Good morning. I inadvertently ordered 2 6' Resistive Tank Level sensors instead of 4-20 ma. The resistive increments is 100 ohms per 1/4" making the range from 0 ohms to 28,000 ohms. I purchased 2 converters online that have an input from 0-50,000 ohms, when the converters arrived i noticed that the input is for a potentiometer. Is there a way to hook this up with a pull up resistor or tying one side to -0v? A schematic of the device is posted below. The probes are 2 wire. Can I make this work?



I appreciate any help i can get.

Thanks in advance Howardhhjr

 

I inadvertently ordered 2 6' Resistive Tank Level sensors instead of 4-20 ma.

Hi Howard
Do you have a datasheet or link for the Resistive Tank level that you could post?
E

 

Description: C4651-Z-110436, Continuous level sensor per user defined specifications.
Resolution 1/4" Resolution
Output Resistive, 100 Ohms per 1/4" (Minimum resistance at fitting)
Stem Material 316 Stainless Steel
Mounting Type External
Mounting Fitting 2" Pipe Plug
Float Type Stainless Steel, 2.13" OD by 2.10" High
Float Specific Gravity 0.55
Sensing Length 67.12"
Stem Length 70"
Top Deadband 1.19"
Bottom Deadband 1.69"
Lead Length 600"
Maximum Temperature 221°F (105°C)
Maximum Pressure 200 PSI
Approvals
UL E150881
UL Approval Location Hazardous Location; Class I, Groups A, B, C, D; Class II, Groups E, F, G; Class III

 

 

That is all I have. There was an online configuration tool no real cut sheet so to speak. Thanks.

 

Hi Howard,
A couple details we need to know.
E

 

I purchased 2 converters online

What are the converters' part numbers?

 

I will test the voltage and see if 0% terminal is common with 0volts on Monday at work. I bought this off of aliexpress as it was the only thing i could find that was above the 28K ohm range of the sensor.

• WS9020
  Type
  Other PLC Peripheral Modules
  Brand Name
  PLCOME
  
  

Thanks in advance

 

What you really need is a specification sheet for both the sensor and the converter, NOT an architects "cut sheet. I can appreciate that you are indeed in a serious bind.
A possible work-around would be to use a constant current regulator to run some small current thru the variable resistance sensor. With ten milliamps that would be 40 millivolts per inch, or at 25 milliamps you would have 100 millivolts per inch, giving you 7000 millivolts ,(7 volts) full scale. You will still need a voltage to 4 to 20 milliamp current converter circuit, no matter what.
DEpending on what voltage range that potentiometer converter can accept, that might work.
Either way you are going to have to create a bit of circuitry to get it to work.
Is this a first project at a new job, by any chance??

 

If the input from the measuring package is to a PLC, depending on what else may be connected to the analog input module, a voltage input module plus a constant current supply could be what you need.
So the question stands, what is that 4-20 ma input on? And usually the current input is converted to a voltage by a precision resistor. If the current input does use an external power source then the scheme I suggested can work with just a buffer amplifier.

 

If the input from the measuring package is to a PLC, depending on what else may be connected to the analog input module, a voltage input module plus a constant current supply could be what you need.
So the question stands, what is that 4-20 ma input on? And usually the current input is converted to a voltage by a precision resistor. If the current input does use an external power source then the scheme I suggested can work with just a buffer amplifier.

The Controller is a Caterpillar EMCP 4.3 CONTROL PANEL it has 3 analog inputs that can be configured as 0-5v, 4-20ma, or resistive input. The problem with the resistive input is that it is 0-2000 ohms. The input can be assigned to internal scaling / mathematical equation and ultimately displayed on the local display with the units of your choice which is also available as a modbus register for remote viewing. I still haven't has time to measure the values that I was asked to provide. Hopefully today i can get to it.

Thanks Howard

 

OK, if the wiring will allow, setting up for the zero to five volts option will make it work. You will need an isolated, and stable regulated current source for a bit less than ten milliamps. That will provide an output of just above zero up to five volts. And given that the controller is "Cat" brand, I am guessing that this is for ether a diesel pump system or a generator system.

 

OK, if the wiring will allow, setting up for the zero to five volts option will make it work. You will need an isolated, and stable regulated current source for a bit less than ten milliamps. That will provide an output of just above zero up to five volts. And given that the controller is "Cat" brand, I am guessing that this is for ether a diesel pump system or a generator system.

