Thanks all!
The idea is to sort something that's cheap as chips to give to engineers installing equipment
After writing everything below I had to come back to this as frankly I was getting a bit pissed off. I've run into this mentality before and in my eyes it comes down to this; either you want to work in the instrumentation business or you don't. If you do you will equip the company with the tools to do the job. I have nothing against the homemade solutions but at some point it is just more sensible to buy what you need. This especially when there are dozens of manufactures making instrumentation for the industry.
- they need something really just to prove cabling and connections, and also maybe set off functions and alarms back at the PLC by frigging the 4-20mA readings it sees.
This one little bit, setting off functions and alarms, implies to me that the only rational course of action is to buy a current loop process calibrator / signal generator. Well you could make one but that is a huge project and it wouldn't be rugged. For less that $2000 you can have a high end solution, there is much to be had below that figure.
You see proving out wiring and so forth is easy, you can lash up anything as long as it creates a useful signal the controller can read. However if you want to test that alarms trip properly, or that the right functions are executed at the right time, you really need an accurate instrument to simulate the signal. Frankly this is why there are so many process calibrators on the market to choose from.
From my perspective you are looking at two different levels of responsibility here. Initial check out of wiring isn't that difficult. Once you need to start verifying alarms an other functionality, you up the game as far as the tools required.
They have a habit of being somewhat rough with anything you give them.......
That is why you buy equipment designed to be used in the field. You can make something of course but then you would likely need to be carrying a good multimeter around, thus having two things that can break.
You might say that process calibrators are expensive but honestly they are just a small delta over the price of a good industrial multimeter. In some cases the process calibrator can replace any multimeter needed by the team.