The purpose of this web page describes the wire weight gage used by the National Weather Service to measure river stage and the typical installation and maintenance procedures for a wire weight gage. The web page also contains a series of digital photographs illustrating the installation process.
Wire Weight Gage
The Type “A” wire weight gage (above) is one of several types of river gages used by the NWS to measure river stage. The gage consists of a drum wound with a single layer of stainless steel cable, a brass weight attached to the end of the cable, a graduated disk, and a veeder counter mounted in a locking metal box. One revolution of the cable drum equals a one-foot length of cable. The disk (graduated in tenths and hundredths of a foot) connects to the veeder counter, which advances with each full revolution of the drum. The cable is made of 0.045-inch diameter stainless steel wire, and fills the drum using a threaded sheave. The wire weights reel assembly is equipped with a ratchet used to lock the drum and cable in any position by means of a pawl.
Hydrologic Program Managers and Hydrometeorological Technician verify the check bar reading of an existing wire weight gage by sliding the check bar (at the bottom of the gage) fully forward toward the drum. When in this position, the cable weight, when released, can rest on the check bar. The check bar reading equals the combined reading from the veeder counter and the number of gradations on the graduated disk when the weight is resting on the bar with no slack in the cable.
Caution #1: Be sure you do not have any loose items in pockets (especially keys!) that may fall out (into the river below) when you lean over the railing.
The figures below show a wire weight gage with the check bar forward, supporting the weight, and the gage with the check bar back position, allowing the weight to descend to the river.
Check bar (forward)
Check bar (back)
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Generally, wire-weight gages mount to the upstream side of a bridge above the main channel of the river or stream. The gages attach using several methods. In some cases, the gage mounts directly to the bridge with bolts. In other cases, the gage mounts using straps to secure it to the railing. Many newer bridges have concrete barriers on each side, necessitating hanger straps designed for the bridge. The owner of the bridge decides the method of mounting the gage. Some bridge owners will allow the wire weight wage to attach into the concrete.
VERY IMPORTANT…Consult with the bridge owner for permission to install a wire weight gage.
It is good practice to check to the wire weight gage adjustments prior to going to the bridge site. It is common to find a new wire weight gage on which the main dial switches from one foot to the next at a gradation other than 0.00. For instance, the dial may change from 23 ft to 24 ft at 23.85 or 24.20. If left unchanged, this can cause confusion for the observer. Check this in the shop if possible.
Use an Allen wrench to loosen the small brass collar on the gage shaft near the dial. Turn the collar to re-tighten on the shaft, and then turn the handle and viewing the new turnover point. It sometimes takes several tries to get it right. The dial should turn over when 0.00 is read by the pointer and the dial has just switched to the next foot.
Caution #2: Never mount a wire weight gage on a bridge without help. Installation of a wire weight is a two-person job. Be sure someone is available to help when installation of a gage is scheduled.
Please note that the NWS is only responsible for the installation and maintenance of NWS owned wire weight gages. Gages owned by other agencies are their responsibility.
After mounting the gage on the bridge, run level from a known elevation point to the gage. Consult with the owner of the bridge for nearby benchmarks or reference elevations. Often, the owner will agree to run a level for the wire-weight check bar. The objective of running a level on the new gage is to develop a check bar setting. The process of determining the check bar elevation is calculated from the "gage zero datum".
The datum of the gage may be a recognized datum, such as mean sea level (MSL), or an arbitrary datum chosen for convenience (so stages are a relatively low numerical value). To eliminate the possibility of negative values of the river stage, the "gage zero" datum selected for operating purposes is set below the elevation of zero flow on the control for all possible conditions.
Maintain a permanent datum so a station uses only one datum for the gage height. To maintain a permanent datum, each gaging site requires at least two reference marks or benchmarks that are independent of the structure on which the gage is mounted. Periodically run levels from reference marks to maintain a fixed datum.
Reference: Bench Mark and Zero Datum
Note, in the figure above, the "gage zero datum" is below the streambed. The next several paragraphs describe how to calculate the "gage zero datum".
If the gage is being installed where a gage was located previously, the previous zero datum is usually used. If installing the gage at a new gage site, the Hydrologic Program Manager must establish zero datum.
