HP5890 oven - problem holding low temperature

[AlexW]

Our HP5890 has recently developed a strange problem. During a normal run (typically ramping from 50 to 280 C) everything is fine, and the oven cools back down to the start temperature. After a while though (usually less than an hour), the oven temperature starts to rise by 5-10 degrees, which eventually leads to a shut down.

Briefly setting the temperature high (so the oven heats) then back to 50 C solves the problem temporarily; turning the oven off and on from the front panel does not. The exhaust doors seem to be opening correctly when cooling, at least from high temperature.

Any ideas to try before I call the engineer? Thanks.

[GasMan]

The fact that you are programming up to 280°C suggests that you will probably have the inlet and detector set to 300°C or more. If you do not have the insulation cups installed around the inlet and detector, you could be getting heat from the inlet and detector getting down into the oven which will raise the oven temperature.

Gasman

[rb6banjo]

Make sure the flapper motor is working right. I have had that do some strange things to me on 5890's over the years. It's easy to change without a service engineer.

[thohry]

Make sure the flapper motor is working right. I have had that do some strange things to me on 5890's over the years. It's easy to change without a service engineer.

Agreed. I once had a similar proplem with 6890. The flap motor is too aged. Alex could also check if something is jammed.

[AlexW]

Thanks for the replies.

I will have a look at the flapper to see if there's anything obvious. It does definitely open fully when cooling from a run, but doesn't when a hold temperature is set. For example, if set to 50 the oven might rise to 58, and the flapper remains shut. If I set the temperature to 60 and then immediately back to 50, it will open and the oven will reach the set temperature (for a while).

Gasman - inlet and detector are both 280 C; most methods have an oven ramp to 250 C max, I'd misremembered 280. Everything was working fine until recently though, and insulation is fitted on the inlet. The detector is an MSD so the transfer line is outwith the oven.

[James_Ball]

Can you tell if the fan it turning as it should, especially when the oven is completely closed up for the low hold temperature?

If the fan is not operating properly, then the latent heat in the metal of the oven and from the heated zones could cause a slow rise in temperature.

Also if a slight rise happens, the flaps in the back should open slightly to allow it to cool, if they don't open just a little that would be the problem. Sometimes they do get out of calibration and think they have moved slightly open when they are still closed. Could also be that the set screws have come loose and the shaft has to turn more than it should before the flaps open, that would leave them closed when the controller thinks they are open.

I have had 6890s that had the opposite problem of not reaching the max temps or ramping as fast as they should because the flaps were slightly open when they were supposed to be closed. Power cycling the GC would reset the flaps to the proper position.

[Consumer Products Guy]

I have had 6890s that had the opposite problem of not reaching the max temps or ramping as fast as they should because the flaps were slightly open when they were supposed to be closed. Power cycling the GC would reset the flaps to the proper position.

I've experienced that on both 5890 and 6890. I've replaced the flap motors on each type. Also on one (believe 6890), I riveted the flat circular flap back onto the shaft, it was falling off (I bought a nice pop rivet tool).

[mckrause]

Flapper seals are probably worn. This is exactly the kind of symptom you would see for that issue, as they leak enough to fool the thermocouple but not enough to keep your heater from overheating the oven.

[ELIkjw]

Make sure the flapper motor is working right. I have had that do some strange things to me on 5890's over the years. It's easy to change without a service engineer.

Agreed. I once had a similar proplem with 6890. The flap motor is too aged. Alex could also check if something is jammed.

I have had odd temp flucuations like this on my 5890. The flapper was the culprit. Plus side is that they are easy to work on. Good Luck.