Tricky situation with Watlow

bdauliya · Mar 27, 2023

Good Eve Gentlemen,

We had this cold room which was converted to hot room before I joined this work. Everthing was fine until a month ago, they noticed the temp is higher than the set value, and alarm went off.

I couldn't find the schematics for this but I see there is a thermocouple connected to Ranco Digital temp controller (which I assume was for cold room before, the setting is set at C), and there is Watlow Series 998 which is supposed to turn heater ON and OFF based on temp set.The temp set is 44C but reaches 47C after some time. Watlow should Off the heater when it reaches Temp 44 right?

Please suggest, what am I missing here?

Ken Roach · Mar 27, 2023

Does the Watlow controller have a thermocouple input of its own ?

What sort of device provides the over-temperature alarm ?

Have you tested the heater(s) to be sure they aren't simply stuck in the full ON state ?

Obviously, get yourself a Watlow 998 manual and examine its installation and configuration. Since the 998 is a dual-channel process controller, make sure that your heating system is connected to the correct channels.

danw · Mar 27, 2023

The Watlow 998 is a dual loop controller. I hated dealing with customers who used the Honeywell dual loop controller because people/operators didn't pay attention to which loop was being displayed. That can be an issue on any dual loop controller because of the techniques used to indicate which loop's variables are being displayed. Caveat Emptor.

Pdf page 6 of the 998 manual goes through the menu keystrokes to show which loop is being displayed. Make sure you're looking at the right loop.

To my knowledge, all single loop controllers, when configured for on/off control have a "Hystersis" value that splits evenly above and below the setpoint. Say the SP is 40° with a hysteresis of 6°, 3° below, 3° above. At 43° the output turns off. At 37°, the output turns on. It's the nature of on/off.

An on/off controller can not maintain an even setpoint. By its very nature, on-off control has to oscillate above and below the setpoint.

If the controller is set for PID control and the temperature maintains a steady value above setpoint, then there is no reset/integral action enabled; the controller is running proportional only and exhibiting the reason for reset action.

The other reason is that the room is so well insulated that it can not lose the heat supplied by the heater so it drifts very slowly down from the on/off turn-off hysteresis point. Chances are the control output has turned off so the heater is not supplying additional heat, but the temperature is maintained because of the room's insulation.

When run in PID, enabled reset action will turn the output off over time, but the heat still has to be lost for the temperature to drop, unless the system can call for AC cooling.

cardosocea · Mar 28, 2023

bdauliya said:
Everthing was fine until a month ago, they noticed the temp is higher than the set value, and alarm went off.

The important question here is did they notice it but it had been like that since always or did they noticed that it actually changed? A lot of times people go past things that aren't quite right but never question it and when they notice it there's a tendency to claim that it wasn't like this the week before when in fact it had been for ages.

bdauliya · Mar 28, 2023

Ken Roach said:
Does the Watlow controller have a thermocouple input of its own ?

What sort of device provides the over-temperature alarm ?

Have you tested the heater(s) to be sure they aren't simply stuck in the full ON state ?

Obviously, get yourself a Watlow 998 manual and examine its installation and configuration. Since the 998 is a dual-channel process controller, make sure that your heating system is connected to the correct channels.

So the thermocouple is wired to Ranco Digital temp controller, 110 VAC HOT from ranco is branched to Heater and a relay, Relay connects to Power supply, that 24 VDC from power supply is given as Input to Watlow.

There's a honeywell temp chart recorder when it exceeds to 50C, that raises the alarm.

I am yet to look inside heater, the thing is they don't want to shut down and keeps running.

bdauliya · Mar 28, 2023

danw said:
The Watlow 998 is a dual loop controller. I hated dealing with customers who used the Honeywell dual loop controller because people/operators didn't pay attention to which loop was being displayed. That can be an issue on any dual loop controller because of the techniques used to indicate which loop's variables are being displayed. Caveat Emptor.

Pdf page 6 of the 998 manual goes through the menu keystrokes to show which loop is being displayed. Make sure you're looking at the right loop.

To my knowledge, all single loop controllers, when configured for on/off control have a "Hystersis" value that splits evenly above and below the setpoint. Say the SP is 40° with a hysteresis of 6°, 3° below, 3° above. At 43° the output turns off. At 37°, the output turns on. It's the nature of on/off.

An on/off controller can not maintain an even setpoint. By its very nature, on-off control has to oscillate above and below the setpoint.

If the controller is set for PID control and the temperature maintains a steady value above setpoint, then there is no reset/integral action enabled; the controller is running proportional only and exhibiting the reason for reset action.

The other reason is that the room is so well insulated that it can not lose the heat supplied by the heater so it drifts very slowly down from the on/off turn-off hysteresis point. Chances are the control output has turned off so the heater is not supplying additional heat, but the temperature is maintained because of the room's insulation.

When run in PID, enabled reset action will turn the output off over time, but the heat still has to be lost for the temperature to drop, unless the system can call for AC cooling.

Thanks for your resonse.
yes I am trying to fiigure out what was the setting on watlow, But can't find any documentation about the whole system.

bdauliya · Mar 28, 2023

cardosocea said:
The important question here is did they notice it but it had been like that since always or did they noticed that it actually changed? A lot of times people go past things that aren't quite right but never question it and when they notice it there's a tendency to claim that it wasn't like this the week before when in fact it had been for ages.

They said the alarm went off, and that has never happened before.

danw · Mar 28, 2023

It is rare in my experience to find documentation on room temperature control, so join the club.

> 110 VAC HOT from ranco is branched to Heater and a relay, Relay connects to Power supply, that 24 VDC from power supply is given as Input to Watlow.

The Watlow 998 has a single digital input, that Watlow calls an Event input. The table below shows the actions that can be chosen when the input is energized. The relay is most likely switching 24V to the Event Input (wired to terminals 23, 24) to call for an action.

You'll have check out the wiring to see what is wired to the Watlow (it was designed as environmental chamber controller, heat/humidity or heat/cool) and check the programming, which you can do while it is running, just don't change any settings.

Is the Watlow even doing anything or is the Ranco now the room temperature controller (which is what your description infers)?

Check and see if the heater elements are powered when the temperature is above setpoint. Or are they off and the temperature is 'coasting' down?

bdauliya · Mar 29, 2023

Thank you Gentlemen for your inputs.

I ended up changing the heater, and that resolved the issue.
It looked like the heater was stuck at ON state.

danw · Mar 29, 2023

You sure it's a bad heater?

If the controller does not call for heat, there should be no energy to generate heat from a heater element. Heater elements fail open.

It is quite common, though, for solid state relays to fail un-safe, that is, closed, and thereby supply power a heater even though the controller is not calling for heat. Can't turn the darn things off when they fail.

Tricky situation with Watlow

bdauliya

Supporting Member

Ken Roach

Lifetime Supporting Member + Moderator

danw

Lifetime Supporting Member

cardosocea

Member

bdauliya

Supporting Member

bdauliya

Supporting Member

bdauliya

Supporting Member

danw

Lifetime Supporting Member

bdauliya

Supporting Member

danw

Lifetime Supporting Member

Similar Topics