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Alternetives to Zahm & Nagel C02 testers?

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  • #16
    Originally posted by bb23 View Post
    I realize this is a stupid question, but I have to ask anyway: what is the preferred method for testing/calibrating the pressure gauge on the taprite co2 tester?

    Thanks for the help!
    Not a stupid question at all!

    Timm has a previous post that shows his test rig. Very simple and quick to use. Basically it's a length of pipe with the pressure standard gauge mounted in a Tee on it, and a bleed down valve at one end, and a pressure-regulated supply connection at the other. There is another T where you mount the gauge under test. I think he uses an adapter so that he can attach a variety of different connections to it. I believe Timm's pressure standard is an electronic gauge that uses a strain gauge element to read pressure, and not a mechanical assembly like a bourdon tube or diaphragm. So theoretically it doesn't tend to drift as much. But basically you carefully remove the taprite gauge, connect it to your test rig, and slowly increase pressure via a CO2 or compressed air regulator, and check to see how far off your taprite gauge is from the standard. Hopefully your gauge can be adjusted. Otherwise you should record the offsets and write them down as a table. For example, your gauge reads 12 psi, but the standard shows 12.5, and at 15 psi the standard reads 15.8. I like to calibrate by both increasing the pressure, and then starting higher, and bleeding the pressure. This can identify hysteresis in the gauge. Ideally you want the gauge to read the exact same whether you got there by increasing or decreasing pressure. Mechanical gauges almost always have some detectable hysteresis.

    In the general case, there are a couple ways to test, and it depends on what you use as pressure standard.

    I've done these types of calibrations in the past in other industries, so I have a deadweight tester (which is a primary pressure standard) and a very accurate wallace and tiernan pressure calibrator. Heise makes a similar one--I used them when I was in the Navy. Here's an example from Cole Parmer It's a small suitcase, and has the valves and pressure regulators and even a CO2 pressure supply built in. It's designed to do on site calibrations, and since you can pressurize the back side of the gauge itself, you can use it for differential pressure calibration (at low pressures), which makes it useful to calibrate a manometer. They are super expensive new (~$6,000), but I see one on ebay right now for $125. These are nice because you can bring it to the gauge and test in situ, if you have a calibration port to connect the tester to. For testing a zahm, of course, it's simple to bring the device to the tester, but if you have pressure gauges in your process piping, then in-situ tests are easier.

    The deadweight tester is used to calibrate your pressure standard, since it's not convenient to use. It's basically a hydraulic hand pump with a platform for weights. The weights are carefully calibrated. You stack them up to test for a specific pressure, which can be REALLY high, if need be. I only ever use it at the low end of the range. Anyway, you carefully pump by hand to float the weights, and at that point you know quite accurately what the pressure is. They're tricky to use, and totally unnecessary for a brewery. Better to send your pressure standard out for calibration periodically.

    Regards,
    Mike Sharp

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    • #17
      Here's the thread on my calibration set-up: http://discussions.probrewer.com/sho...alibration-Day! I think it ran me about $200. I send the reference gauge back to Kodiak once a year for calibration, and it's time to get it in the mail again. Re-cal costs about $80 each time, but it's money well spent, as a brewery runs on its gauges.
      Timm Turrentine

      Brewerywright,
      Terminal Gravity Brewing,
      Enterprise. Oregon.

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