When installing a gage at a new site, first measure the elevation of the deepest part of the channel along the face of the bridge. Survey the elevation with the rod person wading across if the stream is small and shallow.
If the water is too deep or swift for wading, the lowest channel elevation can be determined by performing soundings from the bridge deck. Soundings are measurement of depth or distance from the bottom of channel to the water surface, bridge rail, bridge deck, or other feature of known elevation, using a cloth tape measure and attached weight. Determine the lowest elevation of the channel by subtracting the deepest sounding from a referenced elevation. Choose a zero datum elevation two to three feet lower than the lowest part of the channel. That way if the channel should experience scour in the future, the water surface during low flow will not fall below the zero of the gage. For example, if the lowest part of the channel at the gage site was 950 ft above MSL, choose a zero datum of 947 ft MSL.
If the channel bottom is solid rock, there is no need to set the zero datum of the gage two to three feet below the lowest channel bottom. One foot would be enough. Conversely, if the channel is composed of sandy, loose material, three feet of may not be enough.
Please note: Try to install the wire weight gage during low flow. This will make it easier to get a cross-stream profile and determine the lowest point in the channel.
A benchmark or a reference mark, with a known elevation, is the control for the elevation of the check bar and the zero datum of the gaging site. When setting a wire-weight gage, perform leveling to find the elevation of the check bar and determine the elevation of zero datum.
The figure below shows the leveling rod sitting on the wire-weight gage check bar.
Hold leveling rod on check bar
On occasions such as the replacement of a bridge, a wire weight gage may need to be re-installed and new check bar values established.
When re-installing a wire weight gage at an established wire weight site, the gage zero is usually already established. If this gage zero is still valid, the re-installed gage will be set against this value. If the gage zero is no longer valid, the gage zero must be re-established. The established gage zero elevation is simply subtracted from the surveyed elevation of the re-installed wire weight check bar elevation, or the distance in feet and hundredths of a foot that the check bar is above the gage zero. This distance becomes the check bar reading of the newly installed gage.
Bring the gage back into tolerance if the observed check bar reading differs by 0.03 ft or more from the official check bar established for the station. Official check bar values are on WS Form B-19 or B-44.
Calibrating the gage, setting the check bar to the proper stage
The cable drum is fastened to the handle and shaft assembly by a friction clamp. Loosen the friction clamp to set the check bar reading. This allows the handle and shaft attached to the veeder counter to turn independently of the cable drum. This also allows the observer to hold the cable, weight, and drum in place while obtaining a check bar reading on the counter and graduated disk. (See picture above.)
Caution #3: Hold the cable drum in place to prevent the weight from free fall after releasing the handle shaft assembly from the drum cable assembly.
Use the procedures listed below to set the check bar reading on the gage.
Caution #4: Tighten both screws before letting go of the cable drum.
Handle in wrong position for closing cover
Document the new check bar
reading on the B-44 for the station.
Also, make the new check bar setting available to the Hydrologic Program
When a weight is lost from a wire weight gage, consider two factors: the length of the remaining cable and the condition of the cable. These factors determine the correct method of repair. Unwind and rewind the remaining cable and count the number of feet of cable left (one revolution of the drum equals one foot of cable). Visually inspect the condition of the remaining cable, and replace if the cable fraying or kinking is noted.
Install a new brass weight on the existing cable if the remaining cable is in acceptable condition and sufficient cable remains on the drum to allow the weight to extend several feet below gage zero.
If the remaining cable on the wire weight is unacceptable or if it is too short to extend several feet below gage zero, replace the cable. A drum should be near capacity (100 ft) regardless of the amount required for a reading. Having a full drum of cable allows replacement of weights without addition of new cable.
Cable replacement follows these steps:
Caution #5: Keep tension on the cable while installing the new weight to prevent the cable from tangling.
Adding the weight to the cable
Tightening the nut on the weight
At this time, the gage should be back in operation. Lower the weight to water level and rewind it. Cable should rewind evenly on the drum.
After installation of the new cable and weight, the wire-weight check bar reading must be set as previously described. The gage is now operational.
During routine river stage observations, use the following procedures:
Access a series of pictures illustrating the steps in wire-weight gage installation by clicking here.
Photographs by NWSTC and