Can you give me an example of a bit less than 10ma power supply?

Thanks

 

If pin 2 is 5.0V could you put a 110K resistor in series with the 28K sensor and use the 24.0V supply for a potential divider giving 0 to 4.87V for pin 3?
Or if isolation is essential put a 50K resistor between pins 2 and 3 with the sensor between 3 and 4 and live with the loss of resolution

 

Here is the circuit of a 4-20mA transmitter I have used very successfully for years in industrial control applications.
Set the external pot to min and adjust ZERO to 4mA. Then, the external pot to max and SPAN to 20mA.
Check each way.
You could put your sensor on the bottom half of the pot connector, between MIN and WIPER. Connect WIPER to MAX and change the SPAN trim pot to maybe a 25K.
A similar connection could be used on the module you have but some experimenting will be needed.

 

Can you give me an example of a bit less than 10ma power supply?

Thanks

OK, here is an example circuit description: From your regulated 24 volt source positive, connect the input of a 78L05 voltage regulator IC, Then connect a 10K, 1% tolerance resistor between the regulator common terminal and the regulator output terminal. The regulator output terminal will be the 10 milliamp source positive connection. If the current needs to be adjusted a bit, use a high value trim-pot across the 10K resistor. You may need to add a 470 ohm resistor in series with the 10K if the current needs to be decreased a bit.

 

The Controller is a Caterpillar EMCP 4.3 CONTROL PANEL it has 3 analog inputs that can be configured as 0-5v, 4-20ma, or resistive input. The problem with the resistive input is that it is 0-2000 ohms. The input can be assigned to internal scaling / mathematical equation and ultimately displayed on the local display with the units of your choice which is also available as a modbus register for remote viewing. I still haven't has time to measure the values that I was asked to provide. Hopefully today i can get to it.

Thanks Howard

This normally wouldn't be a feasible option because of the nonlinearity, but since you have the ability to scale the input with an equation then the simplest physical solution is to use a 2kohms resistance input and place a resistance of 2150 ohms in parallel with your 28.8 kohms sensor, as these two resistances in parallel is 2kohms. From empty to full, the output of the sensor will look like this (0-2kohms on y-axis, 0-72 inches on x-axis):

 

Certainly the parallel resistor scheme can work and it is by far the simplest hardware approach, but depending on the resolution of the A/D conversion, (number of bits) it may not be accurate enough. If it is for a fuel gage application to show when the tank is approaching empty but not be accurate enough to allow filling to the maximum capacity.
Certainly the two component constant current scheme will require a well regulated DC supply, but one supply could support all of the gages if they are in a single location.There is a lot of benefit in being linear.

 

Certainly the parallel resistor scheme can work and it is by far the simplest hardware approach, but depending on the resolution of the A/D conversion, (number of bits) it may not be accurate enough. If it is for a fuel gage application to show when the tank is approaching empty but not be accurate enough to allow filling to the maximum capacity.
Certainly the two component constant current scheme will require a well regulated DC supply, but one supply could support all of the gages if they are in a single location.There is a lot of benefit in being linear.

Good point, I am used to the relatively high input resolution (16 or 32 bit) of a PLC, but if this caterpillar controller has only 8 bit (0-254) or even 10 bit (0-1023) resolution then most of the data at the higher end of the range will be questionable. I added some columns to the spreadsheet I used to make the graph previously, and here is what the readings would be like at a few different input resolutions:

