FAQs

Where do I Measure the Flow in a Parshall Flume?

One of the most common questions is where to measure the flow in a Parshall Flume.  The nature of a Parshall Flume is such that there is only ONE correct point of measurement under free-flow.  That point of measurement (called Ha) is 2/3 of the length of the inlet of the flume measured UPSTREAM of the throat (the narrowest part of the flume.  

DO NOT measure in the throat.  That isn’t the correct point and it’ll give you a flow rate LOWER than what’s actually passing through the flume.

The correct point of measurement is indicated on our Parshall Flume Dimensions Drawing.

 

How Accurate is a Parshall Flume?

By themselves, and in controlled conditions, Parshall flumes are accurate to +/- 2-5%.  

Keep in mind, though, that variances in approach / downstream conditions, installation, and measurement reading errors, mean that you shouldn't expect a flume installation to be more accurate than +/-8-10%.

This system accuracy is similar to what you should expect from really any properly installed flume or weir measuring flow in open channels.

 

Is there a 4" Parshall Flume?

No. In developing the Parshall flumes over the years, researchers have developed:  1", 2", 3", and then 6", 9", and larger sizes.  They skipped over the development of a 4" Parshall Flume.  

As Parshall flumes are empirical devices, you cannot simply interpolate the size and flow characteristics of a 4" size flume.

 

How do I maintain my Parshall Flume?

One advantage of flumes is that they require little in the way of maintenance.  You should make an annual inspection should be made.  

During that inspection:

  • Check that the flume hasn’t shifted out of position
  • That the upstream / downstream channels are clear of vegetation and debris
  • And that the flume’s flow surfaces are in good repair
  • Check to make sure no flow is bypassing the flume

Periodically you may want to clean the flume to keep if free of buildup.  Each site is different so the cleaning schedule will vary from site to site.  To clean the flume, simply use a mild and biodegradable soap solution and a medium bristle brush.  

If your flume has a staff gauge, take extra time to make sure that the gauge is clean and readable. If the gauge is in poor shape, now would be a good time to replace it.

 

What if my Parshall Flume has Shifted out of Position?

Flumes in earthen channels can shift out of position.  Frosty heave, erosion, scour, and animal traffic can all contribute to this.

For irrigation / water rights flumes, a good time to check your flume is before the spring thaw.

Ideally if the Parshall Flume has shifted, you would re-level the flume.  However, sometimes that isn’t possible or will cost too much.

If that's the case, a correction has been developed that allows users to correct for both side-to-side (lateral) and front-to-back (longitudinal) settling.

While flumes cast in concrete don't typically shift, they still require periodic inspection of the flume’s general condition. If the flume uses a flow meter to automatically determine the level and flow, a good time to inspection the flume is when you are checking the meter calibration (at least annually).

 

Can a Fiberglass Parshall Flume be Field Repaired?

Generally yes - but off-site is easier. 

When repairing a Parshall Flume in the field, there are a couple of things that have to happen:

  • The flow has to be able to be stopped or bypassed as long as necessary to complete the repair
  • The flume has to be dry and clean
  • The weather has to cooperate - ideally the temperature is 70-75 degrees with no precipitation
    • Colder than that and cure times become longer (if possible at all)

Ideally, you have AC power at the site.  Depending upon the size / nature of the repair, you may need to grind or sand more than what a batteries provide.

 

Can a Galvanized Parshall Flume be Field Repaired?

Surface rust can be repaired in the field.  

When repairing a Parshall Flume in the field, there are a couple of things that have to happen:

  • The flow has to be able to be stopped or bypassed as long as necessary to complete the repair
  • The flume has to be dry and clean

    A battery powered grinder can be used to remove any flaking / surface oxidation.  Once that is done and a a good clean surface is left, simply mask off the area and then apply cold galvanization.