Flow loop unstable
Flow loops have a particular character: very little dead time, a fast response, and a measurement carrying real turbulent noise. That combination makes them sensitive to a small number of specific mistakes, and most unstable flow loops are suffering from one of them.
The good news is that flow loops are the easiest class of loop to work on. They settle in seconds, so a bump test is quick and a change can be assessed almost immediately.
Derivative should be zero
This is the first thing to check on any unstable flow loop. Derivative acts on rate of change, and a turbulent flow measurement has a large rate of change even when the flow is steady. The derivative term multiplies that noise and writes it to the valve.
There is essentially no flow loop that benefits from derivative action. If rate is non-zero, set it to zero before you assess anything else.
Flow loops want low gain and fast reset
This surprises people who work mostly on temperature loops. A flow loop has very little lag, so the process gain the controller sees is high and the controller gain needs to be correspondingly low — well below one is normal.
Reset, on the other hand, can be fast, because the loop responds in seconds. The characteristic flow-loop tuning is a low gain with a reset measured in seconds, and it looks wrong to anyone used to slower loops.
The failure mode to avoid is compensating for the low gain by making reset faster still. Once reset repeats faster than the loop's dead time, the loop hunts regardless of gain.
What causes it, most likely first
Derivative action switched on
most common tuning can fix this
Noise amplification straight to the valve.
- How to confirm it
- Rate is non-zero, output is noisier than the measurement.
- What to do
- Set rate to zero.
Gain far too high
most common tuning can fix this
A flow loop tuned with the gain you would use on a temperature loop will be violently unstable.
- How to confirm it
- Decaying oscillation with a period of seconds after a small setpoint change.
- What to do
- Reduce proportional action substantially.
Reset faster than the dead time
common tuning can fix this
Flow loops have short dead time, but not zero, and reset set faster than it produces a guaranteed cycle.
- How to confirm it
- Slow steady swinging that does not respond to gain changes. PIDSnap's pre-flight check flags this from the loop profile.
- What to do
- Slow the reset.
Equal-percentage trim on a wide-range flow loop
common tuning cannot fix this
Equal-percentage trim makes the process gain vary strongly with load, so no single tuning set works across the range.
- How to confirm it
- The loop is stable at one production rate and hunts at another.
- What to do
- Linear trim is the durable answer for flow control. Output characterisation or gain scheduling is the workaround.
Questions that come up
What gain should a flow loop have?
PIDSnap does not give tuning numbers, and neither should anything else that has not measured your loop. What is safe to say is the shape: flow loops characteristically want a gain well below one and a reset measured in seconds, which looks aggressive on reset and timid on gain compared with slower loops.
Should I filter the flow measurement?
A small amount of filtering is normal on a flow loop and is usually already present in the transmitter. Heavy filtering added later to make a trend look calmer is a warning sign — it adds lag and limits what the loop can ever achieve.
Related
Tuning guides
Last reviewed 2026-08-01.