Resistive heating of mercury? - Page 1
Forum section: electronics / general

author: rexxar
author_rank: Frequent Contributor
timestamp: October 10, 2013, 05:43:53 pm
content: Hi all, I've got an issue with my old-style mercury thermostat.First off, my furnace has an annoying feature that the flame has to be on for 30 seconds before the fan will kick in. What tends to happen though, is the thermostat will be just on the edge of triggering, so it turns on the flame, and it stays on for about 10 or 20 seconds, then kicks off again. What I'm thinking is that it's passing enough current through the mercury that it's heating up slightly, causing the coil to heat up, which breaks the circuit. Then it cools back down, trips, heats up, and the whole thing oscillates every minute or so, wasting gas.Does anyone have any experience with this? Is resistive heating of mercury an actual thing?

author: JackOfVA
author_rank: Unknown
timestamp: October 10, 2013, 06:17:23 pm
content: Thermostats have hysteresis to prevent this sort of on/off cycling. In the old bimetal spring, mercury capsule switch type, there's a device called an "anticipator" that provides hysteresis. It's basically positive feedback so that when the spring/capsule trips to ON, some extra heat is immediately applied or removed from the bimetal spring that drives the spring further into the ON position even before the furnace can fire. The anticipator is usually a simple resistor powered from the 24V thermostat supply. If the anticipator fails, the thermostat can "chatter" back and forth between on/off with random air currents.In air conditioning mode, the anticipator resistor is off until the thermostat trips, and it then immediately adds heat thereby driving the thermostat further into "I need cooling". In heat mode, I believe the anticipator resistor is on normally and when the thermostat trips, the resistor power is removed and hence the thermostat coil cools down, driving it further into "I need heat" mode.I suspect your anticipator is the source of the problem. If you Google anticipator bimetal thermostat I believe you will find additional information.

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 10, 2013, 07:05:40 pm
content: Quote from: JackOfVA on October 10, 2013, 06:17:23 pmIn my experience, that's not quite how it works, but it's close.For a heating system's thermostat, adding heat to the bimetallic strip tends to drive the thermostat to "off", not "on". The thermostat goes "on" when it thinks the temperature is too cold; "off" when it's too hot.The ones I've seen use a magnet to provide positive feedback for hysteresis. When the contacts are nearly closed, a magnet pulls them fully closed. This works because the magnetic attraction is roughly proportional to inverse square of distance, while the bimetallic spring's force is roughly a linear function of displacement. The bimetallic spring then needs to provide a bit of extra force to pull apart the magnetic contacts at the end of the cycle. The heat anticipator is used to provide this extra force. Without it, the room would have to get very hot and the furnace would overshoot the desired temperature before the bimetallic strip would heat up enough to shut the furnace off.The heat anticipator is a simple resistance heater as you've described. To answer rexxar's question, it's not designed to resistively heat the mercury, but it heats the bimetallic strip.In the mechanical thermostats I've seen, there's a screw adjustment that allows you to vary the position of the magnet(s), which controls how tightly the magnets grip the contacts together. This can be used to adjust the cycle time.

author: JackOfVA
author_rank: Unknown
timestamp: October 10, 2013, 07:33:43 pm
content: Quote from: AG6QR on October 10, 2013, 07:05:40 pmI had the heating and cooling configuration reversed but my correction crossed your composition, I suspect.

author: rexxar
author_rank: Frequent Contributor
timestamp: October 10, 2013, 08:23:10 pm
content: It looks like there is a resistive heater in there. It's hidden under the dial, so I didn't notice it before. It looks like a carbon or graphite wiper over a regular strip of metal. I adjusted it so the metal strip is shorter, so that should make it generate less heat, right? It's not supposed to be cold enough this week for the furnace, so I'll have to wait to report back.Thanks for the help!

author: AndyC_772
author_rank: Super Contributor
timestamp: October 10, 2013, 08:55:22 pm
content: P = (V^2)/RMaking the strip shorter reduces R...

author: dfmischler
author_rank: Frequent Contributor
timestamp: October 10, 2013, 09:00:36 pm
content: The anticipator is in there to control overshoot at the end of a heating cycle. Many heating systems still have additional heat that will be delivered to the heated space after the thermostat shuts off (e.g. circulating hot water, or forced hot air with a large heat exchanger) so the system will overshoot the target temperature if the heating cycle is not ended early. Sometimes the anticipator needs tuning to do a good job, especially if the heating system is not accurately sized to the heated space.

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 10, 2013, 09:53:48 pm
content: Quote from: AndyC_772 on October 10, 2013, 08:55:22 pmP=(I^2)*RMaking the strip shorter reduces R...So the question is, is the power to the heat anticipator closer to a constant current source, or a constant voltage source?In the crude systems I've seen, the anticipator is a low-value resistance that is in series with the thermostat switch. That is, it gets its energy from the current which the furnace sends through the thermostat switch. In my experience, it was closer to a constant current source, because the resistance of the anticipator was very small compared to the resistance of the load being switched by the furnace. But I readily concede that this may not be the case in all systems.

author: AndyC_772
author_rank: Super Contributor
timestamp: October 11, 2013, 06:46:14 am
content: I have no idea what the nature of the source is - just wanted to plant the idea that a shorter strip might not automatically equal less heat. If shortening it doesn't help, lengthening it might!