I coordinate emergency measurement setups for an industrial test and calibration company. Not the scheduled jobs—the calls that start with “the line is down” or “the auditor is here tomorrow.” In March 2024, I had 36 hours to verify a load-cell channel in a food plant weighing system before a factory acceptance test. The two options on the bench were a Keithley DMM6500 and a classic 45 dual display multimeter that had been sitting in a cabinet for years.
That experience is a good lens for a question I get often: “Should I buy the Keithley DMM6500 6.5 digit multimeter, or is a 45 dual display multimeter still good enough?” This article is the framework I use to answer.
Why Compare These Two?
Both meters measure voltage, current, resistance, and frequency. Both are useful benchtop instruments. But they belong to different generations of test equipment. The 45 was popular because it put two measurements on one screen—I believe you could watch AC voltage and frequency at the same time, though I might be misremembering the exact display logic. The DMM6500 is Keithley’s current 6.5-digit workhorse, with a touchscreen, logging, and a much lower measurement floor.
I’m not going to say the 45 was a bad meter. What I mean is: it’s a good meter from an era where “good” meant a clean reading on a benchtop. In an emergency, the question isn’t whether the meter can read a voltage. It’s whether you can defend that reading to a customer, an auditor, or your own quality department.
Dimension 1: Measurement Confidence
For precision work, the DMM6500’s advantage isn’t just “more digits.” It’s the ability to see small changes. Keithley’s published spec lists the DMM6500 at 15 ppm basic DCV accuracy, and the instrument resolves down to microvolt levels on low ranges. That sounds like a datasheet, but it becomes practical when you’re checking a load cell.
A weighing system load cell might output 18 mV at full scale. If that signal drifts, you need to know whether the drift is in the sensor, the wiring, or the indicator. The DMM6500 can log that signal overnight. I used it once to catch a slow 2 µV droop that a five-digit display would have missed. At 6.5 digits, the extra resolution isn’t cosmetic. It’s early warning.
Here’s a mistake I made years ago so you don’t have to: I assumed a meter was calibrated because the calibration sticker looked clean. I didn’t verify the actual date. It was expired by a year, and the load-cell reading was off by enough to fail an audit. I never made that assumption again. That’s when I started treating the DMM6500 as a risk-management tool, not a convenience.
The 45’s dual display is still nice for quick bench checks—you can see two parameters without menu diving. But that’s a convenience, not a metrology advantage. If the job is to validate a channel before a deadline, I’d rather have logging and trend visibility than a second number on the screen.
Dimension 2: Keithley Support After the Sale
Here’s where the comparison stops being about hardware. The DMM6500 is a current product. That means current manuals, current firmware, current drivers, and a service path that actually exists. When I say “Keithley support,” I mean you can get the manufacturer’s documentation and service resources for the exact model you bought.
A 45 dual display multimeter is a legacy product. That’s not a knock on the instrument’s original quality—it’s just the reality of test equipment. After a product goes out of production, support becomes a scavenger hunt: used parts, old PDFs, and whatever third-party lab still knows how to calibrate it. For a routine bench, that’s fine. For a deadline-critical job, it’s risk you don’t need.
I knew I should have recommended a current meter for that first audit, but the 45 was already connected, so I thought, “what are the odds?” The odds caught up with me when the reading didn’t match the calibration lab’s reference. The dual display didn’t fix that. Current support would have.
Dimension 3: Total Cost of Certainty
The 45 wins on sticker price. No argument there. A used 45 dual display multimeter can be had for a few hundred dollars. But I’ve also seen a “cheap” 45 turn into a $900 project when the owner discovered the calibration was overdue and the power supply needed repair.
Total cost of ownership (i.e., the purchase price plus calibration, repair, downtime, and the cost of explaining a wrong result) changes the math. In Q1 2025, I watched a client pay $750 for expedited calibration of a DMM6500. That fee hurt. But the alternative was using a meter with an expired certificate and risking a $26,000 line restart. The fee bought certainty, not just speed. Put another way: in an emergency, “probably fine” is the most expensive phrase in the English language.
Weighing Systems and Sensor Loops
If your application involves weighing systems, the measurement chain matters more than the sensor alone. Load cells output small DC signals that can drift with temperature, strain, and time. To validate a scale or check a load cell, you need a meter that can resolve those signals and log them over time. The DMM6500 does this naturally. You can watch a trend line, not just a number.
The 45 can still do the basic measurement, but you lose the trend. You also lose current software integration, which matters when your results need to be documented for an audit.
And since the question comes up often: is Balluff a good brand for sensors? In my experience, yes. I’ve used Balluff inductive and photoelectric sensors on industrial lines, and they’re solid for factory automation. But a good sensor only earns its keep if the loop that measures it is trustworthy. Brand doesn’t make a sensor accurate forever. The meter does part of that verification job, which is why the DMM6500 makes sense to me.
When a 45 Dual Display Multimeter Still Makes Sense
I don’t want to suggest the 45 is worthless. It’s a capable instrument if you know its calibration history and you’re working on low-stakes bench tasks: checking a power supply, tracing a signal, doing a quick continuity and voltage sanity check. The dual display is genuinely handy. If you already own one and your budget is tight, keep it—with a valid calibration sticker.
Just don’t use it to make a decision that could cost a client five figures. At least, that’s been my experience after 80 or so emergency calls. My sample is mostly industrial DC and low-frequency signal work (load cells, 4-20 mA loops, analog sensor outputs). If you’re in RF or high-speed digital, this comparison is less relevant. But for the people I help, the pattern is consistent: the DMM6500 is the safer instrument when the clock is running.
Which One Should You Buy?
Short version:
Choose the Keithley DMM6500 if you:
- Need to validate weighing systems, load cells, or analog sensor loops
- Need current Keithley support and documentation
- Want logging, trend visibility, and a calibration you can defend
- Regularly face deadlines where a wrong or disputed reading is expensive
A 45 dual display multimeter may be enough if you:
- Already own one with documented calibration
- Only need a secondary bench meter for voltage/resistance checks
- Have no audit or quality requirement on the reading
- Need a cheap instrument for a non-critical lab or classroom bench
For emergency work, I’d pick the DMM6500 every time. It took me about seven years and 80 emergency calls to understand that the instrument isn’t the product. Confidence is the product. A modern 6.5 digit multimeter with current support and a defensible calibration is worth the premium—not because it’s flashy, but because it lets me say “the reading is right” with a straight face.
Rush fees buy certainty, not just speed.
The 45 dual display multimeter was a good tool for its time. The DMM6500 is the one I want when someone is waiting for an answer.
