Hello,
I've simple variac that I'd like to put into a box a displaying it's output voltage and current.
It's output can reach 240Vac and 3A
I'm a newbie to this and I was looking at some sensor modules, specifically one based on a ZMPT101B transformer and the other one that uses the ACS712. (and 1602 lcd for display)
Would these be ok for my purposes?
Thanks
The Allegro™ ACS712 provides economical and precise
solutions for AC or DC current sensing in industrial, commercial,
and communications systems
The ZMPT101B module is a compact single-phase AC voltage sensor module based on the minuscule 2mA/2mA precision voltage transformer ZMPT101B made by Qingxian Zeming Langxi Electronic (www.micro-transformer.com).
If you are competent around 240VAC, then yea, go for it.
Personally, as an EE, I have a deep respect for anything over 60 Volts... a respect that has been renewed multiple times over my lifespan.
I'm confident with tube amplifiers actually 
Not at all with arduino, sensors etc...
me too, especially if the output is GU-47B )))
Back on OP, what would be the best way to calculate RMS values for v and i once I got the sensors working?
the sensor will still have to be calibrated, I put it in the voltage range I needed and used the resulting coefficient
With the power line you can assume a sine wave and then RMS is 0.707 times the peak.
In certain cases (like with an AC light dimmer) the current isn't a sine wave, and with inductive loads the voltage & current can be out-of-phase and that makes it more difficult to calculate the power (if you want to calculate power).
Note that the Arduino can be damaged by voltages greater than Vcc (+5V with a "regular" Arduino) or by negative voltages.
Yeah but I guess the sensors would show instantaneous values, not peaks... right?
All things considered for under $10.00 USD I can buy an AC voltage and current meter. This is one example of the many out there.
Yes, you can roll your own using a ZMPT101B Module which includes the transformer and a few parts. For current I would likely just choose a current transformer module or the mentioned ACS 712. That assumes roll your own and tie up an Arduino plus a display.
Ron
Yes, problem with these is they not one of them goes down to 0V. At least not the ones I've seen so far
Good point! If you just want to measure line voltage like 120 / 240 they are fine but for use with a variac not so fine. 
You can get ANALOG moving coil/iron cheap meters that go to 0v .
(You are aware of the zeroing screw on the front for these )
This is the easiest/cheapest/safest method .
£10 off eBay for a 0-300v meter, ammeters similar. Job done
Yes I was evaluating that route too 
Memories of some of the first Variac transformers I worked with. They had both an analog volt and ammeter on the front panel.
Ron