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Multi-Parameter Meter Comparison
As a long time and daily user of multi-parameter water quality meters, I've used almost every kind of meter on the market with just a few exceptions. When you use a meter as much as I do you become intimately familiar with the meters and subtle nuances of each brand and model. Not only have I used the meters, I've also had lengthy conversations with many of the manufacturer's representatives discussing the things I liked about the meters and the things I don't like. In this article, I will summarize six of the top multi-parameter meters and tell you what
I know about each, the things I like and the things I don't like. Every one of these meters has my recommendation as being acceptable, so if you have one keep using it, they are all good meters. My purpose is to here is to tell you some things about these meters you may not have thought about and which one I think provides the best value.
With the exception of one meter, I have used them all at length and I have obtained quotes for the purchase of them all, so I am speaking from my own anecdotal experience. I'm not Consumers Reports and I haven't done extensive research or experiments on the meters. I have just used them in the real world and I am giving you my "user" opinion. So manufacturers don't sue me if I don't rate your meters as high as you would like me to.
First off, all the meters had the following parameters at minimum; pH, temperature, conductivity, turbidity, dissolved oxygen (all optical DO with the exception of one) and oxygen reduction potential (ORP). These parameters are the standard parameters use in the "Low Flow Method" (U.S. Environmental Protection Agency Region 1 Low Stress (low flow) Purging and Sampling Procedure for the Collection of Ground Water Samples from Monitoring Wells, July 30, 1996, Revision 2). I prefer the "optical" dissolved oxygen sensor over the older "clark cell" membranes for various reasons, namely the higher reliability, lower maintenance and longer life of the sensor.
Before we get started, one other important factor, that from my experience most people do not consider, is the size/volume of the flow through cell. The volume of the flow through cell can have a significant impact on the length of time it takes to sample a well. This is rarely considered during the purchasing process however it can make a big difference on sampling efficiency and your bottom line. Let me explain.
The low flow technique is predicated in the concept of sampling water fresh from the aquifer with minimal disturbance or mixing of the standing water column within the monitoring well casing. In addition, the goal is to also sample this water, fresh from the aquifer, without changing its physical or chemical characteristics during the sampling process. To accomplish this, according to the method, you pump at a relativity slow rate (<500cc/min) to avoid mixing within the casing and to avoid draw down of the water table within the well (<0.3 feet). Measurements of the water table elevation are collected frequently during pumping to assure you do not exceed the 0.3 feet of allowable draw-down. If draw down exceeds 0.3 feet, the pumping rate needs to be slowed to keep within the maximum draw-down. There are numerous other factors; however, I'm not going to go into all the scenarios that could come into play.
While pumping, measurements of the parameters listed above are taken at periodic intervals and plotted to observe changes over time. Once the percentage of change for each of the parameters is within a predetermined minimal amount for 3 consecutive readings (this is called "stabilized" or "stability") the filling of the sample bottles can occur.
The big variable here is the sampling interval and this is where I see a lot of "samplers" making mistakes with their sampling method and why the size/volume of the flow cell comes into play. It's a simple concept but one often overlooked. If my flow through cell is 1,000 cc's and I'm pumping at 100 cc/min. It takes a minimum of 10 minutes to change out one volume of the flow through cell. If I'm pumping at 50 cc/min it will take 20 minutes to change out one volume.
Many "samplers" from various companies I've encountered (If I see someone sampling while I'm out and about I usually stop and talk to them) are simply taking a reading, which is used to calculate stability, at a per-determined time interval without factoring in the flow rate. The method they follow calls for simply taking a reading every 5 minutes. However, if your flow cell is 1,000 cc's and your pumping at 50cc/min you have only changed out ¼ of the water in the flow cell and essentially are reading largely the same water as your last reading, giving you the false appearance of stability. If you read the same water over and over you would expect to get the same readings. The sampling interval to determine stability has to be at minimum at least 1 volume of the flow cell (in this scenario it would be 20 minutes), some say you should even factor in the volume of the tubing into your calculation.
So, if I'm on a well that will only yield 50 cc/min while staying within the draw-down limits of 0.3 feet, the smaller my flow through cell is the more frequently I can collect a reading to determine stability. So using the same scenario as above only this time I have a 200 cc flow through cell, I will change out the water in the cell in 4 minutes rather than 20 minutes. Since I have to have three in a row that fall within the acceptable percentage of change, in the first scenario it would take me an 60 minutes to get 3 in a row to reach stability, in the second scenario it will only take me 12 minutes. A significant difference.
YSI 6920 V2
This meter, or should I say "sonde"(sonde is the name for the wand part with all the sensors connected and in contact with the water), is the largest sonde of them all. It's generally sturdy and well made, but large. One of the unique features of this unit is its ability to connect to a variety of reading devices, a hand held PDA device or even directly to a laptop. The unit calibrates fairly easy and is moderately simple to operate. The software that comes with the unit can do data logging. The calibration cup is large and you will go through more calibration fluid over time than you would with other devices, thus increasing its operational costs. One of issues I have with the device is the size of the stock flow through cell. At 900 cc's, it is the largest flow through cell of all the meters evaluated. From a "low flow" sampling efficiency perspective, that makes this unit, for me, the least efficient of the all the meters evaluated. This meter with the six parameters listed above, was quoted to me at $9,200. This does not include an output device like a PDA or a laptop. That must also be included in the total unit price.
