The?Fluke 5790B AC Measurement Standard?is a multi-purpose ac measurement standard that combines the accuracy you’d expect from a thermal transfer standard with the ease of use of a digital multimeter.

In this article, electrical calibration expert Marty Kidd explains why someone needs this type of product. He also explains why owners of an old Fluke 5790A should consider moving up to a new 5790B.

Why does someone need a 5790B?

They need a 5790B measurement standard if they have some kind of ac voltage source that they need to calibrate.

How to calibrate an ac voltage source – 3 options

• Use a thermal voltage converter.?The old way of doing accurate ac voltage measurements is to use a thermal voltage converter (TVC) and run dc into it; make a lot of corrections; run ac through it; compare the numbers; take corrections; do the math. It’s complicated, and very easy to damage the TVC. This is the way this was done when I first came into calibration 30-something years ago.
• Use a long-scale DMM.?In some situations you can use a long-scale DMM like the?8588A Reference Multimeter, but if you need better accuracy you’re going to need something else.
• Use the Fluke 5790B AC Measurement Standard.?The 5790B is kind of between these two alternatives. It has the ease of a long-scale DMM and it’s got accuracy that gets much closer to a thermal voltage converter. For example, at 1 volt 100 Hz you might be at 200 or 250 ppm on a DMM, but you’re going to be down around 24 ppm with the 5790B. So, you’re much more accurate.

What kinds of voltage sources need high accuracy?

The most common use would be to calibrate the ac voltage output of a calibrator like a Fluke?5502A?or?5522A Multi-Product Calibrator?or even a?5730A Multifunction Calibrator. Those customers who have fairly good capability already might have a good resistance standard, they might have a really good measurement standard or a dc voltage reference and divider so they can actually calibrate the other functions of a calibrator, but they don’t have something accurate enough for ac. That’s where a 5790B would come in.

Automate ac voltage source calibration with software

The 5790B is easy like a DMM but it can be automated much more easily. When you’re working with a TVC, it’s all manual. 5790B can be fully automated with?MET/CAL software. There are procedures out there that use 5790B but customers don’t have to use MET/CAL, they could use LabView or something else. That’s actually a feature of both the A and B versions of the 5790.

Higher end labs, very accomplished third-party labs, any of the DoD or defense contractors that have the bigger labs, would be using a 5790B.

Fluke 5790B vs 5790A - Why upgrade?

• Increased risk.?Suppose a customer has a 5790A. The 5790A is, what, 28 years old? They can’t get the parts to repair them anymore. In fact, the number one board has a chip on it that we just don’t have anymore. They can’t replace the board, and that is kind of the heart of the unit.

  So if they’re depending on a 5790A right now, even if it’s working ok, if it goes bad they’re going to have to find out, oh, we can’t repair it, then they have to try to get approvals, which could take a month or two or three. Then they have to order a new 5790B and they have to wait for us to build it and get it shipped to them. They could be down for months. Some customers just can’t afford that.?

  When I talk to customers and I see that they have a 5790A, I recommend to them, get this budgeted and get a new 5790B as soon as you can, because you’re at risk of being down for months if your standard goes bad. This isn’t the type of unit that a lab typically has multiples of it. The typical lab will have one or if they have two, it’s because they have the workload for two. It’s not like people have spares that they can just pull up if one of them goes bad.

• Easier and safer to use.?The 5790B is all new, so it’s going to last a long time. It’s got all new components, new design, new graphical interface that is very easy to use, so it’s actually much nicer to use than the old one. However you configure it, it actually lights up the terminals to tell you what connections to make. A lot of customers really like that, because if they have an external guard, it lights up, and they know they have to connect the guard.
• Fewer errors.?You have the ability to put current shunt information – corrections, calibration details – into the 5790B so that you can actually use, for example the?A40B Current Shunts?with the 5790B and the math is done for you. You get a readout of the corrected current on the 5790B so you don’t have to do the math yourself. It’s really nice that you can select the shunt you’re using, make sure it’s got the current calibration data in there, and it will give you the corrected readout with the shunt.
• More versatile.?The 5790B also works for ac current if you use it with the A40B shunts. It’s very accurate and a lot of customers who are doing calibrators are using the shunts with the 5790B to do the current portion of calibrators.
• DC voltage specification.?Another nice thing about the B compared to the A: The A has got as good as a 6.5-digit DMM on dc voltage, but the A was never spec’d; the spec was “typical.” I know it’s good and I’ve got this measurement I’m doing, I just did some ac stuff and now I’ve just got to do 10 volts dc measurement and I don’t have a DMM right here. Now I’ve got to get up and go find a DMM and bring it over, right? But the B is actually specified for dc, about 24 ppm for dc. That’s the best spec but it’s in that range. You’re down in the tens of ppms for dc voltage. That’s a nice upgrade from the older one.
• Improved design.?The 5790B just looks good. I’m sitting here now looking at it. And for me it looks great. I wish I could have had this as a tech on the bench.
• Increased workload coverage.?You can get a 5790B with a 50 MHz wideband to measure up to 50 MHz. There is a model called 5790B-AF that comes with a cable characterized right at 50 MHz 1 Milliwatt. The reason that’s important is that customers can use it to calibrate the Reference OUT on their power meters. Power meters have a Reference OUT that they use to zero out their RF sensors. For example, the military (Air Force) wanted something that could measure that because the method they were using was just very difficult, challenging and cumbersome. We set this up so they can calibrate that RF OUT on all the various power meters with the 5790B wideband. The fact that you can go to 50 MHz is another nice feature, but you don’t have to; there’s an option to go to 30 MHz as well.

Summary: 9 reasons to purchase a 5790B AC Measurement Standard

1. You calibrate the ac voltage output of a calibrator like a 5502A, 5522A or 5730A.
2. You calibrate the current portion of a calibrator and want to use the 5579B with a current shunt.
3. You think it might come in handy to use the 5790B to measure dc voltage.
4. You want the measurement accuracy of a thermal voltage converter with the ease of use of a long-scale DMM.
5. You have an old 5790A and can’t afford the risk of it needing repairs (and you realize you can save 10% if you trade it in for a new 5790B).
6. You want to make an investment in an all-new instrument that will last a long time.
7. You want the “mistake proof” assurance of the Visual Connection Management terminals.
8. You have a need for the 50 MHz wideband.
9. You want to fully automate the measurement standard with MET/CAL software.