Spoiler: expand to see

Inches	Sensor ohms	apparent ohms	8 bit	10 Bit	16 bit
0​	0​	0​	0​	0​
0.5​	200​	182.9787234​	23​	94​	5996​
1​	400​	337.254902​	43​	173​	11051​
1.5​	600​	469.0909091​	60​	240​	15371​
2​	800​	583.0508475​	74​	299​	19105​
2.5​	1000​	682.5396825​	87​	349​	22365​
3​	1200​	770.1492537​	98​	394​	25236​
3.5​	1400​	847.8873239​	108​	434​	27784​
4​	1600​	917.3333333​	117​	470​	30059​
4.5​	1800​	979.7468354​	125​	502​	32104​
5​	2000​	1036.144578​	132​	531​	33952​
5.5​	2200​	1087.356322​	139​	557​	35630​
6​	2400​	1134.065934​	145​	581​	37161​
6.5​	2600​	1176.842105​	150​	603​	38563​
7​	2800​	1216.161616​	155​	623​	39851​
7.5​	3000​	1252.427184​	160​	641​	41040​
8​	3200​	1285.981308​	164​	658​	42139​
8.5​	3400​	1317.117117​	168​	674​	43159​
9​	3600​	1346.086957​	172​	689​	44109​
9.5​	3800​	1373.109244​	175​	703​	44994​
10​	4000​	1398.373984​	178​	716​	45822​
10.5​	4200​	1422.047244​	181​	728​	46598​
11​	4400​	1444.274809​	184​	739​	47326​
11.5​	4600​	1465.185185​	187​	750​	48011​
12​	4800​	1484.892086​	189​	760​	48657​
12.5​	5000​	1503.496503​	192​	770​	49267​
13​	5200​	1521.088435​	194​	779​	49843​
13.5​	5400​	1537.748344​	196​	787​	50389​
14​	5600​	1553.548387​	198​	795​	50907​
14.5​	5800​	1568.553459​	200​	803​	51398​
15​	6000​	1582.822086​	202​	810​	51866​
15.5​	6200​	1596.407186​	204​	817​	52311​
16​	6400​	1609.356725​	205​	824​	52735​
16.5​	6600​	1621.714286​	207​	830​	53140​
17​	6800​	1633.519553​	208​	836​	53527​
17.5​	7000​	1644.808743​	210​	842​	53897​
18​	7200​	1655.614973​	211​	848​	54251​
18.5​	7400​	1665.968586​	212​	853​	54590​
19​	7600​	1675.897436​	214​	858​	54916​
19.5​	7800​	1685.427136​	215​	863​	55228​
20​	8000​	1694.581281​	216​	868​	55528​
20.5​	8200​	1703.381643​	217​	872​	55816​
21​	8400​	1711.848341​	218​	876​	56094​
21.5​	8600​	1720​	219​	881​	56361​
22​	8800​	1727.853881​	220​	885​	56618​
22.5​	9000​	1735.426009​	221​	889​	56866​
23​	9200​	1742.731278​	222​	892​	57106​
23.5​	9400​	1749.78355​	223​	896​	57337​
24​	9600​	1756.595745​	224​	899​	57560​
24.5​	9800​	1763.179916​	225​	903​	57776​
25​	10000​	1769.547325​	226​	906​	57985​
25.5​	10200​	1775.708502​	226​	909​	58186​
26​	10400​	1781.673307​	227​	912​	58382​
26.5​	10600​	1787.45098​	228​	915​	58571​
27​	10800​	1793.050193​	229​	918​	58755​
27.5​	11000​	1798.479087​	229​	921​	58933​
28​	11200​	1803.745318​	230​	924​	59105​
28.5​	11400​	1808.856089​	231​	926​	59273​
29​	11600​	1813.818182​	231​	929​	59435​
29.5​	11800​	1818.637993​	232​	931​	59593​
30​	12000​	1823.321555​	232​	934​	59747​
30.5​	12200​	1827.874564​	233​	936​	59896​
31​	12400​	1832.302405​	234​	938​	60041​
31.5​	12600​	1836.610169​	234​	940​	60182​
32​	12800​	1840.802676​	235​	942​	60319​
32.5​	13000​	1844.884488​	235​	945​	60453​
33​	13200​	1848.859935​	236​	947​	60583​
33.5​	13400​	1852.733119​	236​	949​	60710​
34​	13600​	1856.507937​	237​	951​	60834​
34.5​	13800​	1860.188088​	237​	952​	60955​
35​	14000​	1863.77709​	238​	954​	61072​
35.5​	14200​	1867.278287​	238​	956​	61187​
36​	14400​	1870.694864​	239​	958​	61299​
36.5​	14600​	1874.029851​	239​	960​	61408​
37​	14800​	1877.286136​	239​	961​	61515​
37.5​	15000​	1880.466472​	240​	963​	61619​
38​	15200​	1883.573487​	240​	964​	61721​
38.5​	15400​	1886.609687​	241​	966​	61820​
