Please help a newbie set up OMRON!

Proportional band is the percent of process variable (span of temperature, in this case) it takes to swing the output from 0% to

100% as the measurement pases through set point. It is the inverse of gain. So a 10% proportional band setting means that it takes a 10% of input span change to instantaneously change a zero% output to 100% or vice versa.

That period is the cycle time of the output that varies in duty cycle to vary the output percentage. In this case, 20 seconds will wear your output relay a tenth as much as a 2 second output cycle, and will probably work fine, since the thermal time constant of 50L of water is a lot longer than that, so a single power cycle will not produce a significant thermal ripple.

Normal means higher input produces higher output. Reverse means a lower input produces a higher output. Reverse is appropriate for heating. (lower temperature produces higher power)

On/off mode is similar to the thermostat on your furnace. It outputs full power till the setpoint is exceeded, then turns completely off, till the temperature falls below setpoint. This mode wears the output relay the least, but overshoots temperature, somewhat, since there is heat energy stored in the heating elements that keeps heating the water a little while, after the power is switched off. But it is simple and reliable.

Is there a manual tuning mode for the PID control (turn off the auto tune)? If you can find that, I can help you tune the PID response to control at setpoint with little overshoot or oscillation.

Hi John.

You seem to be in the habbit of helping me, especially on the electronics group. Many thanks.

Thanks for the explanations. The unit is quite basic and isn't manually configurable.

The data sheet has this to say; proportional band: 3 to 20% (in PID mode) automatically adjusted acording to the rise time of the controlled system. reset time: 4 minutes (in PID mode).. rate time 0.4 minutes (in PID mode). proportional period: 2 or 20 seconds, switch selectable sampling period: 500ms.

Not sure what reset time could mean?

With this automatic adjustment, how long does it take? Will it need to 'learn' again after being switched off?

My useage is for a home brewery, heating the brewing liquor, which needs to be kept farely accurately within 1C.

Initially, I'll fill the vessel up to 50L, and bring up to temperature

- about 72C. I'll then dump 15-20 Litres in to another vessel, and I'll need to keep the remainder at a slightly higher temperature of about 76C for about an hour and a half, after which time the remaining liquid will get run off.

The vessel is plastic (HDPE I think), with two layers of reflective coated bubble wrap insulation. I intend to use dual kettle elements to bring up to temperature, and perhaps run just one element for keeping at temperature.

Am I best running on the 2 second propotional period when the water gets down lower?

Regards, Mark

It seems to me that You need a SSR (Solid State Relay) to drive power to heating elements with this PID parameters.

MarkMc wrote: (snip)

I think I agree. This is the most detailed sheet i could find:

It goes back to the primitive days of proportional only controllers (with no time sense). With proportional only control, you must have a constant error to produce a constant, non zero output. Then they added a manual output bias adjustment that the operator could set, to get the error to zero, with an arbitrary output power level. The reset time was the amount of time the system took to resettle after an adjustment was made, before the operator could evaluate whether or not his last adjustment had forced the error to zero. In a PID controller, an error integrator (the I in PID) continuously creates this output bias value, but it also must not get ahead of the system settling time. Its integration speed is still sometimes labeled in units of reset time, or resets per time. Slower settling systems require a longer reset time, to keep the integration term from impatiently over correcting the output bias term and pushing the controller into an integrator cycle. Slower is safer.

I can't answer that.

How much of a hurry are you to get to this temperature?

Are you going to rely on remembering to turn the heat off before this step?

The relay contact output is rated for less than 3 amperes, resistive, so it is not up to driving these elements, directly. You will have to add a booster relay, either mechanical or solid state.

If you are willing to wait longer for warm-up, you might connect them in series for the low wattage keep warm situation. this will lower the total wattage to half of one heater's rating. A heavy duty double pole, double throw switch could perform the switch from parallel to series (or one heater to two in series).

I doubt it. Even one liter of water will not change temperature much in 20 seconds, especially if you lower the wattage for the lower volume.

Putting some numbers in ...

1 kg (1 litre) of water needs 4200 J to heat it by 1 deg C. So if you have a 1 kW (1000 J/s) element it will take 4 seconds to to raise the temp of 1 litre by 1 deg C. For 50 litres. the time/deg is 4 x 50 = 200 sec; for 50 deg temperature rise (20-70): total time is 4 x 50 x 50 = 10,000 seconds = 3 hours.