Megger & Test Instrument FAQs: Insulation Testers, Oscilloscopes, Surface Roughness, and Pipettes

1. What’s the difference between the Megger MIT310 and MIT320 insulation testers?

The short answer: test voltage range. The MIT310 is a 1 kV model (good for low-voltage cables and motors), while the MIT320 goes up to 5 kV (for high-voltage switchgear and transformers). I’ve used both—or rather, I’ve mostly used the MIT310 for routine checks, but when we had a 3.3 kV motor repair rush job last year, the MIT320 saved us.

If I remember correctly, the MIT320 also has a slightly bigger display and better battery life. But honestly, for 90% of field work, the MIT310 is more than enough. Don’t overbuy unless you’re regularly testing above 1 kV. I once assumed higher voltage meant better accuracy—turned out the MIT310 was actually more precise at low resistance readings. Live and learn.

2. Can I trust a used oscilloscope for critical measurements?

It depends. I still kick myself for an incident in 2023: we grabbed a “fully calibrated” used Tektronix from a surplus dealer. The price was tempting—$600 vs. $2,000 new. But the calibration certificate was two years old and the vertical offset was off by 3%. We shipped a product with a flawed waveform, and the client caught it. Cost us $8,000 in rework and a dent in our reputation.

To be fair, not all used scopes are bad. But here’s what I now require:

• A recent calibration certificate (within 12 months)
• Original accessories (probes, manuals)
• A return policy (at least 30 days)

I’ve bought three used scopes since that disaster—and only one was worth keeping. The other two had worn inputs. So yes, you can trust a used oscilloscope, but only if you verify everything. Otherwise, new from a reputable brand (like Megger or Tektronix) is safer.

3. How do I use an Eppendorf pipette correctly?

You’d think it’s simple: press the plunger, aspirate, dispense. But I’ve seen experienced lab managers make mistakes. The key is to pre-wet the tip two or three times before taking your actual sample—this equilibrates the air cushion and improves accuracy by 0.5–1% according to Eppendorf’s own documentation.

If I remember correctly, the proper technique is:

1. Press to the first stop, immerse the tip 2–4 mm into the liquid.
2. Slowly release the plunger (wait 1 second).
3. Dispense by pressing to the first stop, then pause, then press to the second stop to blow out the last drop.

The most frustrating part? People forget to check the tip for cracks. A tiny crack at the tip side can cause a 10% error. I once assumed all tips were identical—nope. Use only genuine Eppendorf tips or validated equivalents for critical work.

4. What does a surface roughness tester measure and how do I choose one?

Surface roughness testers (like the Mitutoyo SJ-series or even portable units) measure Ra, Rz, Rmax, and other parameters. The most common is Ra (average roughness). If you’re in machining, you’ll use it to verify that a finished part meets spec. A client once rejected a batch of shafts because the roughness was 0.8 µm instead of the required 0.6 µm. With a portable tester, we caught it before shipping.

When choosing a tester, look at:

• Range – Can it measure from 0.01 µm to 50 µm? Most standard units do.
• Stylus tip – Diamond stylus with 5 µm radius is typical.
• Standards compliance – ISO 4287, ISO 21920, etc.

I recommend getting a tester with a built-in printer or data export. I still regret buying one without—now I have to manually copy readings into our QMS. To be fair, it’s a budget model, and it works. But the extra $200 for data logging would have paid for itself in time saved.

5. How does test equipment quality affect my company’s image?

This is the one question most people don’t think to ask, but they should. The quality of your test gear is a direct reflection of your professionalism. When a client walks into your lab and sees a shiny Megger MIT310 and a Fluke 87V on the bench, they feel you’re serious. When they see a no-name multimeter with peeling rubber, they wonder if your measurements are as sloppy as your tools.

I’ve seen this play out: a prospective client visited our facility, and our service manager had just upgraded to a Megger MIT320 for cable testing. The client asked about it, and our tech explained the 5 kV range. That conversation closed the deal—the client later said it showed we invest in our work.

The $50–100 difference between a budget insulation tester and a Megger? It’s not a cost; it’s an investment in trust. I’d rather spend $800 on a Megger that will last a decade than $400 on a generic that drifts after two years. That said, if your budget is tight, a used Megger from a trusted source is a solid middle ground.

6. What if I need a test instrument urgently—next-day or even same-day?

In my role coordinating instrument procurement for a mid-sized engineering firm, I’ve handled over 200 rush orders in the last five years. Yes, it’s possible to get a Megger insulation tester or a used oscilloscope delivered quickly—but you’ll pay a premium. Here’s what I’ve learned:

• Same-day: Only possible if you’re near a major distributor (e.g., Grainger, McMaster-Carr, or local instrumentation supplier). Expect +40–60% markup.
• Next-business-day: Many online instrument suppliers offer expedited shipping for $30–80 extra. Standard is usually 3–5 days.

One time, a client’s Megger MIT320 failed on a Thursday when we were due on-site Friday. I assumed overnight shipping would work—didn’t verify that the carrier actually delivered to our area on Fridays. It arrived Monday. We borrowed a unit from a competitor (embarrassing). Now I always check carrier service maps before ordering rush. Looking back, I should have ordered the backup unit weeks earlier.

The moral: if your projects depend on specific test tools, keep a spare or know your fastest supplier. The certainty of having the right instrument on time is worth far more than saving a few bucks on standard shipping.