39​	15600​	1889.577465​	241​	967​	61918​
39.5​	15800​	1892.479109​	241​	969​	62013​
40​	16000​	1895.316804​	242​	970​	62106​
40.5​	16200​	1898.092643​	242​	972​	62197​
41​	16400​	1900.808625​	242​	973​	62286​
41.5​	16600​	1903.466667​	243​	975​	62373​
42​	16800​	1906.068602​	243​	976​	62458​
42.5​	17000​	1908.616188​	243​	977​	62542​
43​	17200​	1911.111111​	244​	978​	62623​
43.5​	17400​	1913.554987​	244​	980​	62703​
44​	17600​	1915.949367​	244​	981​	62782​
44.5​	17800​	1918.295739​	245​	982​	62859​
45​	18000​	1920.595533​	245​	983​	62934​
45.5​	18200​	1922.850123​	245​	984​	63008​
46​	18400​	1925.060827​	245​	986​	63080​
46.5​	18600​	1927.228916​	246​	987​	63151​
47​	18800​	1929.355609​	246​	988​	63221​
47.5​	19000​	1931.44208​	246​	989​	63289​
48​	19200​	1933.489461​	247​	990​	63357​
48.5​	19400​	1935.49884​	247​	991​	63422​
49​	19600​	1937.471264​	247​	992​	63487​
49.5​	19800​	1939.407745​	247​	993​	63551​
50​	20000​	1941.309255​	248​	994​	63613​
50.5​	20200​	1943.176734​	248​	995​	63674​
51​	20400​	1945.011086​	248​	996​	63734​
51.5​	20600​	1946.813187​	248​	997​	63793​
52​	20800​	1948.583878​	248​	998​	63851​
52.5​	21000​	1950.323974​	249​	999​	63908​
53​	21200​	1952.034261​	249​	999​	63964​
53.5​	21400​	1953.715499​	249​	1000​	64019​
54​	21600​	1955.368421​	249​	1001​	64074​
54.5​	21800​	1956.993737​	250​	1002​	64127​
55​	22000​	1958.592133​	250​	1003​	64179​
55.5​	22200​	1960.164271​	250​	1004​	64231​
56​	22400​	1961.710794​	250​	1004​	64281​
56.5​	22600​	1963.232323​	250​	1005​	64331​
57​	22800​	1964.729459​	251​	1006​	64380​
57.5​	23000​	1966.202783​	251​	1007​	64429​
58​	23200​	1967.65286​	251​	1007​	64476​
58.5​	23400​	1969.080235​	251​	1008​	64523​
59​	23600​	1970.485437​	251​	1009​	64569​
59.5​	23800​	1971.868979​	251​	1010​	64614​
60​	24000​	1973.231358​	252​	1010​	64659​
60.5​	24200​	1974.573055​	252​	1011​	64703​
61​	24400​	1975.894539​	252​	1012​	64746​
61.5​	24600​	1977.196262​	252​	1012​	64789​
62​	24800​	1978.478664​	252​	1013​	64831​
62.5​	25000​	1979.742173​	252​	1014​	64872​
63​	25200​	1980.987203​	253​	1014​	64913​
63.5​	25400​	1982.214156​	253​	1015​	64953​
64​	25600​	1983.423423​	253​	1016​	64993​
64.5​	25800​	1984.615385​	253​	1016​	65032​
65​	26000​	1985.790409​	253​	1017​	65070​
65.5​	26200​	1986.948854​	253​	1017​	65108​
66​	26400​	1988.091068​	253​	1018​	65146​
66.5​	26600​	1989.217391​	254​	1018​	65183​
67​	26800​	1990.328152​	254​	1019​	65219​
67.5​	27000​	1991.423671​	254​	1020​	65255​
68​	27200​	1992.504259​	254​	1020​	65290​
68.5​	27400​	1993.57022​	254​	1021​	65325​
69​	27600​	1994.621849​	254​	1021​	65360​
69.5​	27800​	1995.659432​	254​	1022​	65394​
70​	28000​	1996.68325​	255​	1022​	65427​
70.5​	28200​	1997.693575​	255​	1023​	65460​
71​	28400​	1998.690671​	255​	1023​	65493​
71.5​	28600​	1999.674797​	255​	1024​	65525​
72​	28800​	2000.646204​	255​	1024​	65557​

 

Was it so difficult to visualize a two component circuit that it was given up???Using a constant current thru the sensor can easily deliver a very linear voltage, and if the input circuit can also be set for zero to five volts then that is certainly the way to go. Of course, if the supply is only 24 volts with no adjustment, then it would be better to add a series resistor to reduce the heating in the regulator.