Hach Hydrolab MS5
The MS5 sonde is small. 1.75 in diameter and appears solid and well made. It is able to connect with a couple different reader units. The most popular one is the Surveyor, which I have used, but there is also a pocket PC (a PDA device) that seems simpler and has more capabilities such as plotting change over time on a graph which helps determine stability. The calibration on this device is fairly simple and straight forward. The manual is easy to read with lots of illustrations to help you skim through it faster. The calibration cup is relatively small thus will save on calibration fluid. The flow through cell comes in at 200 cc, making it tied for the smallest flow through cell of all the meters available. This 200 cc flow through cell is only available on the MS5 model, not some of their other similar but larger models. The price quoted to me for the MS5 with the parameters needed, the PDA reader and the roughly 200 cc flow through cell was$9,256.
The Horiba sonde is a large unit; however, it is very sturdy and rugged. It has the largest diameter of the meters evaluated. In addition it is the only meter I am evaluating that does not come with an optical DO option. I think this is the biggest downfall for this meter. "Clark cell" DO requires more maintenance and regular replacement increasing the long term operations costs of the unit. I much prefer the simplicity of optical DO. The Horiba brand is the brand I am most familiar with and the brand I have owned the most of. I've used, to the point of wearing out, several U-22's and one U-53. I'm very accustom to the meters. Calibration is easy with the U53 however the calibration cup is somewhat large, increasing costs over time. The flow through cell is also very large coming in at roughly 600 cc's. Too large for my need so I made my own flow through cell that was just under 400 cc's. The unit comes with a very usable display reader and can also do data logging. The current retail price of a unit with a flow through cell, according to some dealer websites, is
$4,805. Remember that is for a unit with a "clark cell" DO not optical DO, which is not available as yet from Horiba.
Insitu Troll 9500 XP
The Troll 9500 is a well-made high quality sturdy meter and very reliable. However don't let it roll off a counter and smash on the ground, it will cost you about $2,000 to get fixed....speaking from experience (I still cry thinking about it). I've used the Troll extensively and found it performed well with no issues. The unit operates with the assistance of the Rugged Reader a hand held device. It's a great reader in that it logs data, graphs it and even calculates stability based on your input parameters. My only complaint about the Troll 9500 software is that it does not allow you to adjust flow rate, thus sampling frequency on the fly. Once you set your sampling/data logging frequency, you are stuck with it until the end of the sample, unless you restart the sampling process. Calibration is easy and the technical and sales support staff at Insitu are exceptional. The flow through cell on the smaller XP unit is around 200 cc's. This unit was quoted to me with the Rugged Reader and a flow through cell for approximately $10,015. The quote I am referencing is rather dated so prices may have been adjusted since then.
The AquaRead is the smallest of the meters by a significant margin. It appears to be well-made, mostly of aluminum with some plastic parts. I've used the AquaRead extensively and currently have 2 of them which we use almost daily. They perform well with very few issues. The unit operates with the assistance of a AquaRead hand held device with a built in GPS. It logs data and can be connected to a PC; however my only complaint about the AquaRead hand held is that you can't see all the parameters on one screen at the same time. You have to cursor from page to page to read the parameters. Calibration is easy and the technical and sales support staff at AquaRead are very good and helpful. The flow through cell on the AquaRead is 200 cc's, tied as one of the smallest available. A new AquaRead was quoted to me recently with a with the hand reader with GPS and a flow through cell for $4,225. This makes it the least expensive and smallest meter and flow through cell on the market.
The Manta 2 by Measurement Specialties (formerly Eureka) is the only meter which I have never used. In addition, I don't know of anyone that has used one. It appears to be a decent unit. The unit I was quoted came with a Windows based hand held display that could perform data logging. Both the sonde and the display appeared to be a decent in looks and function based on the literature provided. Although no specific cc's on the flow through cell were provided, it appears to be somewhat small but likely over 200 cc's. The Manta 2 was quoted to me recently with a the hand reader and a flow through cell for $7,725.
I base my pick on the following; price, flow through cell size and long term operational costs. The AquaRead comes in first place in all three categories and is my pick for multi-meters right now as it is clearly currently the best value out there.
Keep in mind that technology is ever changing, even though they currently finish tops in my book at this point, doesn't mean they will always stay there. Technology and manufacturing changes quickly. If you are interested in learning more about the AquaRead multi-meters you can contact the North American distributor at Waterra.
I can't guarantee they will quote you what I was quoted or that you will be as happy as I have been with their meters. I'm not affiliated with Waterra or AquaRead in any way other than as a customer. I'm simply offering you my experience, perspective and opinion.
My Additional Thoughts and Dreams
I would like to see the manufacturers develop a unit that worked with an off the shelf tablet such as the iPad or similar Android or Windows tablet (or even smart phones). I would like to then see applications developed for use on the tablets which could be customized and downloaded like most current smart device apps are. The connection from the sonde to the tablet could be via Wi-Fi or Bluetooth. The development of the sonde that is operable across multiple device platforms using a downloadable application opens up a world of data management and reporting possibilities. If the manufacturers won't do it.....I guess I'll have to do it myself. :-) Stay tuned!