RTD sensors and ethanol

[Sorghumrunner]

We are putting together the temperature displays for our still, and I've got three points I want to measure with digital readout, the rest will just be standard thermometers. I'm looking at RTD sensors to go in the 1.5" sanitary fittings on the still vapour space, doubler vapor space, and at the spirits outlet before the parrot. I want to run these wires just outside the room to a control box with temperature displays.

What do I need to do regarding the wiring/conduit at the still to keep it safe? I'm assuming this is a Class 1 Group D classified area. Do I have to have exp. proof boxes here? Trying to keep cost down, but keep it safe.

Chris

[nick jones]

Chris,

I'm sure that all of this is spelled out very clearly in the National Electric Code, and that your electrician will be able to answer more authoritatively that I will, and please DO NOT TAKE MY MUSINGS AS PROFESSIONAL ADVICE because I'm not a licensed electrician, but that being said...

I think that the application that you speak of could be Intrinsically Safe by nature, or quite easily rendered Intrinsically Safe so as to negate the need for exp. proof conduit and enclosures. I rarely link to Wikipedia, but their entry on Intrinsically Safe states:

"The primary concept behind intrinsic safety is the restriction of available electrical and thermal energy in the system so that ignition of a hazardous atmosphere (explosive gas or dust) cannot occur. This is achieved by ensuring that only low voltages and currents enter the hazardous area, and that no significant energy storage is possible."

I'd think that an RTD is a pretty low voltage and current... Something to consider anyway...

Nick

P.S. speaking of RTDs, IMHO 3-wire RTDs are the only way to go. Take all of those 2-wire RTDs to the scrap yard with all of your spare catalytic converters; they're trash. The 3-wire compensates for ambient temperature fluctuations, the 2-wire doesn't. Pretty big difference if you ask me.

[Sorghumrunner]

Thanks Nick. I figured that it was 'intrinsically safe' but then I saw McMaster selling probes with exp. proof conduit fittings, so that threw me.

[Jedd Haas]

Nick, the 3 wires are to compensate for the resistance of the wire. In the case of a Pt100 sensor, the sensor is a piece of platinum wire with a nominal resistance of 100 Ohms. The resistance of the wire is perhaps an Ohm or two, (depending on wire length and gauge) which, although small, is enough to affect the reading. Here are a couple links that go into more detail:

http://ongengineering.blogspot.com/2013/03/2-wires-3-wires-or-4-wires-rtd.html

http://control.com/thread/1236021381

[nick jones]

Nick, the 3 wires are to compensate for the resistance of the wire. In the case of a Pt100 sensor, the sensor is a piece of platinum wire with a nominal resistance of 100 Ohms. The resistance of the wire is perhaps an Ohm or two, (depending on wire length and gauge) which, although small, is enough to affect the reading. Here are a couple links that go into more detail:

http://ongengineerin...-wires-rtd.html

http://control.com/thread/1236021381

Jedd,

I'm not sure If I'm understanding you right, but one should verify the accuracy of any RTD upon commissioning, and make adjustments to the temperature controller or read out as necessary. In both 2-wire and 3-wire setups, this will correct for any resistance caused by the lead wires.

The difference between the 2-wire and 3-wire is that after this initial commissioning, the 3-wire will automatically adjust for a CHANGE in the resistance of the lead wires (due to ambient temperature change of the lead wires). Whether or not "an ohm or two" (or twenty) is a significant error is up for the individual distiller to decide, obviously. This table at the engineering toolbox shows a drastic increase in the CHANGE in resistance at around 24 gauge wire (not sure if they're talking about solid or stranded). My own experience has shown me that the difference, especially in an environment with variable ambient temperatures like a distillery, is significant. After all, I'm not interested in the difference between 50 deg F mash and 172 deg f vapor. I'm interested in the difference between 172.7 deg F vapor and 173.5 deg F vapor.

Nick

[Jedd Haas]

What you are describing is a side effect of the resistance compensation for 3 wire RTDs. All wires change resistance with temperature; the point of having 3 wires is to take the resistance of the wires, whatever they happen to be at any given moment, out of the reading. According to the first link I provided, 2 wire RTDs do not compensate for lead resistance, while 4 wire RTDs are even better.

[nick jones]

What you are describing is a side effect of the resistance compensation for 3 wire RTDs.

True. This "side effect" is why 2-wire RTDs are trash, IMHO. Make your own call between 3-wire or 4-wire, but stay away from 2-wire if you can. Speaking of staying away from 2-wire, keep in mind that a surprising number of temperature controllers and readouts are not compatible with anything but 2-wire setups (like the ones on Carl stills). Again: trash. IMHO, that is.

Nick