed on the front (like Fluke 52 II), and serial info on the back.

Inside, both models share similar hardware, and even some firmware overlaps — the main difference is in the channel handling and software.

Details

Temperature range (depends on probe)

J: –210 °C to 1200 °CK: –200 °C to 1372 °CT: –250 °C to 400 °CE: –150 °C to 1000 °C

Accuracy

±(0.05% + 0.3 °C) above –100 °C

Resolution

0.1° up to 999.9°, then 1° above that

Display

Backlit dual-line LCD (T1, T2, or T1–T2 on the 52 II)

Functions

MIN, MAX, AVG with time stamps, electronic offset, relative time clock

Power

3 × AA batteries (about 1,000 hours of use)

Operating range

–10 °C to 50 °C

Size / weight

173 × 86 × 38 mm (~0.9 lb)

Safety ratings

CE, CSA, TÜV certified — EN61010 compliant

Here are a few things people often discuss about the 51/52 II online:

1. “My readings jump around or flutter — is it broken?”

Usually it’s just a loose thermocouple connection or a worn-out probe. Clean the connector or try a new probe before assuming the meter’s bad.

2. “Can I fix it myself?”

Yes, often! Common fixes include cleaning or replacing battery terminals (they can corrode) and adjusting small internal trim pots for calibration. Some techs have even rebuilt dead units from parts.

3. “How long do these last?”

Some owners report 15–25 years of regular use — they’re very durable if not exposed to moisture or physical shock.

4. “How do I calibrate it?”

Use a temperature simulator (like a Fluke 5520A) or compare against a known reference thermometer. For best results, it’s often sent to a calibration lab.

5. “What’s the difference between 52 II and newer models?”

Mainly data logging, USB connection, and wider thermocouple compatibility. The 52 II focuses on accuracy and simplicity — no software or wireless features.

If you want more technical or official info, these are worth checking out:

• User Manual (Fluke 51/52 II) — setup, buttons, specs, care tips
• Fluke Official PDF
•  Series Overview (51–54 II) — feature comparison across the whole family
• Fluke Comparison Sheet
• Datasheet (Fluke 52 II) — full range, accuracy tables, dimensions
• Fluke Direct Datasheet
•  Service / Repair Info — schematics, part numbers (on Manualslib or Element14 forums)
• Always use matching thermocouple types for T1 and T2 if comparing readings.
• The “offset” function is great for probe calibration or correcting minor errors.
• Replace batteries yearly — weak batteries cause erratic readings.
• Store the unit somewhere dry; humidity and corrosion are the biggest killers.
• If you want more probe options, Fluke’s 80PK series (pipe clamp, surface, air, immersion) are all compatible.