At least once a year. The USC Manual of Cross-Connection Control (10th edition) says field test kits should be checked for accuracy at least once a year and recalibrated when off by more than 0.2 psid. Arizona's rule points to the 9th edition (1993) for test procedures. Mesa and Glendale require it yearly. The tester owns that duty.
What the standards say
The standard most programs lean on is the USC Manual of Cross-Connection Control. Arizona's backflow rule, R18-4-215, names the Ninth Edition (December 1993, "and no future editions or amendments") for test procedures, so the wording below from the 10th edition is the current industry standard, not Arizona's adopted text. Texas's environmental agency quotes the manual's 10th edition in its gauge guide: "Field test kits should be checked for accuracy at least once a year, and re-calibrated when inaccuracy exceeds the current error tolerance, ± 0.2 pressure differential in pounds per square inch (psid)."
AWWA's Manual M14 says much the same, per the same guide: field test equipment accuracy "should be verified at least annually," and if it is out of tolerance, the equipment "shall be calibrated and brought into acceptable accuracy tolerances."
Kit makers agree. Watts says backflow test kits "should typically be recalibrated every 12 months," and that "if your kit undergoes heavy or frequent use, recalibration every six months may be advisable."
| Source | Interval it gives | Who it puts on the hook |
|---|---|---|
| USC Manual, 10th ed. | Accuracy check at least once a year; recalibrate past ±0.2 psid | Not stated; the tester carries it in practice |
| AWWA Manual M14 | Verify accuracy at least annually | The tester using the kit |
| Watts (kit maker) | About every 12 months; six months under heavy use | The kit owner |
| Mesa City Code 8-1-12 | Calibrate annually; report expires after one year | Recognized testers |
| Glendale Backflow Tester Policy | Calibrate annually; reports sent each year | Testers and companies on the list |
What Arizona and Valley cities require
Arizona's rule, R18-4-215, says every required assembly "shall be tested at least annually," following "Section 9 of the Manual of Cross-Connection Control," by a certified general tester. The rule itself doesn't set a calibration interval or mention test kits. The cities fill that gap.
- Mesa: Recognized testers "must conduct a calibration of all backflow prevention test equipment annually through an agency accepted by the Department" (City Code 8-1-12). Mesa's code adds that "An equipment accuracy report dated more than one year old will be considered expired." A tester without a current report gets removed from Mesa's list.
- Glendale: "All test equipment used in testing of backflow prevention assemblies shall be calibrated annually." Calibration reports go to the city's cross-connection office each year. Missing reports can get the equipment, the tester or the company pulled from the recognized list.
- Phoenix: To get on the city's list, a company submits "Test Kit Calibrations" with its paperwork. The tester application lists each test instrument by manufacturer, model and serial number, along with the calibration company and date, with copies attached. The test report instructions (TRT 00335) ask for the tester's "certification number, and test kit serial number" at sign-off. The calibration date itself lives on the tester's file, not on the report.
For the rest of the tester rules, see who can test backflow assemblies in Arizona.
Accuracy check vs. calibration
They're two different steps. The Texas guide explains it well. An accuracy test compares the field gauge to a standard gauge. Calibration means "adjusting the gauge mechanism so that the gauge indicates within specified accuracy limits." So a kit that passes its yearly accuracy check may not need adjusting at all.
The USC method, as quoted there, checks the kit against a reference traceable to NIST at 12.0, 8.0, 5.0, 2.0 and 1.0 psid. "Exceeding the accuracy of ± 0.2 psid should be cause for rejection." Those low points matter, because many pass marks on a test report sit near the bottom of that scale. Our guide to reading a backflow test report shows where each number lands.
Texas adds two rules of its own: gauges can only be calibrated by a company listed as a service center by a backflow gauge manufacturer, and a tester can't accuracy-test their own gauge. Those are Texas rules, not Arizona's, but they show how seriously regulators take it.
What to do as the owner
You don't calibrate anything. The tester does. But you can check it in about a minute:
- 1Ask for the calibration sheet for the kit used on your test.
- 2Match the serial number on the sheet to the test kit serial number on your report.
- 3Check the date. If it's more than 12 months old, ask before the report is filed. In Mesa, that report is already expired.
- 4Keep the sheet with the report in your assembly file, so you have both halves if a reading is ever questioned.
For what else to ask before you book, see how to hire a backflow tester and read the report. Records advice is in how long to keep backflow test records.
HQ Plumbing & Air's certified techs test and certify domestic and irrigation backflow assemblies and file the results with the city. We're licensed and insured, and estimates are free.