Re: Liquid level indicator



MarkMc wrote:
> Hi Chris
>
> FF - yes of course, just didn't twig! Obviously, too much home brew
> has killed some brain cells!
>
> Thanks for the info about the MOV, I've heard of varistors, but never
> knew what they were.
>
> The relay is a tiny (ultra miniature they call it) PCB mount thing.
> I'm not sure if the cover is removable. I can vandalise one as part of
> 'research' I suppose!
>
> The .pdf file on the Maplin site is new - it wasn't there when I bough
> my unit a few months ago. Would've been useful. I've been developing
> a cct for temperature control using a PIC microcontroller which
> interfaces to the FE33L, and spits out an RS232 signal for a PC to read
> - I'm using the FE33 mainly as a tempsensor and LCD display - all of
> which I'd probably implement using discrete components for version 2
> now I've learned a bit about PIC's.
>
> I did have a .pdf file of the manual for FE33L, but I can't find it (a
> USB memory stick got trashed), otherwise I'd stick it on the web site.
> If I get time, perhaps I can scan in the paper version. It's very low
> on detail though, and certainly isn't a data*** and doesn't have a
> schematic. It basically has two outputs for temperature control - too
> hot or too cold, so you can control a heat and cool source.
>
> By the way, the relay (YX97 - maplin) is rated to 10A
>
> The temperature probe is the FE34M, and I have one of them. I wouldn't
> want to put it in hot wort. Hence I'd like to make something from s/s
> if that's at all possible.
>
> I just looked through my Maplin order history, and I see that I ordered
> RG23 500v 0.1uF capacitors, and also RG22Y Contact Suppressor 100R and
> 0.1uF in series 250v max.
>
> Using your suggested 'relay logic' setup, would I need to put two
> suppressors in the cct, one across each, or can I still stick with your
> original suggestion of having these across the load? Any reason for
> your change here?
>
> The heat source will be controlled by RS#655-622, as will the pump.
> Well, that's the intention anyway. But I am yet to decide on what to
> use for the heat source. I haven't yet found my s/s containers for
> this application, so the size will dictate the heat source to a certain
> degree.
>
> I was thinking about using an electric kettle element 2kW or similar.
> I will be needing to heat about 25L of liquid from ~50C to 65C, fairly
> rapidly, while recirculating with the pump.
>
> Sorry Chris, what is GFCI?
>
> I'm getting an old coal shed converted to use as the brewery, and it'll
> have it's own consumer unit, earth rod and RCD main switch...
>
> Cheers,
> Mark

Hello again. The folks at Maplin were kind enough to email an
attachment with more information on the FE33L temperature module. If
you want, you can have them forward it to you also.

The use of this module for real temp control looks iffy, but you might
get barely passable results with the British can-do spirit and a little
luck. Unfortunately, bacteria also have that spirit of derring-do, and
your FE34 sensor probably won't cut it. By putting it in a s/s sheath,
you'll probably be doing a good job of insulating it from the liquid.
That will slow down your temperature response big-time, which is
probably a fatal flaw.

All is not lost, though. They've probably got a commercial thermistor
(temperature-variable resistor) as the temp sensor, and it's more than
likely a standard value. There's no schematic in the .pdf, but I'd
guess they're probably asserting a logic high on a pin, and then
measuring current through the sensor with a series resistor. Either
that, or they're doing an R-C charging thing like the POT instruction
with the BASIC Stamp. It might be possible for you to measure the
thermistor resistance at room temperature, then measure again at a
higher set temperature to get a handle on its temperature coefficient
(delta ohms per degree). Or call Maplin and ask nicely (although I'm
not sure they'd be interested in undercutting their market for their
1.99 GBP sensors). It's probably a standard value, and also probably
available as a purchase part from some vendor somewhere. If you're
really lucky, it might be a value you can get by cannibalizing a
digital thermometer. As another possibility, you might be able to
purchase a bead thermistor, and affix it to the inside of a thin sealed
s/s tube with thermally conductive epoxy, but this sounds a little
doubtful. I'm the most uneasy about creating a situation where we
contaminate the brew. That can get people sick -- not a good thing. A
final possibility might be affixing the bead thermistor to the outside
of the tank. If it's copper or brass, it's a great conductor of heat.
If you blanket the air side, and use thermally conductive epoxy to
affix the thermistor to the outer wall, you might not even have to put
anything inside the vat. I like that option best.

The FE33.pdf from Maplin shows how to interface the 1.5V logic from the
temp control module to transistors and relays. I'd definitely watch
your current drive here. If you've got 0.8V across a 10K resistor,
you're only squirting 80 microamps into the base of the transistor.
That isn't going to be enough, realistically, to drive a decent sized
relay. You could use it to drive a solid state relay, or if you're
still going to use a large relay to drive your heaters, you might use
the first transistor to drive another one, which then drives the relay
like this (view in fixed font or M$ Notepad):


12V
+
|
.-. 12V
22K| | +
| | |
'-' |
| ___ |<
o-|___|--| PNP
| 8.2K |\
| |
1.5V ___ |/ .---o
LOW o-|___|-o-| NPN | |
Signal 10K | |> | C|
.-. | 1N4- C|RY2
| | | 002^ C|
10K| | === | |
'-' GND | |
| === ===
=== GND GND
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de



But, having shown that you can do something, you should then ask
yourself whether it's smart. From what I've heard about brewing,
you'll have to be consistently within a couple of degrees if you want
things to work right. To be honest, you're not going to reliably get
that accuracy or repeatability with this setup. This setup won't work
well on a consistent basis. Even with a rock solid 1.5V supply, a
precision thermistor matched to the characteristics of the FE34,
controlled ambient temperature, and everything else optimized, I don't
think you're going to get better than +/-5 degrees C or so
repeatability. That's 60 to 70 degrees C, and the extremes will
probably ruin your batch. It might be better to think this one out,
and work on a control setup which will do justice to the rest of your
effort, time and money. In for a penny, in for a pound (GPB).

There's nothing wrong with manually turning on the heat and monitoring
with a thermometer, just like the Bavarians of yore. And if you want
to have some kind of automated temp controller, there are units
available on ebay which will do a much better job than this little
module. Control of temp to within 1 degree C or so is very possible.
You can then use a standard precision thermistor with your temp
controller (you might even be able to buy it with the controller) to do
the job.

By the way, a GFCI is a Ground Fault Circuit Interrupter, and it's a
requirement any time you're using line voltage in a wet place. In the
event that there's a ground fault, the GFCI will cut power to reduce
the risk of electrocution. If you're using electric heaters on a metal
vat with liquid inside, you have to use one of these.

Good luck. Take some time, and think 'er over.

Good luck
Chris

.

Quantcast